ANALYTICAL  CHEMISTRY. 


F.  P.  TEE  AD  WELL,  PH.D., 

Profe&or  of  Analytical  Chemistry  in  the  Polytechnic  Institute  of  Zurich. 


AUTHORIZED   TRANSLATION    FROM    THE   GERMAN 
BY 

WILLIAM  T.  HALL,  S.B., 

Assistant  Professor  of  Chemistry,  Massachusetts  Institute  of  Technology. 


VOLUME  II. 
QUANTITATIVE   ANALYSIS. 


THIRD  EDITION,  REVISED  AND  ENLARGED 
TOTAL   ISSUE,    EIGHT   THOUSAND 


NEW  YORK: 

JOHN  WILEY  &  SOSTS. 
LONDON:  CHAPMAN  &  HALL,   LIMITED. 


Copyright,  1904,  1910,  1911, 

BY 

WILLIAM  T.  HALL. 

First  arid  Second  Editions  entered  at  Stationers'  Hall. 


THE  SCIENTIFIC  PRESS 
CRT   DRUMMOND   AND   COMPANY 
BROOKLYN,     N!    V. 


TRANSLATOR'S   NOTE. 


THIS  translation  has  been  made  from  the  second  German 
edition,  but  Professor  Treadwell  has  kindly  indicated  quite  a 
number  of  changes  which  he  intends  to  make  in  the  third  edition. 
Since  it  has  been  my  aim  not  so  much  to  prepare  an  exact  literal 
translation  as  to  publish  a  book  which  will  be  useful  to  English- 
speaking  students,  I  am  under  great  obligations  to  several  of  my 
friends  and  colleagues  for  suggesting  certain  other  changes.  That 
part  of  the  proof  relating  to  Gravimetric  Analysis  has  been  care- 
fully read  and  criticised  by  Professor  Henry  Fay,  that  relating 
to  Volumetric  Analysis  by  Professor  F.  Jewett  Moore,  and  Pro- 
fessor Augustus  H.  Gill  has  twice  read  the  chapter  on  Gas  Analysis. 
I  have  also  received  valuable  assistance  in  reading  the  proof  from 
Messrs.  R.  S.  Williams,  F.  R.  Kneeland  and  J.  R.  Odell,  all  of  the 
Massachusetts  Institute  of  Technology.  I  am  indebted  to  Mr. 
A.  R.  Jackson,  of  Winthrop,  for  several  drawings. 

WILLIAM  T.  HALL. 
1  HUSETTS  INSTITUTE  OF  TECHNOLOGY, 
April,  1904. 


In  preparing  the  second  edition,  the  text  has  been  compared 
with  the  fourth  German  edition,  and  certain  additions  have 
been  made  which  are  not  found  in  the  German  text. 

W.  T.  H. 

March,  1910. 

The  third  edition  has  been  compared  with  the  fifth  German 
edition.  In  the  preparation  of  dilute  acids  the  volume  of  con- 
centrated acid  is  always  referred  to  the  volume  of  water  added. 
Thus  sulphuric  acid  (1:4)  signifies  one  volume  of  concentrated 
acid  diluted  with  four  volumes  of  water.  With  solutions  of 
solids,  the  first  number  signifies  the  weight  in  grams  of  the 
solid  and  the  second  the  volume  of  the  solvent  in  cubic  centi- 
meters. W.  T.  H. 
June,  1911.  iii 


241336 


TABLE  OF  CONTENTS. 


INTRODUCTION. 

PAGE 

Gravimetric  and  Volumetric  Analysis 1 

Direct  and  Indirect  Analyses 2 

Weighing 6 

Reduction  of  Weighing  to  Vacuo 13 

Testing  of  Weights 15 

Filtration  and  Washing  of  Precipitates 18 

Drying  and  Igniting  of  Precipitates 21 

Evaporation  of  Liquids 30 

Drying  Substances  in  Currents  of  Gases 33 

Preparation  of  the  Substance  for  Analysis 35 

Recrystallization 35 


PART    I. 
GRAVIMETRIC  DETERMINATION  OF  THE  METALS. 


V  (ALKALIES). 

Potassium  ......................................................  38 

Sodium  .........................................................  43 

Separation  of  Potassium  from  Sodium  ...........................   43,  50 

Lithium  ........................................................  53 

Determination  of  Lithium,  Potassium,  and  Sodium  ...............  .....  53 

Ammonium  .....................................................  57 

Magnesium  ......................................................  65 

Separation  of  Magnesium  from  the  Alkalies  .........................  68 

GROUP  IV  (ALKALINE  EARTHS). 

Calcium  ........................................................  70 

Strontium  .......................................................  72 

Barium  .........................................................  74 

Separation  of  Calcium  from  Magnesium  ...............      ...........  76 

v 


vi  TABLE    OF  CONTENTS. 

PAGE 

Separation  of  Strontium  from  Magnesium 78 

Separation  of  Barium  from  Magnesium 79 

Separation  of  the  Alkaline  Earths  from  One  Another 79 

GROUP  III. 

Aluminium 82 

Iron 87 

Titanium 100 

Chromium 102 

Uranium 106 

Separation  of  Group  III  from  Group  IV 107,  147 

Separation  of  Iron  from  Aluminium 107 

Separation  of  Iron,  Aluminium,  and  Phosphoric  Acid Ill 

Separation  of  Iron  from  Chromium 113 

Separation  of  Aluminium  from  Chromium 114 

Separation  of  Iron  from  Titanium 114 

Separation  of  Aluminium  from  Titanium 116 

Separation  of  Uranium  from  Iron  and  Aluminium 119 

Manganese 120 

Nickel 129 

Cobalt 138 

Zinc 140 

Separation  of  Manganese,  Nickel,  Cobalt,  and  Zinc  from  the  Alkaline 

Earths 147 

Separation  of  the  Bivalent  from  the  Other  Metals  of  the  Ammonium 

Sulphide  Group 149 

Separation  of  Zinc  from  Nickel,  Cobalt,  and  Manganese 1 56 

Separation  of  Manganese  from  Nickel  and  Cobalt 161 

Separation  of  Cobalt  from  Nickel 161 

Separation  of  Nickel  from  Zinc 165 

Separation  of  Nickel  from  Manganese 165 

Separation  of  Nickel  from  Iron 166 

Removal  of  Ferric  Chloride  by  Ether 167 


GROUP  II. 

(a)  Sulpho-Bases. 

Mercury 168 

Lead 174 

Bismuth 179 

Copper 182 

Cadmium 189 

Separation  of  the  Sulpho-Bases  from  the  Preceding  Groups 192 


TABLE  OF  CONTENTS.  vii 


Analysis  of  Brass 193 

Separation  of  the  Sulpho-Bases  from  One  Another 194 

(6)  Sulpha-Adds. 

Arsenic 205 

Antimony 218 

Tin 228 

Separation  of  Arsenic,  Antimony,  and  Tin  from  Members  of  the  Ammo- 
nium Sulphide  Group 235 

Separation  of  Arsenic,  Antimony,  and  Tin  from  Mercury,  Lead,  Copper, 

Cadmium,  and  Bismuth 235 

Analysis  of  Bronzes 236 

Separation  of  the  Sulpho- Acids  from  One  Another 241 

Analysis  of  Bearing  Metal 252 

Gold 257 

Platinum , 268 

Separation  of  Gold  from  Platinum 271 

Analysis  of  Commercial  Platinum 272 

Selenium 277 

Tellurium 279 

Separation  of  Selenium  and  Tellurium  from  the  Metals  of  Groups  III, 

IV,  and  V 279 

Separation  of  Selenium  and  Tellurium  from  Metals  of  Group  II 280 

Separation  of  Selenium  from  Tellurium 282 

Molybdenum 284 

Tungsten 288 

Separation  of  Molybdenum  from  Tungsten 293 

Analysis  of  Wolframite 296 

Analysis  of  Tungsten  Bronzes .  .  T- 298 

Separation  of  Tungsten  from  Tin 300 

Separation  of  Tungstic  Acid  from  Silica 302 

Vanadium 303 

Separation  of  Vanadium  from  Arsenic  Acid 306 

Separation  of  Vanadium  from  Phosphoric  Acid 307 

Separation  of  Vanadium  from  Molybdenum 308 

Analysis  of  Vanadinite 308 

Determination  of  Vanadium  and  Chromium  in  Iron  Ores  and  Rocks.  .  . .   310 

Determination  of  Vanadium  and  Chromium  in  Pig  Iron 312 

Determination  of  Vanadium,  Molybdenum,  Chromium,  and  Nickel  in 

Steel 313 

GROUP  I. 

Silver..  .   317 


viii  TABLE  OF  CONTENTS. 

GRAVIMETRIC  DETERMINATION  OF  THE   METALLOIDS. 

GROUP  I.* 

PAGE 

Hydrochloric  Acid 320 

Analysis  of  an  Insoluble  Chloride 323 

Free  Chlorine 324 

Chlorine  in  Organic  Compounds 325 

Hydrobromic  Acid 329 

Hydriodic  Acid 330 

Separation  of  the  Halogens  from  One  Another 331 

Hydrocyanic  Acid 337 

Determination  of  Hydrocyanic  Acid  in  the  Presence  of  Halogen  Hydride  339 

Sulphocyanic  Acid 339 

Determination  of  Sulphocyanic  and  Hydrocyanic  Acids 342 

Determination  of  Sulphocyanic  Acid  and  Halogen  Hydrides 342 

Hydroferrocyanic  Acid 342 

Hydroferricyanic  Acid 344 

Hypochlorous  Acid 344 

GROUP  II. 

Nitrous  Acid 344 

Hydrosulphuric  Acid,  H2S 347 

Analysis  of  Tetrahedrite 359 

Acetic  Acid 371 

Cyanic  Acid 371 

Determination  of  Cyanic,  Hydrocyanic,  and  Carbonic  Acids 371 

Hypophosphorous  Acid 372 

GROUP   III. 

Sulphurous  Acid 373 

Selenous  and  Tellurous  Acids 374 

Phosphorous  Acid 374 

Carbonic  Acid ." 375 

Determination  of  Carbon 398 

Determination  of  Carbon  and  Hydrogen  in  Organic  Substances 414 

Dumas  Method  for  Determining  Nitrogen 422 

Oxalic  Acid 427 

Boric  Acid 428 

Molybdic  Acid , 433 

Tartaric  Acid 433 

Meta-  and  Pyrophosphoric  Acids 433 

lodic  Acid 433 

*  For  the  Division  of  Acids  into  Groups  cf.  Vol.  I. 


TABLE  OF  CONTENTS.  ix 

GROUP  IV. 

PAGE 

Phosphoric  Acid 434 

Determination  of  Phosphorus  and  Silicon  in  Iron  and  Steel 440 

Separation  of  Phosphoric  Acid  from  the  Metals 448 

Thiosulphuric  Acid 450 

GROUP  V. 

Nitric  Acid 451 

Chloric  Acid 460 

Perchloric  Acid 462 

GROUP  VI. 

Sulphuric  Acid 464 

Hydrofluoric  Acid 471 

Separation  of  Phosphoric  and  Hydrofluoric  Acids 474 

Separation  of  Fluorine  from  the  Metals 481 

Separation  of  Fluorine  from  the  Acids 482 

Hydrofluosilicic  Acid 483 

GROUP  VII. 

Silicic  Acid 485 

Analysis  of  Silicates 491 

Determination  of  Zirconium  and  Sulphur  in  Rocks 505 

Analysis  of  Chromite 509 

Determination  of  Thorium  in  Monazite 510 

Determination  of  Water  in  Silicates.  .  .  512 


'PART  n. 

VOLUMETRIC   ANALYSIS. 

Measuring  Instruments 514 

Normal  Volume  and  Normal  Temperature 516 

Calibration  of  Measuring  Flasks 522 

Calibration  of  Pipettes 524 

Calibration  of  Burettes 527 

Normal  Solutions .  .  .  530 


I.   ACIDIMETRY   AND   ALKALIMETRY. 

Indicators 538 

Alkalimetry 558 

Acidimetry 571 


X  TABLE  OF  CONTENTS. 

II.  OXIDATION   AND   REDUCTION   METHODS. 

PAGE 

Permanganate  Methods 596 

Potassium  Dichromate  Methods 641 

lodimetry 644 

Reduction  Methods 697 

III.  PRECIPITATION   ANALYSES. 

Determination  of  Silver 702 

Determination  of  Halogens 707 

Determination  of  Cyanogen 710 

Determination  of  Sulphocyanic  Acid 712 

Determination  of  Sulphuric  Acid 714 

Determination  of  Phosphoric  Acid 718 

Determination  of  Nickel 720 

Determination  of  Copper 724 

Determination  of  Lead .  .                                                                                .  726 


PART   III. 

GAS  ANALYSIS. 

The  Collection  and  Confinement  of  Gas  Samples 730 

Calibrating  Gas  Measuring  Instruments 743 

Purification  of  Mercury 747 

Determination  of  Carbon  Dioxide 750 

Ethylene 751 

Benzene 752 

Acetylene 754 

Separation  of  the  Heavy  Hydrocarbons 756 

Oxygen 757 

Carbon  Monoxide 762 

Hydrogen 770 

Methane 774 

Analysis  of  Illuminating  and  Producer  Gas 775 

Technical  Gas  Analysis 786 

Method  of  Hempel 768 

Method  of  Winkler-Dennis 794 

Orsat's  Apparatus 797 

Bunte's  Apparatus 798 

Analysis  of  Gases  which  are  Absorbed  by  Water 800 

Nitrous  Oxide 800 

Nitric  Oxide .  802 


TABLE   OF  CONTENTS.  XI 

PAGE 

Nitrogen 806 

Analysis  of  Gases  by  Titration 808 

Chlorine 808 

Hydrochloric  Acid 814 

Sulphur  Dioxide 815 

Hydrogen  Sulphide 816 

Ethylene 818 

Gas- Volumetric  Methods 822 

Determination  of  Ammonia  in  Ammonium  Salts 822 

Determination  of  Nitrous  and  Nitric  Acids 825 

Hydrogen  Peroxide  Methods 826 

Standardization  of  Permanganate  Solutions 827 

Determination  of  Cerium  in  Soluble  Salts 828 

Silicon  Fluoride 828 

Determination  of  Fluorine 829 

Determination  of  Vapor  in  Gas  Mixtures 831 

APPENDIX. 

Specific  Gravity  of  Acids 838 

Specific  Gravity  of  Alkalies 840 

Tension  of  Aqueous  Vapor 842 

Heats  of  Combustion  of  Gases 845 

Tables  for  Calculating  Analyses 847 

International  Atomic  Weights 849 

Table  of  Chemical  Factors 850 

Logarithms 854 

INDEX 859 


QUANTITATIVE  ANALYSIS. 


INTRODUCTION. 

THE  purpose  of  a  quantitative  analysis  is  to  determine  the 
quantity  of  the  constituents  present  in  a  compound  or  in  a  mix- 
ture. The  methods  to  be  employed  depend  upon  the  nature  of 
the  substances  to  be  determined,  so  that  in  every  case  a  qualitative 
analysis  should  precede  the  quantitative  one.  In  quantitative  analy- 
sis we  distinguish  two  essentially  different  methods  of  procedure : 

I.     GRAVIMETRIC  ANALYSIS  (Analysis  by  Weight). 
II.     VOLUMETRIC  ANALYSIS  (Analysis  by  Volume). 

In  the  case  of  gravimetric  analysis  we  separate  the  component 
to  be  determined  from  a  solution  in  t£*e  form  of  an  insoluble  com- 
pound of  known  chemical  composition,  and  then  determine  the 
weight  of  this  compound;  from  this  we  can  calculate 'the  amount 
of  the  substance  present. 

Suppose,  for  example,  that  we  have  for  analysis  a  sample 
of  barium  chloride.  The  amount  of  barium  present  can  be 
determined  by  dissolving  a  weighed  amount  of  the  chloride  in 
water,  precipitating  the  barium  from  the  solution  by  the  addi- 
tion of  sulphuric  acid  and  weighing  the  insoluble  barium  sulphate 
formed. 

If  we  start  with  a  grams  of  barium  chloride  and  obtain  p  grams 
of  barium  sulphate,  the  amount  of  barium  present  may  be  calcu- 
lated as  follows: 

BaS04:Ba=p:s. 

s  =  15-^77-  •  P  =  weight  of.  barium  in  a  gm.  of  barium  chloride. 


3V    :  '.;-•,  INTRODUCTION. 

It  is,  however,  customary  to  express  the  results  in  percentages; 
therefore  in  this  case  we  have 


x=  .£•=  per  cent,  barium. 

BaSO4   a 

In  the  case  of  volumetric  analysis  the  constituents  are  not 
weighed,  but  they  are  determined  by  measuring  the  amounts  of 
reagents  of  known  strength  which  react  with  them. 

Suppose  that  we  have  a  sample  of  caustic  soda  which  contains 
some  sodium  chloride  as  an  impurity  and  that  we  desire  to  know  how 
much  caustic  soda  there  is  in  100  gms.  of  the  mixture.  A  portion 
of  the  substance  weighing  a  gms.  is  dissolved  in  water,  some  methyl 
orange  is  added  and  hydrochloric  acid  of  known  strength  is  then 
run  into  the  solution  from  a  burette  until  the  alkali  is  just 
neutralized,  this  point  being  reached  when  the  yellow  color  of 
the  solution  changes  to  pink.  If  t  c.c.  of  hydrochloric  acid 
were  necessary,  of  which  1  c.c.  contained  exactly  a  gms.  of 
HC1;  it  is  evident  that  to  neutralize  the  caustic  soda  contained 
in  a  gms.  of  the  mixture  a-t  gms.  of  HC1  were  used  up,  and  it 
follows  : 


5=~HC1  —  a'^==Sms-  NaOH  in  a  gms.  of  the  mixture;  in  100  gms. 
NaOH 


100-  NaOH-  a  -t 
x=  -  Tj?Tj  -  =  per  cent  NaOH. 


We  will  first  consider 

A.     GRAVIMETRIC  METHODS. 
These  are  divided  into 

(a)  Direct  Analyses. 

(b)  Indirect  Analyses. 

In  the  case  of  a  direct  analysis  the  substance  to  be  determined 
is  separated  from  the  solution  in  the  form  of  an  insoluble  com- 
pound and  weighed. 


GRAVIMETRIC  METHODS.  3 

The  determination  of  barium  in  barium  chloride  was  an  example 
of  a  direct  analysis. 

The  indirect  method  depends  upon  the  fact  that  when  two  or  more 
substances  are  made  to  undergo  the  same  chemical  treatment  they  ex- 
perience a  relatively  different  change  of  weight. 

For  example,  suppose  that  we  have  a  mixture  of  the  chlorides 
of  sodium  and  of  potassium  and  desire  to  determine  the  relative 
amounts  of  each  of  the  two  substances  in  the  mixture.  For  this 
purpose  a  portion  of  the  mixture  (a  gms.)  is  weighed,  dissolved 
in  water,  the  chlorine  precipitated  as  silver  chloride,  and  the 
weight  of  the  latter  determined  (p  gms.).  From  these  data  it 
is  possible  to  calculate  the  amount  of  sodium  chloride  and  of  potasr 
sium  chloride  that  was  present  in  the  mixture. 

If  we  let  x  represent  the  amount  of  the  sodium  chloride,  y 
the  amount  of  the  potassium  chloride,  a  the  amount  of  silver 
chloride  formed  from  x  gms.  of  sodium  chloride,  and  /?  the 
amount  of  silver  chloride  formed  from  y  gms.  of  potassium 
chloride,  then 

XaCl"    KC1 

x    -f   y  =  a 

AgCl    AgCl 
a    +    p  =  p. 

We  have,  therefore,  two  equations  with  apparently  four  un- 
known quantities,  but  a  and  /?  can  be  expressed  in  terms  of  x 
and  y: 

NaCl:AgCl=z:a 


AgCl        ,  AgCl 

and  -  however,  are  known  quantities;  they  are  simply 


the  quotients  of  the  molecular  weights  in  question. 

If  we  designate  by  m  the  fraction  ^,  p.  and  by  n  the  frao- 

AgCl 
tion  we  have 

x+  y  =a 
mx+ny=p 


4  INTRODUCTION. 

and  from  this  we  can  calculate 

p  —  n-a 

x  =  — and  y  =  a—x 

m—n 

or 

_JL.  n 

m—n        m—n 

All  indirect  analyses  may  be  calculated  by  means  of  this  last 
general  equation. 

In  the  above  example 

_AgCl_143:34  _AgCI_  143,34 

in — AT   .^i —  _0  ift  — 4.-±<j£\j}   n —  T^m  —  T  4  m  — i.y^^o 
NaCl      ob.46  IvCl      74.56 

and 

m  —  n  =  0.52  97. 

If  these  values  are  substituted  in  the  above  equations  we 
obtain 

s=1.888-p-3.628-a. 

Consequently,  in  order  to  determine  the  amount  of  sodium 
chloride  in  the  original  mixture  it  is  only  necessary  to  determine 
the  values  a  and  p,  then  multiply  them  by  the  coefficients  3.628 
and  1.888  respectively,  and  subtract  the  first  product  from  the 
last. 

Although  this  method  appears  so  simple  and  attractive  on  paper, 
impossible  values  are  often  obtained  in  practice,  so  that  it  is  always 
necessary  to  be  very  cautious  about  using  an  indirect  method. 

The  experimental  errors  which  are  unavoidable  in  such  an 
analysis  are  multiplied  by  the  value  of  the  coefficients;  thus  in 
the  above  case  the  actual  error  in  the  determination  of  the  weight 
a  is  multiplied  by  3.63  ...  and  the  error  in  determining  the  weight 
of  the  silver  chloride  (p)  is  multiplied  by  1.89  ... 

It  is  clear,  therefore,  that  an  indirect  analysis  becomes  more 
accurate  in  proportion  as  the  coefficients  are  small  and  when  the 
error  in  determining  a  and  p  is  slight. 

In  the  above  example  the  coefficients  are  relatively  small  and 
consequently  good  results  are  to  be  expected,  and  experiment 
shows  this  to  be  the  case. 

Example:  A  mixture  weighing  0.5480  gm.  (a)  and  consisting  of 


GRAVIMETRIC  METHODS.  5 

0.4966  gm.  sodium  chloride  (x)  and  0.0514  gm.  potassium  chloride 
(y)  yielded  1.3161  gm.  of  silver  chloride  (p),  but  from  the  values 
of  a  and  p  we  can  calculate  those  of  x  and  y: 

x=  1.888-1.3161  -3.628-0.5480 
=  0.4963  gm.  sodium  chloride; 
j/  =  0.0517  gm.  potassium  chloride. 

The  calculated  values,  therefore,  show 

99.92  per  cent,  of  the  true  value  for  the  sodium  chloride, 
100.6    per  cent,  of  the  true  value  for  the  potassium  chloride. 

Although  the  above  results  are  satisfactory,  it  must  be  borne 
in  mind  that  the  analysis  was  carried  out  with  chemically  pure 
substances.  If  this  were  not  so,  as  is  usually  the  case  in  prac- 
tice, the  results  would  be  far  less  accurate. 

The  same  analysis  may  be  performed  in  a  much  more  simple 
manner  than  as  above  described,  by  weighing  the  mixture  of  the 
chlorides  in  a  platinum  crucible,  then  changing  them  to  sulphates 
(by  treatment  with  sulphuric  acid  and  evaporating  off  the  excess 
of  the  latter)  and  again  weighing.  In  this  case  the  actual  experi- 
mental error  is  slight  and  excellent  results  might  be  expected. 

We  have 

Nad      KC1 
x    +    y  =  a 


m-n-  0.0464 
Now 

(1)  x  +  y  -a 

(2)  mx+ny  =  p 
and 

(3)  x== 


m—  n         m—n 


6  INTRODUCTION. 

Substituting  the  values  for  m  and  n  in  equation  (3  we  obtain 
z  =  21.547-;p-25.181a. 

In  this  case  the  coefficients  are  very  large,  so  that  the  analytical 
error  is  multiplied  enormously  in  the  calculation,  so  much  so  that 
it  is  impossible  to  obtain  even  approximate  values  except  when 
the  mixture  is  composed  of  about  equal  parts  of  the  two  chlorides. 
Example:  In  a  mixture  containing  about  equal  parts  of  the 
two  salts  there  was  found 

99.64  per  cent,  of  the  sodium  chloride  present; 
100.76  per  cent,  of  the  potassium  chloride  present. 

In  a  mixture  containing  considerable  sodium  chloride  and  Jiwtib 
potassium  chloride  there  was  found 

(a)     95.0  per  cent,  of  the  sodium  chloride  present; 
x      148.0  per  cent,  of  the  potassium  chloride  present. 
(6)     96.8  per  cent,  of  the  sodium  chloride  present; 
129.9  per  cent,  of  the  potassium  chloride  present. 

The  values  obtained  are,  therefore,  worthless. 

In  the  case  of  a  direct  analysis  the  small  unavoidable  errors 
exert  a  much  less  influence  upon  the  result,  so  that  a  direct  deter- 
mination should  always  be  preferred. 

Only  in  those  cases  where  a  direct  method  is  unknown  should  one 
resort  to  an  indirect  analysis! 

OPERATIONS. 

The  principal  operations  of  quantitative  analysis  are  those 
of  weighing,  filtration,  and  the  washing,  drying,  and  ignition  of 
precipitates. 

Weighing. 

The  balance,  as  used  for  purposes  of  quantitative  chemical 
analysis,  is  shown  in  Fig.  1. 

It  consists  of  a  horizontal  lever  with  two  arms  of  equal 
length,  and  in  order  to  be  serviceable  it  must  be  accurate  and 
sensitive. 

It  fulfils  the  first  condition  if 

(1)  The  arms  of  the  lever  are  equally  long; 


WEIGHING.  7 

(2)  The  point  of  support  (the  fulcrum)  lies  above  the  centre 
of  gravity; 

(3)  The  fulcrum  (a  knife-edge)  and  the  knife-edges  from  which 
the  pans  are  suspended  lie  in  the  same  plane   and  are  parallel 
to  one  another. 

The  balance  is  more  sensitive  the  greater  the  displacement 
of  the  position  of  equilibrium  brought  about  by  the  addition  of 
a  small  weight,  e.g.  one  milligram. 


FIG.  1. 

The  sensitiveness,   or  sensibility,   may   be  expressed  by  the 
equation  : 


tan  a  * 


q-d 


in  which  p  is  the  weight  added,  I  the  length  of  the  balance-arm, 
q  the  weight  of  the  beam,  and  d  the  distance  between  the  centre 
of  gravity  and  the  point  of  support. 

The  sensitiveness  of  the  balance  is  greater,  therefore,  the 
heavier  the  weight  added,  the  longer  the  beam,  the  lighter  the 
beam,  and  the  shorter  the  distance  between  the  centre  of  gravity 
and  the  point  of  support. 


*  a  is  the  angle  through  which  the  pointer  moves  on  the  addition  of  the 
small  weight. 


&  INTRODUCTION. 

For  convenience  in  determining  the  position  of  the  balance, 
a  pointer  is  fastened  to  the  beam  which,  when  the  equilibrium  is 
established,  rests  at  the  zero  of  a  scale  on  an  ivory  plate  below. 

The  object  to  be  weighed  is  placed  upon  the  left  scale-pan  and 
the  weights  upon  the  right  pan;  the  beam  is  lowered  and  the 
balance  set  in  slight  motion,  by  producing,  with  the  hand,  a 
gentle  draft  of  air  upon  one  of  the  pans.  If  the  correct  weight 
has  been  added,  the  pointer  will  swing  to  the  same  number 
of  scale  divisions  to  the  right  of  the  zero  that  it  does  to  the  left, 
provided  that  it  does  so  when  there  is  nothing  in  either 
scale-pan,  which  is  usually  not  the  case.  It  is  to  be  noted  that 
when  the  zero-point  of  the  balance  (i.e.,  the  point  of  the  scale 
at  which  the  pointer  rests  when  the  balance  is  in  equilibrium 
with  nothing  in  either  scale-pan)  coincides  with  the  zero  of  the  scale, 
it  may  change  during  the  course  of  the  day,  so  that  disregard  of 
this  fact  may  lead  to  a  considerable  error. 

The  cause  of  the  displacement  of  the  zero-point  is  that  the 
first  condition  for  the  accuracy  of  a  balance  is  not  fulfilled.  On 
account  of  unequal  warming  the  arms  become  of  unequal  length. 

In  order  that  accurate  weighings  may  be  obtained,  it  is  necessary 
to  make  them  independent  of  any  inequality  in  the  lengths  of  the 
arms,  which  can  readily  be  done,  as  the  following  consideration 
will  show.  In  the  case  of  a  lever,  equilibrium  takes  place  when 
the  statical  moments  are  equal. 

By  statical  moment  is  understood  the  product  of  the  force 
into  the  length  of  the  lever-arm,  and  the  length  of  the  lever-arm 
is  the  perpendicular  distance  from  the  axis  of  revolution  (the 
fulcrum)  to  the  line  of  action  of  the  force. 

If  an  object,  whose  weight  Q  (Fig.  2)  is  to  be  ascertained,  is 
placed  upon  the  left  balance-pan  and  equilibrium  is  established 
(the  pointer  rests  at  zero)  by  putting  weights  amounting  to  P 
gms.  in  the  right  balance-pan,  then 

(1)  Ql=Plt. 

If  now  the  object  Q  is  placed  on  the  right-hand  balance-pan  and 
the  balance  again  brought  to  the  state  of  equilibrium  by  placing 
weights  upon  the  left-hand  balance-pan,  in  this  case  the  weights 


WEIGHING.  9 

will  not  as  a  rule  amount  to  P  gms.,  but  to  Pl  gins.     Since,  how- 
ever, equilibrium  has  been  reached,  we  have 


(2) 


I' 


FIG.  2. 


If  equation  1  is  multiplied  by  equation  2,  we  obtain 


The  true  weight  is  obtained,  tnerefore,  by  taking  the  geometric 
mean  of  the  two  values.  For  practical  purposes,  however,  it  is 
sufficiently  accurate  to  take  the  arithmetical  mean,  in  which  case 
the  true  weight  of  the  object  would  be: 


This  method  of  obtaining  the  true  weight  independent  of  the 
lengths  of  the  balance-arms  is  known  as  that  of  double  weighing. 

The  same  end  is  obtained  by  Borda's  method  of  substitution. 

According  to  this  method  the  object  to  be  weighed  (Q)  is  coun- 
terbalanced (tared)  by  means  of  shot,  sand,  weights,  etc.,  the 
object  Q  is  then  removed  and  equilibrium  with  the  tare  is  again 
established  by  placing  weights  upon  the  scale-pan.  We  have, 
then,  as  a  result  of  the  first  weighing, 


10  INTRODUCTION. 

and  from  the  second  weighing, 

Pl-Tlt 

from  which  it  follows; 


The  latter  method  is  used  chiefly  in  weighing  large  objects. 

For  ordinary  analytical  work  the  weighing  is  made  by  the 
method  of  swings. 

First  of  all  the  zero-point  of  the  balance  is  determined  by  setting 
the  balance  in  motion  (without  any  load  in  either  pan),  observing 


I' 


FIG.  3. 

and  recording  the  turning-points,  or  extreme  positions,  of  the 
pointer  on  the  scale  of  an  uneven  number  of  swings  (say  five*) 
and  taking  the  mean  of  the  readings.  In  order  to  give  the  same 
algebraic  sign  to  all  the  observed  readings  it  is  best  to  number 
the  divisions  on  the  scale  from  left  to  right  from  0  to  20  so  that  the 
zero-point  in  case  both  balance-arms  were  of  equal  length  would 
be  numbered  10. 

The  next  thing  to  be  determined  is  the  sensitiveness  of  the 
balance  for  the  object  to  be  weighed.  For  this  purpose  the  object 
is  placed  in  the  left-hand  balance-pan,  and  by  placing  weights  in 
the  right-hand  pan  equilibrium  is  established  as  nearly  as  possible 

*  The  first  two  swings  are  inaccurate  on  account  of  the  jar  in  shutting 
the  balance-door,  etc.,  so  that  they  are  disregarded. 


WEIGHING.  1  1 

and  the  point  of  rest  of  the  pointer  on  the  scale  is  determined  as 
above.  An  additional  weight  of  1  mgm.  is  added,  or  removed  if 
the  object  was  too  light  before,  and  the  point  of  rest  is  again  deter*- 
mined. 

The  difference  (d)  between  this  and  the  previous  point  of  rest 
gives  the  sensitiveness  of  the  balance.  Assuming  the  zero-  point  to 
lie  at  10.22,  the  first  point  of  rest,  obtained  with  a  load  of  19.723 
gms.,  to  be  at  9.80,  and  the  point  of  rest  with  a  load  of  1  mgm.  less 
(i.e.,  with  a  load  of  19.722  gms.)  to  lie  at  12.32,  then  the  sensitive- 
ness of  the  balance  will  amount  to  12.32  —  9.80  =  2.52  scale  divisions. 

As  the  zero-point  of  the  balance  was  at  10.22  and  the  point 
of  rest  with  a  load  of  19.723  gms.  was  9.80,  it  follows  that  the 
object  was  lighter  than  the  weights  in  the  right-hand  pan,  and 
in  fact  the  excess  of  weights  in  the  pan  was  sufficient  to  move 
the  point  of  rest  10.22  —  9.80  =  0.42  divisions  on  the  scale.  This 
amount  can  be  calculated  from  the  determination  of  the  sensitive- 
ness of  the  balance  as  follows: 

Since  2.52  of  the  scale  divisions  correspond  to  1  mgm.,  then  0.42 
of  the  scale  divisions  correspond  to  the  weight  which  must  be  sub- 
tracted from  19.723  gms.  in  order  to  obtain  the  true  weight;  there- 
fore 

2.52:1  =  0.42:  x 

0  42 
£=•  =  0.17  mgm.,  or  about  0.2  mgm. 


The  true  weight  of  the  body  in  air  is  consequently 
19.723  -  0.0002  =  19.7228  *  gms. 

In  making  a  weighing  one  should  always  accustom  himself  to 
note  the  observations  methodically,  as  follows: 

Assume  that  a  platinum  crucible  is  to  be  weighed. 


*  As  most  analytical  balances  will  scarcely  detect  with  certainty  less 
than  ^o  mgm.,  the  weight  is  expressed  only  to  the  fourth  decimal.  If  the 
fifth  decimal  place  in  a  calculation  amounts  to  six  or  more,  the  number  in  the 
fourth  decimal  place  is  increased  one. 


12 


INTRODUCTION. 


Zero-point. 

I.  Point  of  Rest  with  Load 
of  12.052  gms. 

II.  Point  of  Rest  with  Load 
of  12.053  gms. 

Left. 

Right. 

Left. 

Right. 

Left. 

Right. 

4.2 
4.6 
5.1 

17.6 
17.1 

5.8 
6.2 
6.6 

18.7 
18.3 

3.5 
3.8 

4.2 

15.8 
15.4 

Sum=13.9 
Mean  =4.  63 

34.7 
17.35 
4.63 

18.6 
6.2 

37.0 
18.5 
6.2 

11.5 
3.83 

31.2 
15.60 
3.83 

Sum  of  both  means=  21  .98 
Mean                       =10.99 

24.7 
12.35 

19.43 
9.71 

Sensitiveness =  12.35  —  9.71  =  2.64  scale  divisions. 
12.35-10.99=  1.36  scale  divisions.  • 

1.36  : 2.64=  0.5  mgm. 
Weight  of  crucible=  12.052 +  0-0005  =  12.0525  gms. 

The  sensitiveness  of  a  balance  varies  slightly  with  the  load. 
It  is  simplest  to  determine  once  for  all  the  sensitiveness  for  50 
gms.,  20  gms.,  IQ^ms.,  5  gms.,  and  2  gms.,  place  a  card  in  the 
balance  with  the  resists  obtained  and  use  the  numbers  as  required. 

In  this  way  the  sensitiveness  of  a  balance 


with  a  load  of 
50  gms. 
20      " 
10      " 

5      " 

2      " 


was  found  to  equal 
2.23  scale  divisions 
2.28  "  " 

2.64     "  " 

2.66      "  " 

2.66      "  " 


The  determination  of  the  zero- point,  however,  must  be  made 
with  every  weighing.  If  a  number  of  weights  are  to  be  made 
one  after  another  it  suffices  to  determine  the  zero-point  at  the 
beginning  and  at  the  end  and  to  use  the  mean  of  the  two  deter- 
minations. In  case  of  very  heavy  loads,  however,  the  zero- point 
should  be  determined  before  and  after  each  weighing  and  the 
mean  value  used. 


REDUCHON  OF  WEIGHING*  TO   YACUO.  13 


Reduction  of  Weighing  to  Vacuo. 

Since  most  of  our  we'ghings  are  made  in  the  air  with  brass 
weights,  we  are  constantly  introducing  an  error  due  to  the  dis- 
placement of  air.  This  error  is  so  small  that  it  can  be  disregarded 
in  ordinary  analyses;  in  the  case  of  the  most  accurate  work, 
however,  as  in  atomic  weight  determinations,  calibrations  of 
measuring  vessels,  etc.,  it  should  never  be  neglected.  In  such 
cases  the  apparent  weight  must  be  reduced  to  vacuo  as  follows: 
1  c.c.  of  air  at  15°  C.  and  760  mm.  pressure  weighs  0.0012  gm.=  /. 

The  specific  gravity  of  brass  is  8.0  =  s'.* 

The  specific  gravity  of  the  substance  weight  =  s. 

The  body  that  weighs  po  gms.  in  vacuo  will  be  balanced  by 
p  gms.  in  the  air. 

The  loss  in  weight  of  the  substance  is  —  A  gms. 

s 

The  loss  in  weight  of  the  brass  weights  is  -^  X  gms. 

s 

The  total  loss  therefore,  ./&  J-t£  A     The  weight  of  the 

(l-- 
\       * 


substance  in  vacuo  is: 

,  Po*     pk 
Po=P  +  L  and 


1 

s 


On  account  of  the  small  values  of  the  fractions  —  and  — . , 

s  s 

sufficient  accuracy  is  obtained  by  simplifying  the  expression  so 
that  it  becomes 

Po= 


Instead  of  making  the  computation,  the  following  table  of 
Kohlrausch  may  be  used : 

*  Brass  has  a  density  of  8.4,  but  the  density  of  most  analytical  weights  is 
nearer  8.0. 


INTRODUCTION. 


REDUCTION  OF  A   WEIGHING  MADE   WITH    BRASS    WEIGHTS   TO    VACUO. 
METHOD    OF    F.  KOHLRAUSCH. 


s 

k 

s 

k        s 

k 

0.7 

+  1.56 

2.0 

+  0.45 

8 

-0.00 

0.8 

1.35 

2.5 

0.33 

9 

0.017 

0.9 

1.18 

3.0 

0.25 

10 

0.030 

1.0 

1.05 

3.5 

0.19 

11 

0.041 

1.1 

0.94 

4.0 

0.15 

12 

0.050 

1.2 

0.85 

4.5 

0.12 

13 

0.058 

1.3 

0.77 

5.0 

0.09 

14 

0.064 

1.4 

0.71 

5.5 

0.07 

15 

0.070 

1.5 

0.65 

6.0 

0.05 

16 

0.075 

1.6 

0.60 

6.5 

0.03 

17 

0.079 

1.7 

0.56 

7.0 

0.02 

18 

0.083 

1.8 

0.52 

7.5 

0.01 

19 

0.087 

1.9 

0.48 

8.0 

+  0.00 

20 

0.090 

2.0 

+  0.45 

21 

-0.093 

fc  =  1.20( — )  mgm.     If  a  substance  of  specific  gravity  s 

\S        o.U/ 

weighs  g  grams  in  the  air,  then  g  -  k  mgms.  are  to  be  added  to  the 
weight  in  air  in  order  to  obtain  the  weight  in  vacuo. 


TESTING  OF   WEIGHTS. 


Testing  of  Weights. 

Although  it  is  now  possible  to  buy  nearly  perfect  weights,  yet 
their  accuracy  should  always  be  tested. 

For  almost  all  analytical  purposes  it  matters  not  whether  the 
50  gm.  weight  weighs  exactly  50  gins.,  but  it  is  essential  that  the 
individual  weights  represent  the  corresponding  relations  to  one 
another. 

In  most  sets  of  weights  the  following  are  found:  50  gm.,  20  gm., 
10 gm.,  10'  gm.,  5  gm.,  2  gm.,  1  gm.,  1'  gm.,  1"  gm.,  0.5  gm.,  0.2  gm., 
0.1  gm.,  0.1'  gm.,  0.05  gm.,  0.02  gm.,  0.01  gm.,  0.01'  gm.;  a  rider 
(weighing  either  10  or  12  mgms.  according  as  to  whether  the  bal- 
ance-arm is  divided  into  10  or  12  equal  parts  between  the  fulcrum 
and  the  point  of  suspension  of  the  right-hand  balance-pan);  and 
usually  smaller  weights  weighing  5,  2,  1,  1,  1  mgms. 

The  weights  may  be  tested  in  the  following  manner :  * 

The  assumption  is  first  made  that  the  sum  of  the  larger  weights 
is  equal  to  100  gms.  =  100,000  mgms.,  and  the  weights  of  the  single 
pieces  obtained  by  the  method  of  double  weighing  are  compared 
with  one  another,  e.g. : 

50  gm.  wt.  agaiijst  20+ 10+  KX+5+2+ 1+ 1'+ 1" 

and  it  is  found: 

Left  Right 

(1)  50gm.  +  0.31mgm.  =(20+10+10'+...) 

Left  Right 

(2)  (20+ 10+ 10'+ .  .  . )  =50  gm.  +  0.61  mgm. 

from  which  it  follows: 

,  0.31  mgm. +  0.61  mgm. 
50  gm.+ s_JE s__  =50  gm.-f-0.46  mgm. 

=  (20+10+10'+...) 
or 

50  gm.  =  (20+ 10+  10'+ .  .  . )  -0.46  mgm. 

*  Kohlrausch;  "teitfaden  der  prakt.  Phys.,  \VAuflage,  p.  34.  -See  also 
T.  W.  Richards,  Journ.  Am.  Chem.  Soc.  (1900)  XXII,  144,  where  the  method 
used  at  Harvard  is  described. 


!6  INTRODUCTION. 

The  other  weights  are  compared  in  the  same  way,  obtaining,  for 
example, 

50gm.  =  (20+10+10'+.  .  Ogm.  +  Amgm. 
20   "    =  10+10'  +  B     " 

10'  "    =  10  +C     "     * 

(5+2+1+...)=  10  +D     " 

m  which  A,B,C,  and  D  may  be  either  positive  or  negative. 

The  sum  of  the  weights  (50+20+10+10'+.  .  .)  was  assumed 
to  equal  100  gms.,  and  with  the  help  of  the  preceding  equations 
each  weight  is  expressed  in  terms  of  the  10  gm.  weight;  then 

10XlO  +  A+25+4C.+  2Z>=(50+20  +  10  +  .  .  .)  =  100  gms.  and 
the  10  gm.  weight  itself: 

A+2B+4C+2D 


10=10- 


10 


c    A+2B+4C  +  2D 
If  we  let  S=  -  —  -  ,  then  we  obtain 

10-10  gm.-S 

10'  =10  gm.-S  +  C 
5+2  +  l  +  l'  +  l"=10  gm.-S  +  D 

20  =  20  gm.-2S  +  B  +  C 
50  =  50  gm.-5S  + 
=  50  gm.+J.4 

The  sum  JA  +  B+2C+D  —  5S  should  equal  0,  which  serves  as 
a  test  for  the  accuracy  of  the  observations. 

The  5  gm.  weight  is  now  compared  with  the  2+l+l/+l"  in 
exactly  the  same  way,  with  the  result  that 


2  =  1+1'  +6 

l'  =  l  +c 


*  It  is  well  to  mark  the  weights  of  the  same  denomination  so  that  they 
may  be  distinguished  from  one  another. 


TESTING   OF  WEIGHTS. 
According  to  the  preceding  work 

5+2+1+1'+!"  =  10,000-3+ Z> 


consequently 
and 


10Xl+a+26+4c+  2d=  10,000 -S+Z) 
o  +  26+  4c+  2d+  S-D 


1  =  1000- 


10 


If  we  let      s 


10 

l  =  1000-s 
l'  =  1000-s+c 


,  we  obtaio 


2=2000-2s+6+c 

5  =  5000-5s+a+6+2c+d 

In  the  same  way  the  smaller  weights  are  tested  until  finally 
the  following  correction  table  is  obtained. 


TABLE  FOR  CORRECTIOX  OF  WEIGHTS. 


2  +  0.1+0.1' 


50  =  50  g.  +  *A 
20  =  20g.-2S  +  £  +  C 
10  =10g.-S 
10'=10g.-5  +  C 

5-2-1  +  1'  +  !"  = 
,__  , 
=  10  g.  -.S  +  D 

=  1  g.-5  +  a 

0.1 

5 
2 
1       =10g.  —  S  +  D 
1' 
1" 

5   =  5g.  —  5s  +  a+6  + 
2  =  2g.-2s  +  6  +  c 

1  =  1  g.-s 
1'  =1  g.-s  +  c 
1"=1  g.-s  +  rf 

0.5  =1  g.  —  5s'  +  .3  + 

0.2  =or2gT-2l'+r  + 
+  3 
0.1  =0.1g.-s' 
0.1'  =  0.1g.-s'  +  ^ 

0.05  =  0.5g.  —  5s"  + 
0.02  =  0.02  g.-  2s" 

O.OL=0.01  g.-s" 
0.01'  =  0.01g.-s" 
Rider  =  0.01  g.-s" 

1+ 

+ 

-t-o 

Sum  =  100  g. 

10g.-5+D 

1  g.-s  +  a                         O.lg.-s'  +  ff 

*  J  =  0.05  +  0.02  +  0.01+0.01'  +  Rider.      (Rider  =  0.01  g.) 

The  milligram  weights  may  be  standardized  in  exactly  the 
same  manner.  It  is  much  more  convenient,  however,  and  the 
accuracy  attained  is  almost  exactly  the  same,  if  instead  of  using 
these  very  small  weights  the  rider  is  hung  upon  the  whole 
divisions  of  the  balance-arm  in  order  to  obtain  the  weight  in 
milligrams;  for  the  estimation  of  the  fractions  of  the  milligram  it 
is  better  to  calculate  them  from  the  sensitiveness  of  the  balance. 


1 8  INTRODUCTION. 

The  weights  should  never  be  touched  with  the  -fingers,  but  should 
always  be  lifted  by  means  of  the  pincers  provided  with  each  set,  and 
nothing  should  be  placed  on  or  removed  from  the  balance-pans  with- 
out arresting  the  balance,  i.e.,  raising  the  mechanical  supports  so 
that  the  beam  no  longer  rests  upon  its  knife-edges. 

Filtration  and  Washing  of  Precipitates. 

How  large  should  the  filter  be  and  how  many  times  should  the 
precipitate  be  washed? 

With  regard  to  the  latter  question  it  is  evident  that  the  pre- 
cipitate should  be  washed  until  the  soluble  matter  is  completely 
removed.  It  is  clear,  however,  that  this  point  will  never  be  reached 
because  a  part  of  the  solution  always  remains  on  the  filter,  but  it 
is  not  difficult  to  make  the  amount  of  the  dissolved  substance 
remaining  so  small  as  to  be  negligible.  When  the  amount  of 
dissolve:!  substance  remaining  on  the  filter  is  so  small  that  it  could 
not  be  detected  by  our  balance,  we  consider  the  precipitate  to  be 
completely  washed. 

The  aim  should  be  not  only  to  remove  all  of  the  soluble  matter, 
but  to  accomplish  this  with  as  little  wash  water  as  possible. 

No  precipitate  is  absolutely  insoluble,  so  that  it  is  clear  that 
every  unnecessary  excess  of  wash  water  causes  harm  by  removing 
a  fraction  of  the  precipitate,  and  the  greater  the  excess  of  the  wash 
water  the  greater  the  amount  of  the  precipitate  dissolved. 

The  amount  of  wash  water  to  be  used  depends  largely  upon 
the  nature  of  the  precipitate  itself.  Amorphous,  gelatinous  pre- 
cipitates always  require  more  washing  than  crystalline,  granular 
ones.*  As  a  rule,  it  may  be  said  that  the  process  of  washing 
must  be  continued  until  the  substance  which  is  being  washed 
out  can  be  no  longer  detected  in  the  last  filtrate.  In  case  the 
filtrate'  must  be  used  for  another  determination,  it  is  obvious  that 
the  filtrate  should  not  be  tested  too  soon.  When  should  the 
filtrate  be  tested? 

Let  us  assume  the  filter  to  hold  10  c.c.,  the  solution  to  drain 

*  The  reason  why  some  precipitates  require  more  washing  than  others 
is  due  to  the  fact  that  the  degree  of  adsorption  varies.  (Cf.  Ostwald,  Die 
wissenschaftl.  Grundl.  der  analyt.  Chem.,  p.  19.) 


FILTRATION  AND   W 'ASHING   OF   PRECIPITATES.  19 

to  the  last  drop  from  the  paper,  the  amount  of  the  solution  held 
back  by  the  precipitate  and  filter  to  be  1  c.c.  and  to  contain 
0.1  gm.  of  the  solid  substance  which  is  to  be  removed  by  wash- 
ing. 

The  filter  is  filled  to  the  upper  edge  with  wash  water  and 
allowed  to  drain  to  the  last  drop  n  times,  until  not  more  than 
5/100  mgm.  of  the  substance  to  be  removed  by  washing  remains. 

According  to  our  assumption.  9  c.c.  drain  off  and  1  c.c.  remains 
behind;  we  have  consequently: 

Removed  by  the  There  remains  after  the 

1st  washing,  0.1-9/10  gm.  1st  washing  0.1-1/10  gm. 

2d        "         0.1- 9/10- 1/10  gm.  2d        "        0.1- 1/10- 1/10  gm. 

3d        "         0.1-9/10.(l/10)2gm.  3d        "        O.M/10-(1/10)2  gm. 


nth      "         0.1-  9/10-  (l/10)n-lgm.    nth       "        0.1-  1/10-  (1/10)«-  1  gm. 

After  washing  n  times,  therefore,  the  amount  removed  by 
washing  is  the  sum  of  the  decreasing  geometric  series  of  which  the 
first  term  is  0.1-9/10  and  the  constant  factor  is  1/10. 

In  case  n  =  4,  the  sum  of  the  series  is 


10 

After  washing  the  precipitate  four  times,  therefore,  0.09999  gm. 
of  the  impurity  has  been  removed.  According  to  the  assumption 
that  there  was  originally  0.1  gm.  of  this  substance,  there  remains 
in  the  precipitate  only  0.00001  gm.,  or  in  other  words  a  negligible 
amount. 

Consequently,  the  filtrate  should  be  tested  qualitatively  for 
the  substance  to  be  removed  only  after  the  precipitate  has  been 
washed  four  times. 

Often  the  washing  will  be  found  to  have  been  complete  after  the 
fourth  washing,  but  as  a  rule  this  will  not  be  the  case,  and  in  many 
cases  it  will  be  found  necessary  to  repeat  the  operation  for  from 


20 


INTRODUCTION. 


ten  to  twenty  times.  In  the  processes  which  are  described 
it  will  usually  be  stated  how  far  to  carry  the  washing. 

Now  with  regard  to  the  second  point,  how  should  a  precipitate 
be  washed  with  the  least  possible  amount  of  wash  water?  Accord- 
ing to  the  above  consideration  it  is  necessary  to  wash  every  pre- 
cipitate at  least  four  times,  in  which  case  the  filter  should  be  en- 
tirely filled  each  time,  and  it  is  evident  that  the  size  of  the  filter- 
paper  will  influence  the  amount  of  wash  water  used. 

The  filter,  therefore,  should  be  made  as  small  as  possible, 
irrespective  as  to  whether  there  is  little  or  much  liquid  to  filter. 
The  size  of  the  filter  used  should  be  regulated  entirely  by  the  amount 
of  the  precipitate  and  not  at  all  by  the  amount  of  the  liquid  to  be  fil- 
tered. The  mistake  should  not  be  made,  however,  of  using  too 
small  a  filter.  The  precipitate  should  never  reach  the  upper 
edge  of  the  paper;  about  5  mm.  should  remain  free,  and  even  in 
this  case  the  filter  should  not  be  so  completely  filled  as  in  Fig.  4,  a. 


FIG.  4. 

It  is  better  to  have  the  filter  filled  about  as  much  as  is  shown  in 
Fig.  4,  b,  in  order  that  sufficient  room  is  left  for  the  wash  water. 

The  use  of  too  large  filters  is  one  of  the  inexcusable  ana- 
lytical errors. 


THE  DRYING  AND  BURNING   OF  PRECIPITATES.  21 


The  Drying  and  Igniting  of  Precipitates. 

Before  a  precipitate  can  be  weighed  it  must  be  absolutely  dry. 
Those  precipitates  which  do  not  undergo  a  change  of  weight  on 
ignition  are  treated  as  follows: 

(a)    THE  PRECIPITATE  IS  IGNITED  DRY. 

This  method,  in  which  the  precipitate  is  separated  from  the 
filter,  the  filter  burnt  by  itself,  the  ash  added  to  the  main  part 
of  the  precipitate  and  the  mixture  then  ignited  to  constant 
weight,  is  used  in  those  cases  when  the  ignited  substance  will  be 
reduced  by  the  burning  paper,  e.g.,  in  the  case  of  precipitates  of 
silver  chloride,  lead  sulphate,  bismuth  oxide,  etc. 

In  order  to  perform  this  operation  it  is  first  necessary  that  the 
filter  and  precipitate  should  be  completely  dried  at  100°  C.  For 
this  purpose  the  funnel  containing  the  filter  is  carefully  covered 
with  a  piece  of  filter-paper,*  placed  in  a  drying-closet  (preferably 
one  that  is  heated  by  steam)  and  dried  at  100°  C.  When  per- 
fectly dry,  a  weighed  crucible  is  placed  upon  a  piece  of  glazed 
paper  of  about  20  sq.  cm.  (Fig.  6,  left)  and  the  dry  precipitate  is 
carefully  shaken  into  the  crucible,  removing  it  from  the  paper  as 
completely  as  possible  by  gentle  rubbing  with  a  platinum  spatula. 
Any  small  particles  of  the  precipitate  which  may  have  fallen 
upon  the  glazed  paper  are  brushed  into  the  crucible  with  the 
aid  of  a  feather  (Fig.  6).  Small  particles  of  the  precipitate  will 
still  always  adhere  to  the  paper  and  these  must  be  weighed. 
In  order  to  accomplish  this,  the  filter  is  burnt  and  the  ash  obtained 
is  either  weighed  by  itself  or  mixed  with  the  main  part  of  the  pre- 
cipitate and  weighed  with  it.f 

*  Wet  a  common  cut  filter,  stretch  it  over  the  ground  top  of  the  funnel, 
and  then  gently  tear  off  the  superfluous  paper.  The  cover  thus  formed 
continues  to  adhere  after  drying.  Fresenius,  Quant.  Chem.  Analysis. 

t  By  using  filter-paper  which  has  been  carefully  washed  with  hydro- 
chloric and  hydrofluoric  acids,  it  is  permissible  to  neglect  the  weight  of 
the  ash  from  the  filter  itself.  With  an  unknown  paper  it  is  necessary  to 
determine  tfye  v/eight  of  the  ash  by  a  separate  experiment  and  then  correct 
the  weight  of  the  precipitate  obtained. 


22 


INTRODUCTION. 


The  combustion  of  the  filter,  to  which  small  particles  of  the 
precipitate  still  adhere,  is  best  accomplished  by  the  method  pro- 
posed by  Bunsen  as  follows :  The  filter  is  folded  together  so  that 
the  precipitate  occupies  the  position  indicated  in  the  shaded  part 
of  Fig.  5,  a,  and  then  it  is  further  folded  as  indicated  by  ft  and  7-  of 
Fig.  5  to  a  narrow  strip.  The  paper  is  then  rolled  between  the 


b 


FIG, 


fingers  as  indicated  by  d,  beginning  at  6,  so  that  the  portion  of 
the  filter  which  is  free  from  the  precipitate  is  on  the  outside.  The 
roll  is  now  enveloped  with  a  previously  ignited  heavy  platinum 
wire,  the  wire  is  supported  (as  indicated  in  Fig.  6)  by  means  of  a 


FIG.  6. 

cork  in  the  opening  of  a  porcelain  plate  and  the  filter  is  ignited  by 
means  of  the  gas-flame.  The  flame  is  at  once  taken  away  and 
the  paper  allowed  to  burn  quietly.  If  carbonized  particles  still 
remain,  the  gas-flame  is  applied  repeatedly  until  it  is  no  longer 
possible  to  make  the  particles  glow  any  more.  (Too  strong  ignition 
should  be  avoided.)  The  ash  is  then  added  to  the  contents  of  the 


THE  DRYING  AND  BURNING   OF  PRECIPITATES. 


crucible  by  gentle  shaking  and  the  final  use  of  the  feather.  The 
cover  is  placed  on  the  crucible,  which  is  heated  at  first  with  a  smaU 
flame,  the  temperature  being 
gradually  increased  until  the 
prescribed  temperature  of 
ignition  for  the  given  precip- 
itate is  reached.  The  flame 
is  finally  removed,  the  cruci- 
ble allowed  to  cool  some- 
what, and  while  still  warm, 
but  not  glowing,  is  placed 
in  a  desiccator  (Fig.  7). 

After    cooling    (at   least 
three  quarters  of  an  hour  for 
porcelain  crucibles  and  20  minutes  for  platinum  ones)  the  crucible 
and  its  contents  are  weighed. 

Many  precipitates  (silver  chloride,  lead  sulphate,  etc.)  are  some- 
what reduced  to  metal  by  the  above  treatment.  As,  however, 
these  metals  are  difficultly  volatile,  there  will  be  no  loss  of  the 
metal,  only  of  the  anion  (chlorine  in  the  case  of  silver  chloride 
and  SO4  in  the  case  of  lead  sulphate).  This  loss  may  be  readily 
replaced.  The  metal  in  the  crucible  is  moistened  with  a  few  drops 
of  nitric  acid  to  dissolve  it.  a  few  drops  of  hydrochloric  acid  (in 
the  case  of  a  silver  chloride  precipitate),  or  of  sulphuric  acid  (in 
the  case  of  lead  sulphate)  are  added,  and  after  evaporating  off  the 
excess  of  the  acid  the  crucible  is  weighed.  The  only  danger  in  this 
method  is  that  in  burning  the  filter  the  ash  is  heated  too  hot,  so 
that  some  of  the  reduced  metal  melts  and  alloys  with  the  platinum 
wire.  If,  however,  the  filter-paper  is  rolled  up  as  was  directed, 
there  is  always  some  paper  free  from  precipitate  between  the  precipi- 
tate and  the  platinum  wire,  yielding  an  ash  which,  although  its 
weight  is  inappreciable,  is  still  sufficient  to  protect  the  wire  and 
prevent  the  reduced  metal  from  coming  in  contact  with  it,  provided 
it  is  not  heated  strongly  enough  to  melt  the  metal. 

Many  precipitates  (Mg(XH4)AsO4.  K2PtCl6,  etc.)  are  changed  • 
so  much  by  this  treatment  that  it  would  be  impossible  to  obtain 
correct  results.     In  such  cases  the  filter  cannot  be  burnt,  but 
it  is   previously   dried   at   a  definite   temperature   and   weighed; 


24  INTRODUCTION. 

afterwards  the  precipitate  and  filter  are  again  dried  at  the  same 
temperature  and  weighed  again. 

In  order  to  dry  the  filter,  it  is  placed  in  a  drying-closet  * 
(Fig.  8a)  upon  a  watch-glass  and  near  an  open  weighing  beaker, 
the  temperature  is  brought  to  the  desired  point  and  kept  there, 
with  the  help  of  the  thermo-regulator  T ',  for  \  to  1  hour.  By 
means  of  tongs  the  filter  is  quickly  placed  in  the  weighing  beaker, 
and  the  latter  in  a  desiccator  filled  with  calcium  chloride  (Fig.  7), 
where  it  is  kept  for  exactly  1  hour.  It  is  then  covered,  removed 
from  the  desiccator,  allowed  to  stand  in  the  air  near  the  balance  for 
20  minutes  and.  then  weighed.  The  heating  and  weighing  is  re- 
peated once  more  in  exactly  the  same  way  until  two  consecutive 
weighings  do  not  differ  by  more  than  0.0002-3  gm. 

The  precipitate  is  now  collected  upon  the  filter  and  after  drying 
the  filter  in  the  funnel  at  100°  C.  the  filter  and  its  contents  are 
removed  from  the  funnel  and  dried  in  exactly  the  same  way  as 
before. 

The  same  result  is  much  more  simply  and  accurately  accom- 
plished by  the  use  of  the  Gooch  Crucible. 

This  consists  (as  is  shown  in  Fig.  9,  page  25)  of  a  cru- 
cible with  a  perforated  bottom.  The  crucible  is  provided 
with  an  asbestos  filter,  weighed  after  drying  at  the  pre- 
scribed temperature,  then  the  precipitate  is  filtered  off 
into  the  crucible,  which  is  again  dried  and  weighed.  The 

*  The  drying-closet  shown  in  Fig.  8a  is  fitted  with  six  removable  porce- 
lain plates  which  prevent  any  oxide  falling  from  the  metallic  closet  walls 
upon  the  substance  to  be  dried,  rendering  it  impure.  The  upper  plate  has 
two  holes  bored  in  it  through  which  thermometer  and  thermo-regulator  are 
placed.  This  upper  plate  is  fastened  to  the  top  of  the  closet  as  follows: 
A  glass  rod  provided  with  a  broad  rim  rr  and  bulging  out  at  aa  is  pushed  up 
through  the  opening  P  of  the  porcelain  plate  (Fig.  86)  and  K  of  the  upper 
closet  wall,  and  this  is  fastened  by  placing  an  asbestos  ring  A  between  aa 
and  K. 

The  bottom  plate  rests  upon  a  heavy  iron  wire  so  that  it  does  not 
come  directly  in  contact  with  the  bottom  of  the  closet. 

As  the  plates  can  be  easily  taken  out,  it  is  possible  to  clean  them  without 
difficulty.  The  only  part  of  the  apparatus  that  wears  out  is  the  bottom, 
so  that  it  is  best  to  have  the  closet  so  that  it  may  be  renewed  from  time 
to  time  without  taking  the  apparatus  to  pieces. 

Several  forms  of  electric  ovens  are  also  in  use.  These  require  little  atten- 
tion and  can  be  regulated  to  almost  any  desired  temperature. 


THE  DRYING  AND  BURNING  OF  PRECIPITATES.  25 


FIG.  a 


INTRODUCTION. 


use  of  these  crucibles  permits  such  accurate  and  rapid  work  that 
it  is  worth  while  to  describe  the  method  of  using  them  more  in 
detail. 

Preparation  of  Asbestos  Filters. 

Some  long-fibred,  soft  asbestos  is  cut  into  pieces  J  cm.  long, 
and    digested    with    concentrated    hydrochloric    acid    upon    the 
water  bath  for  an  hour.     A  good  sample  of  asbestos  will  then 
be  separated  into  very  small  fibres.     The  mass  is  collected  in  a 
funnel  with  a  platinum  cone,  or  upon  a  filter-plate,  and  washed 
with  water.      After  drying,  the  asbestos  may  be  ignited,  but  for 
most  purposes  this  is  not  only  unnecessary  but  disadvantageous. 
For  the   preparation  of  a  Gooch  filter,  a  small  flock   of   tlie 
material   is   shaken  with  water   in    a 
flask,     so     that    a    thin     emulsion    is 
formed.     A  piece  of  thin  rubber  tubing 
(Fig.    10)    is   stretched   over   a   funnel 
and  the   crucible   T  is  placed  in  the 
opening.     The  funnel  should  be  la~g? 
enough     so     that     the      crucible     is 
suspended    by    the     rubber    without 
touching    the    sides     of    the    funnel. 
Enough    of    the    emulsion    is    poured 
through    the    crucible    to    produce    a 
layer  of  1  to  2  mm.  thickness,  a  small 
filter-plate  (Fig.  9,  P)  is  placed  upon 
this  layer  and  some  more  of  the  emul- 
sion   is     poured     into     the     crucible. 
Water  must  now  be  passed   through 
the   crucible  until   no   asbestos   fibres 
run   through,   and    in    order    to    see 
them  the  liquid  is  poured  into  a  small 
beaker.     Usually  such  a  filter  is  pre- 
pared and  used  with  a  gentle  suction,* 
but    in   many    cases    it   filters    more 
rapidly  than  paper  without  it. 

FIG.  10.  The  crucible  is  now  dried  at  the 

proper    temperature    and    afterwards 
*  Too  great  a  suction  should  not  be  employed  during  the  filtration,  for 


THE  DRYING  AND  BURNING    OF  PRECIPITATES.  27 

weighed.  The  drying  and  weighing  is  repeated  until  a  constant 
weight  is  obtained,  when  about  half  a  liter  of  water  is  once  more 
I  through  the  crucible  (in  order  to  be  sure  that  no  asbestos 
fibres  run  through)  and  the  crucible  is  again  dried  and  weighed, 
after  which,  if  the  weight  is  constant,  the  crucible  is  ready  for  the 
filtration. 

The  same  crucible  can  be  used  for  a  large  number  of  determina- 
tions. When  the  amount  of  the  precipitate  in  the  crucible  be- 
comes too  large,  the  upper  part  can  be  carefully  removed  and  the 
crucible  again  used. 

If  it  is  desired  to  ignite  a  precipitate  contained  in  a  Gooch 
crucible,  it  is  placed  (as  shown  in  Fig.  11)  within  a  larger  porce- 
lain crucible  and  heated  at  first 
gently  and  finally  more  strongly, 
and  when  necessary  it  can  even 
be  heated  over  the  blast-lamp. 

For  many  purposes  it  is  prefer- 
able to  use  instead  of  the  Gooch 
crucible  a  glass  tube  with  an. 
asbestos  filter.  This  is  particu- 
larly desirable  when  it  is  neces- 
sary to  heat  the  precipitate  in  a 
gas-stream. 

The  so-called  Munroe  crucible,*  in  which  the  filtering  medium 
consists  of  a  porous  felt  of  spongy  platinum,  is  a  modification  of 
the  Gooch  crucible  which  permits  rapid  and  accurate  work.  The 
felt  is  prepared  by  igniting  a  carefully-dried  layer  of  ammonium 
chloroplatinate,  which  has  been  poured  over  the  bottom'  of  a 
platinum  Gooch  crucible  in  the  form  of  an  alcoholic  sludge  while 

in  that  case  the  precipitate  or  even  the  asbestos  itself  will  be  so  compressed 
that  the  nitration  will  be  prolonged  and  the  washing  made  more  difficult. 
By  having  the  crucible  suspended  free  by  the  rubber,  the  possibility  of  em- 
ploying too  much  suction  is  avoided,  for,  as  soon  as  this  has  reached  a 
certain  tension  the  air  is  forced  between  the  rubber  and  the  sides  of  the 
crucible,  so  that  we  have  the  effect  of  a  safety-valve  to  a  certain  extent. 

*C.  E.  Munroe,  J.  Anal.  Chem.,  2,  241;  Chem.  News,  58,  101.  See 
also  W.  O.  Snelling,  J.  Am.  Chem.  Soc.,  31,  456,  and  O.  D  Swett,  ibid.  31, 
928.  The  last  reference  gives  a  table  of  suitable  solvents  for  removing  ignited 
precipitates  from  the  Munroe  crucible. 


28 


INTRODUCTION. 


the  crucible  is  held  against  several  layers  of  filter  paper.  The 
felt  can  be  shaped  to  the  crucible  during  the  ignition  and  subse- 
quently burnished  lightly  with  a  glass  rod  of  suitable  form.  In 
case  imperfections  develop,  the  felt  should  be  saturated  again 
with  chloroplatinic  acid,  the  crucible  slowly  lowered  into  a  moder- 
ately concentrated  solution  of  ammonium  chloride,  washed  with 
alcohol,  dried  and  ignited. 

The  use  of  an  electric  furnace,  Fig.  12,  is  very  convenient  for 
igniting  the  crucible  and  its  contents,  especially  in  the  case  of 


FIG.  12. 


those  precipitates  which  are  likely  to  undergo  change  on  coming 
in  contact  with  a  reducing  flame. 


(6)    THE    PRECIPITATE    IS    IGNITED    WET. 

Those  precipitates  which  do  not  suffer  any  permanent  change 
by  the  action  of  the  products  of  combustion  of  the  filter  may  be 
ignited  wet.  The  precipitate  is  allowed  to  drain  as  much  as 
possible  and  while  still  moist  the  filter  and  precipitate  are  placed 
in  a  platinum  crucible,  the  paper  being  pressed  down  against  the 


THE  DRYING  AND   BURNING    OF  PRECIPITATES. 


29 


sides  of  the  crucible.  The  crucible  is  placed  in  an  inclined  position 
upon  a  triangle  (Fig.  13),*  with  the  cover  inclined  against  the 
upper  edge  of  the  crucible  and  resting  on  the  triangle.  The  flame 
of  the  burner  is  directed  against  the  cover,  which  quickly  dries 


FIG.  13. 

the  filter,  then  scorches,  carbonizes,  and  finally  burns  it.  The 
flame  is  then  slowly  moved  backwards  under  the  crucible  until 
finally  the  crucible  is  subjected  to  the  whole  heat  of  the  burner, 
after  which  it  can  be  heated  over  the  blast-lamp  if  necessary. 

*  In  Fig.  13,  the  inner  triangle  is  platinum  wire,  the  outer  triangle  is  heavy 
iron  wire.  Triangles  of  fused  silica  or  of  nickel-chromium  alloy  are  suitable, 
but  platinum  alloys  with  iron,  so  that  a  hot  crucible  should  never  be  placed 
in  contact  with  iron  wire. 


INTRODUCTION. 


THE  EVAPORATION  OFLIQUIDS. 

Liquids  are  usually  evaporated  upon  the  water-bath.  In 
order  to  prevent  anything  from  falling  into  the  evaporating-dish 
it  is  well  to  cover  it  with  an  evaporation-funnel,  as  shown  in 
Fig.  14. 


FIG.  14. 


FIG.  15. 


The  funnel  is  suspended  above  the  dish  by-  means  of  a  porce- 
lain fork  fastened  to  the  iron  rod  (covered  with  hard  rubber)  which 
is  attached  to  the  water-bath. 

In  case  the  laboratory  is  provided  with  a  glass -covered  hood 
with  a  good  draft  the  use  of  the  funnel  is  unnecessary. 

If,  however,  the  hood  is  directly  connected  with  the  chimney 
it  often  happens  that  on  a  windy  day  a  considerable  amount  of 
dust  falls  into  the  hood. 


THE  EVAPORATION  OF  LIQUIDS.  31 

In  order  to  prevent  this,  the  author  has  made  use  of  the  fol- 
lowing contrivance  which  has  worked  very  satisfactorily  for  some 
years.  The  hood  is  provided  with  a  glass  roof,  aa,  Fig.  15,  and  about 
15  cm.  below  there  is  a  second  glass  plate  bb  which  does  not  quite 
touch  the  inner  wall  of  the  hood  but  is  about  3  cm.  away  from  it 
'iroughout  its  whole  length.  Between  the  two  plates  there  pro- 
it  clay  pipe  R,  about  15  cm.  in  diameter  and  about  5  cm. 
tbove  the  inner  edge  of  the  lower  glass  plate,  leading  directly 
into  the  chimney  K,  in  which  there  is  a  small  gas-flame  (not  shown 
in  the  illustration).  Any; dust,  sand,  etc.,  from  the  chimney  falls 
upon  the  plate  bb ;  none  c$n  get  into  the  hood. 

In  the  evaporation  of*  liquids  on  the  water-bath  in  weighed 
platinum  crucibles  or  dishes,  the  platinum  should  not  come  in 
contact  with  copper  or  glass  rings.  As  a  rule,  porcelain  rings  should 
be  used.  In  case  the  crucible  is  smaller  than  the  ring,  use  is  made 


FIG.  16. 


FIG.  16a. — Water-bath  with  porcelain  ring, 
platinum-brass  cone,  and  crucible. 


of  a  truncated  brass  cone  turned  back  at  the  base  (Fig.  16),  and 
lined  with  thin  platinum  foil.  This  is  suspended  in  the  ring  and. 
the  crucible  placed  within  the  cone  (Fig.  16a). 

During  evaporation  many  substances  have  the  property 
of  "  creeping  "  over  the  edge  of  the  crucible  or  dish,  often  causing 
a  slight  loss  of  the  substance;  furthermore  there  is  often  "bump- 


INTRODUCTION. 


ing,"  so  that  in  some  cases  the  entire  contents  are  thrown  out  of 
the  crucible  (cf.  the  determination  of  boric  acid  according  to  the 
method  of  Gooch).  Both  of  these  phenomena  can  be  readily 
prevented  as  follows: 

The  crucible,  at  the  most  not  more  than  two-thirds  filled  with 
liquid,  is  placed  in  the  cylindrical  tin  or  brass  spiral  kk  (Fig.  17). 


FIG.  17. 

The  first  two  windings  of  the  metallic  spiral  come  into  close  con- 
tact with  the  sides  of  the  crucible  above  the  liquid,  while  the  re- 
maining windings  should  not  touch  the  crucible.  When  steam  is 

passed    through    the    spiral    the     . 

upper  part  of  the  crucible  is  \. 
warmed  first,  so  that  there  is  no 
spattering,  and  furthermore  by 
keeping  the  upper  edge  hot  during 
the  whole  of  the  evaporation  all 
"creeping"  of  the  substance  is 


FIG.  18. 

a = asbestos  ring. 
6=asbestos  plate. 


avoided.     In  this  way  it  is  possible  to  evaporate  off  alcohol  rapidly 
without  boiling  the  liquid. 

In  case  it  is  desired  to  evaporate  high-boiling  liquids,  such  as 
sulphuric  acid,  amyl  alcohol,  etc.,  the  crucible  is  either  heated 
cautiously  over  the  free  flame  (continually  moving  it  back  and 
forth)  or  else  the  crucible  is  placed  in  an  air-bath,  which  can  be 
prepared  in  some  such  way  as  is  represented  by  Fig.  18. 


DRYING   SUBSTANCES  IN  CURRENTS  OF  GASES.  33 


Drying  Substances  in  Currents  of  Gases. 

Substances  may  be  dried  at  a  high  temperature  in  a  current  of 
air  or  of  carbonic  acid  in  a  number  of  different  ways.  An  oil-bath 
provided  with  a  number  of  copper  tubes  (Fig.  19)  may  be  used. 
The  substance  contained  in  a  small  "boat"  is  placed  in  a  glass 
tube  and  the  latter  in  one  of  the  copper  tubes.  The  gas  is  now 
passed  through  one  or  more  of  the  empty  tubes  (so  as  to  warm  it), 
and  then  through  the  tube  containing  the  substance. 


FIG.  19. 
B=tube  wittt  thermometer  for  measuring  the  temperature  of  the  gas-stream. 

In  order  to  heat  a  crucible  in  a  current  of  carbon  dioxide, 
use  can  be  made  of  Paul's  drying  oven  (Fig.  20).  The  crucible 
is  placed  in  the  glass  pipe  R  and  the  pipe  and  copper  cylinder  K 
are  covered  with  watch-glasses.  Dry  carbon  doxide  is  conducted 
through  the  stem  of  the  pipe,  and  the  oven  can  be  heated  to  any 
desired  temperature. 

In  case  it  is  desired  to  evaporate  off  a  liquid  in  a  flask  and  to 


•34 


INTRODUCTION. 


ignite  the  residue  at  a  given  temperature  it  is  necessary  to  proceed 
somewhat  as  follows: 


FIG.  20. 


ft    b 


FIG.  21a. 


FIG.  216. 


The  solution  is  placed  in  the  open  Erlenmeyer  flask  K  and 
evaporated  as  far  as  possible  over  the  free  flame.     The  flask  is 


PREPARATION   OF    THE  SUBSTANCE  FOR  ANALYSIS.          35 

then  placed  in  a  metal  beaker  suspended  in  an  oil-bath  (Fig.  2 la), 
and  dry  air  is  sucked  through  the  spiral  copper  tube  kk  as  shown  in 
the  illustration.  Fig.  216  shows  the  separate  parts  of  the  apparatus. 

PREPARATION  OF  THE  SUBSTANCE  FOR  ANALYSIS. 

It  is  very  difficult  to  give  general  rules  for  the  preparation  of 
substances  for  analysis,  for  it  is  necessary  to  proceed  differently 
in  different  cases.  For  a  scientific  analysis  (i.e.,  one  in  which  it 
is  desired  to  determine  the  atomic  composition  of  a  substance) 
it  is  necessary  to  choose  pure  material  for  the  analysis.  Although 
this  sounds  so  simple  it  is  often  one  of  the  most  difficult  conditions 
to  fulfil.  Many  substances  are  hygroscopic  and  absorb  moisture 
from  the  air,  which  can  be  removed  by  heating  the  substance  or 
by  simply  allowing  it  to  stand  in  a  desiccator  over  calcium  chlo- 
ride, provided  the  substance  itself  undergoes  no  change  by  this 
treatment.  Many  substances  containing  water  of  crystallization 
cannot  even  be  dried  in  a  desiccator,  but  must  be  analyzed  air-dry. 
In  all  cases  it  is  necessary  to  determine  whether  the  substance  to 
be  analyzed  possesses  a  constant  weight. 

For  technical  analyses,  the  purpose  being  to  determine  the  cost 
or  selling  price  of  an  article  or  to  control  its  manufacture,  the  sub- 
stance must  be  analyzed  as  it  is.  In  such  a  case  the  sample  should 
represent  as  far  as  possible  the  average  composition  of  the  product. 
For  our  work  we  are  concerned  chiefly  with  scientific  analyses 
and  the  first  substances  to  be  analyzed  are  easily  crystallized 
from  water. 

Many  commercial  salts  are  prepared  extremely  pure  and  could 
be  analyzed  directly;  in  most  cases,  however,  we  obtain  them  after 
they  have  stood  for  some  time  in  the  air  and  after  they  have  been 
handled  somewhat,  so  that  they  are  not  so  pure  as  when  freshly 
prepared.  Consequently  in  case  it  is  desired  to  test  the  accuracy 
of  an  analytical  process,  the  purity  of  a  commercial  sample  should 
never  be  taken  for  granted.  The  substance  should  be  purified  by 

Recrystallization. 

Ten  or  fifteen  grams  of  the  commercial  salt  are  dissolved  in 
the  least  possible  amount  of  hot  water  (it  is  best  to  use  not  quite 


36  INTRODUCTION. 

enough  water  to  completely  dissolve  the  substance)  and  the  hot 
solution  is  rapidly  poured  through  a  plaited  filter  contained  in  a 
funnel  the  stem  of  which  has  been  broken  off  (Fig.  22).  This 
serves  to  remove  all  dust  or  other  insoluble  impurity.  The  filtrate 
is  received  with  constant  stirring  in  an  evaporating-dish  and  is 
rapidly  cooled  by  placing  the  dish  in  a  larger  one  containing  cold 
water. 


FIG.  22. 

By  means  of  the  rapid  cooling  and  constant  stirring,  the 
salt  is  obtained  in  the  form  of  a  crystalline  powder,*  which  is 
filtered  off  by  pouring  through  a  funnel  provided  with  a  perforated 
platinum  cone.  The  mother-liquor  is  removed  as  much  as  possible 
by  means  of  suction.  The  purity  of  the  substance  is  then  tested 
qualitatively  by  means  of  some  suitable  reaction.  In  case  it  is 
still  not  quite  pure,  the  same  process  of  recrystallization  must  be 
repeated  until  the  presence  of  no  impurity  can  be  detected. 

The  pure  but  still  moist  substance  is  placed  upon  a  layer  of 
several  thicknesses  of  clean  filter-paper,  covered  with  another  sheet 
of  the  same  and  allowed  to  stand  for  twelve  hours  at  the  ordinary 
temperature.  One  or  two  grams  of  the  substance  are  then  weighed 
upon  a  tared  watch-glass,  placed  upon  a  dry  glass  plate,  covered 
with  another  watch-glass  and  allowed  to  stand  for  several  hours 
more.  If  the  substance  shows  no  change  in  weight  it  is  ready  for 

*  Large  crystals  would  be  obtained  by  allowing  the  solution  to  cool 
slowly,  but  they  are  not  desirable,  as  they  usually  contain  more  enclosed 
mother-liquor  than  do  the  smaller  crystals. 


PREPARATION  OF   THE  SUBSTANCE  FOR  ANALYSIS.          37 

analysis.  Otherwise  it  must  be  dried  in  the  air  until  it  no  longer 
shows  a  change  in  weight.  It  is  not  permissible  to  dry  the  sub- 
stance in  a  desiccator  except  in  those  cases  in  which  the  substance 
will  not  lose  water  of  crystallization.  Deliquescent  substances 
of  course  should  not  be  allowed  to  remain  exposed  to  the  air  for 
very  long.  Such  substances  must  be  quickly  dried  upon  a  porous 
plate  and  transferred  as  soon  as  possible  to  a  flask  provided  with  a 
closely  fitting  ground-glass  stopper.  Further  rules  for  the  prepara- 
tion of  the  substance  for  analysis  will  be  given  under  the  special 
cases. 


PART    I. 

GRAVIMETRIC  ANALYSIS. 


A.  GRAVIMETRIC  DETERMINATION  OF  THE  METALS 

(CATIONS). 

METALS  OF  GROUP  V. 
POTASSIUM,  SODIUM,  LITHIUM,  AMMONIUM,  AND  MAGNESIUM 

POTASSIUM,  K.     At.  Wt.  39.10. 

Forms:*  KC1,  K,S04,  K,PtCl6,  and  KC104. 

i.  The  Determination  as  Chloride. 

THIS  compound  is  chosen  for  the  determination  of  potassium 
when  it  is  already  present  as  such,  or  in  case  the  salt  to  be  analyzed 
may  be  changed  to  the  chloride  by  evaporation  with  hydrochloric 
acid.  If  the  potassium  is  present  in  the  form  of  its  sulpha  to  it 
may  be  transformed  to  the  chloride  by  precipitation  with  barium 
chloride  (see  silicate  analysis);  if  it  is  present  as  the  phosphate, 
the  phosphoric  acid  may  bo  precipitated  as  basic  ferric  phosphate 
(Vol.  I,  p.  328,  Ed.  II);  or,  finally,  if  it  is  present  as  chromate  the 
CrO4  ions  may  be  reduced  to  chromic  ions  by  evaporation  with 
hydrochloric  acid  and  alcohol  and  then  precipitated  by  ammonia 
and  filtered  off. 

In  almost  all  of  these  cases  it  is  a  question  of  separating  the 
potassium  chloride  from  the  aqueous  solution  and  in  most  cases 
of  separating  it  from  ammonium  chloride  as  well. 

First  of  all  the  solution  is  evaporated  to  dryness  on  the  water- 
bath  in  a  platinum  dish  (or  if  necessary  a  thin  porcelain  dish  may 

*  Under  this  lu«:uling  will  be  given  in  every  case  the  symbols  of  the  com- 
pounds suitable  for  the  determination  of  the  element  in  question. 

38 


DETERMINATION  OF  POTASSIUM  AS   CHLORIDE.  39 

be  substituted),  taking  the  precaution  of  stirring  the  liquid  fre- 
quently with  a  heavy  platinum  wire,  as  soon  as  the  salt  begins 
•  •parate  out,  in  order  to  hasten  the  evaporation  of  the 
enclosed  water.  In  spite  of  long-continued  heating  and  con- 
tinual stirring,  however,  it  is  not  possible  to  completely  expel  all 
of  the  water  enclosed  within  the  crystals;  this  is  effected  by  cover- 
ing the  dish  with  a  watch-glass  and  drying  in  the  hot  closet  for 
an  hour  or  two  at  130°- 1. 50°  C.  The  covered  dish  is  then  placed 
upon  a  platinum  triangle  and  cautiously  heated  over  a  free 
flame,  holding  the  burner  in  the  hand  and  imparting  to  it  a  fanning 
motion.  The  dish  is  kept  covered  as  long  as  a  decrepitating  sound 
can  be  heard.  The  cover  is  then  taken  off,  any  ammonium  chloride 
on  it  is  removed  by  careful  heating,  and  it  is  then  placed  upon 
another  clean  watch-glass.  The  dish  is  then  heated  again  over  the 
constantly  moving  flame  until  the  vapors  of  ammonium  chloride 
cease  to  be  given  off,  care  being  taken  not  to  heat  the  potassium 
chloride  too  strongly  on  account  of  its  volatility.  Any  potassium 
chloride  remaining  on  the  cover  is  then  washed  into  the  dish  by 
means  of  a  little  water,  the  salt  in  the  dish  is  brought  into  solution 
by  rotating  this  water  in  the  dish  and  the  almost  ever-present 
carbon  particles  (from  the  carbonization  of  pyridine  bases  usually 
present  to  a  slight  extent  in  the  ammonia  and  ammonium  chloride) 
are  filtered  off  through  a  small  filter  into  a  weighed  platinum  cruci- 
ble. A  few  drops  of  HC1  are  added,  the  contents  of  the  crucible 
evaporated  to  dryness  on  the  water-bath,  again  covered  and  allowed 
to  remain  in  the  drying-closet  for  one  to  two  hours  at  130°-150°  C. 
and  once  more  heated  over  the  free  flame  until  all  decrepitation 
has  ceased,  when  the  crucible  is  allowed  to  cool  in  a  desiccator  and  is 
weighed.  After  this  the  crucible  is  again  heated  for  a  few  moments 
over  the  free  flame  so  that  the  bottom  of  the  crucible  becomes  a 
dark  red  (the  cover  of  the  crucible  must  not  be  lifted  during  this 
operation) ;  it  is  allowed  to  cool,  and  is  again  weighed.  The  proc- 

repeatod  until  a  constant  weight  Is  obtained. 
In  the  case  of  every  analytical  operation  the  heating  and  weigh- 
ing must  always  be  repeated  until  two  consecutive  weights  are 
the  same.  Therefore,  whenever  the  terms  "heated"  (or  "ignited ") 
and  "weighed"  are  used  in  this  book,  it  is  to  be  always  understood 
that  a  constant  weight  is  to  be  obtained. 


40  GRAVIMETRIC  ANALYSIS. 

This  method  is  capable  of  yielding  exact  results. 

Example:  Determination  of  Potassium  in  Potassium  Bichromate. 
— Commercial  potassium  bichromate  usually  contains  potassium 
sulphate  as  impurity.  The  salt  is  therefore  purified,  as  described 
on  p.  35,  by  recrystallizing  three  times  from  water,  placing  the 
moist  crystals  in  an  evaporating-dish,  heating  on  the  water- 
bath  with  constant  stirring  and  finally  drying  to  constant  weight 
in  an  oil-bath  at  130°  C.  (cf.  p.  33)  in  a  current  of  dry  air. 

The  dry  substance  is  then  weighed  upon  a  tared  watch-glass, 
placed  in  a  300-c.c  porcelain  evaporating-dish,  treated  with  10  c.c. 
of  concentrated  HC1  and  5  c.c.  of  alcohol,  covered  with  a  watch- 
glass  and  warmed  upon  the  water-bath  until  the  solution  becomes 
a  pure  emerald-green.  Any  solution  which  may  have  spattered  up 
on  the  cover-glass  is  washed  into  the  dish  by  means  of  a  stream 
of  water  from  the  wash-bottle  and  the  solution  is  then  evaporated 
to  dryness.  About  2  c.c.  of  concentrated  HC1  and  200  c.c.  of  water 
are  now  added,  the  liquid  is  heated  to  boiling  and  precipitated 
with  the  least  possible  excess  of  ammonia,  filtered  and  washed 
with  hot  water  until  1  c.c.  of  the  filtrate  evaporated  upon  the  cover 
of  a  platinum  crucible  leaves  no  residue.  If,  however,  on  making 
this  test  a  residue  remains,  it  must  be  redissolved  in  water  and 
added  to  the  rest  of  the  filtrate.  After  the  washing  is  found  to 
be  complete,  the  filtrate  is  evaporated  to  dryness  as  previously 
described,*  the  ammonium  chloride  expelled,  and  the  residue  of 
potassium  chloride  is  weighed. 

If  a  is  the  amount  of  potassium  bichromate  taken,  and  p  the 
weight  of  potassium  chloride  obtained,  the  amount  of  potassium 
present  in  the  potassium  bichromate  may  be  calculated  as  follows : 

KC1:K       =p:s 
74.56:39.10  =  p:s 

39  10 

s=='p  =  weight  of  potassium  in  a  gm.  of  bichromate  and  in 
74.00 

percentage 

100X39.10  p 
X=       74.56      '—I**  <*nt.  K. 

*  Frequently  a  little  Cr(OH)3  separates  out  during  the  evaporation;  it 
must  be  filtered  off  and  washed  free  from  the  solution. 


DETERMINATION  OF  POTASSIUM  AS  POTASSIUM  SULPHATE.  41 

It  is  customary  to  cany  out  the  analysis  in  duplicate  and  to  be 
satisfied  only  when  two  closely  agreeing  results  are  obtained,  of 
which  the  mean  is  taken  as  the  true  value.  According  to  the  above 
method  results  are  obtained  which  are  slightly  lower  than  the  theo- 
retical value,  but  this  should  not  amount  to  more  than  0.15  per 
cent,  and  the  two  "check"  determinations  should  not  differ  by 
more  than  0. 1  per  cent,  from  one  another. 

2.  Determination  of  Potassium  as  Potassium  Sulphate. 

This  method  is  chosen  when  the  potassium  is  already  present 
in  solution  as  the  sulphate,  or  when  it  is  in  such  a  form  that  it  can 
be  readily  changed  to  sulphate  by  evaporation  with  sulphuric 
acid;  it  is  most  frequently  used  for  determining  the  amount  of 
potassium  in  combination  with  organic  acids. 

Since  the  sulphate  of  potassium  is  much  less  volatile  than  the 
chloride,  it  is  advisible  to  choose  this  method  in  case  no  other  metal 
is  present.  On  the  other  hand,  when  it  is  necessary  to  separate 
potassium  from  sodium,  it  is  preferable  to  have  the  potassium  in 
the  form  of  the  chloride. 

Example:  Determination  of  Potassium  in  Potassium  Bichromate. 
—About  0.5  gm.  of  the  purified  and  dried  salt  is  weighed,  as  described 
under  1,  into  a  300-c.c.  porcelain  evapora ting-dish,  treated  with  20 
c.c.  of  a  freshly  prepared,  saturated,  aqueous  solution  of  sulphur 
dioxide*  and  5  c.c.  of  double-normal  sulphuric  acid.  The  dish  is 
covered  with  a  watch-glass  and  warmed  on  the  water-bath  until 
there  is  no  further  evolution  of  gas  perceptible,  when  the  cover-glass 
is  rinsed  off,  removed  and  the  solution  evaporated  almost  to  dryness. 
About  200  c.c.  of  water  are  now  added  and  the  chromium  is  precipi- 
tated from  the  boiling  solution  by  means  of  the  slightest  possible 
excess  of  ammonia.  The  precipitate  is  filtered  off  and  washed 

*  The  solution  of  sulphur  dioxide  may  be  prepared  as  follows:  Into  a 
300-c.c.  Erlenmeyer  flask  about  150  c.c.  of  a  saturated  sodium  bisulphite 
solution  are  placed,  and  concentrated  sulphuric  acid  is  slowly  added  from 
a  drop-funnel,  causing  a  lively  evolution  of  SO2  gas.  This  gas  is  passed  first 
into  a  small  wash-bottle  containing  water  and  then  into  another  flask  of 
distilled  water,  which  is  kept  cool  by  placing  it  in  a  larger  vessel  filled  with 
cold  water.  When  the  evolution  of  the  SO2  begins  to  slacken,  it  can  be 
accelerated  by  gentle  wanning. 


42  GRAVIMETRIC  ANALYSIS. 

until  it  can  be  shown  by  the  test  applied  under  1  that  it  is  com- 
pletely free  from  the  solution.  The  nitrate,  containing  both  potas- 
sium and  ammonium  salts,  is  evaporated  in  a  platinum  dish  to  dry- 
ness,  the  ammonium  sulphate  is  removed  by  gentle  ignition  (the 
salt  melts  and  gases  are  evolved),  the  residue  is  dissolved  in  as  little 
water  as  possible  and  transferred  to  a  weighed  platinum  crucible. 
After  being  evaporated  on  the  water-bath  to  dryness  the  bottom 
of  the  crucible  is  heated  by  means  of  a  free  flame  to  dull  redness 
until  SO3  vapors  cease  to  come  off.  The  crucible  is  allowed  to  cool 
in  a  desiccator  and  then  weighed.  A  piece  of  ammonium  carbonate 
the  siz?  of  a  pea  is  placed  in  the  crucible  (see  below),  which  is 
again  heated  and  weighed,  the  process  being  repeated  until  a  con- 
stant weight  is  obtained. 

If  a  is  the  weight  of  substance  taken  and  p  the  weight  of  the 
K2SO4  obtained,  then  the  percentage  of  potassium  in  the  potassium 
bichromate  may  be  calculated  as  follows  : 

K2SO4:K2      =p:s 
174.27:  78.20  =  p:s 
78.20 


78.20 


100X78.20  p 
and  x  =      17427     •  ~  =  per  cent.  K. 

In  order  to  determine  the  amount  of  potassium  in  organic  salts, 
a  weighed  sample  is  placed  in  a  large  platinum  crucible,  moistened 
with  a  little  concentrated  sulphuric  acid,  and  heated  over  the 
free  flame  exactly  as  in  the  case  of  igniting  a  moist  precipitate 
(p.  29),  placing  the  crucible  in  an  inclined  position  and  directing 
the  flame  against  the  cover  of  the  crucible.  Thick,  white  fumes  of 
sulphuric  acid  are  soon  evolved;  as  soon  as  these  begin  to  diminish 
in  quantity  the  flame  is  gradually  brought  toward  the  base  of 
the  crucible,  finally  heating  it  to  a  dull  red  until  no  more  vapors 
are  given  off.  The  mass  remaining  in  the  crucible  now  consists 
of  KjSO^  and  K2S2O7.  The  latter  compound  can  be  converted 
by  stronger  ignition  into  K2SO4  with  loss  of  SOS,  but  as  this  proced- 
ure involves  a  slight  loss  of  potassium  it  is  preferable  to  add  a  little 
solid  ammonium  carbonate,  by  means  of  which  the  excess  of  sul- 


SEPARATION    OF  POTASSIUM  FROM  SODIUM.  43 

phuric  acid  is  converted  into  ammonium  sulphate,  which  is  readily 
volatile  and  can  be  driven  off  at  a  much  lower  temperature. 

3.  Determination  of  Potassium  as  K>PtCl3  and  as  KC1O4. 

These  determinations  are  only  employed  when  it  is  necessary 
to  effect  a  separation  of  potassium  from  sodium.  We  will,  there- 
fore, first  consider  the  determination  of  sodium  itself  and  after- 
wards the  separation  of  the  two  metals. 

SODIUM,  Na.    At.  Wt.  23.00. 

Sodium,  like  potassium,  is  determined  in  the  form  of  its  chloride 
and  of  its  sulphate,  and  the  same  precautions  which  were  discussed 
under  potassium  hold  in  the  case  of  sodium.  It  may  be  men- 
tioned, however,  that  NaCl  and  Na^SC^  are  more  difficultly  fusible 
and  much  less  volatile  than  the  corresponding  potassium  com- 
pounds. 

Separation  of  Potassium  from  Sodium. 

The  solution  should  contain  salts  of  no  other  metals  with  the 
exception  of  ammonium  salts.  In  order  to  separate  the  sodium 
and  potassium  they  should  both  be  present  as  chlorides,  the  com- 
bined weight  of  which  being  first  ascertained.  The  mixture  is 
then  dissolved  and  the  potassium  precipitated  out  either  as 
chloroplatinate  or  as  perchlorate.  From  the  weight  of  the  pre- 
cipitate, the  corresponding  amount  of  potassium  chloride  can  be 
calculated,  which  value  is  deducted  from  the  weight  of  the  com- 
bined chlorides;  this  gives  the  weight  of  sodium  chloride  origin- 
ally present.  The  sodium,  therefore,  is  determined  by  differ- 
ence. 

A.  Separation  of  the  Potassium  as  K2PtClg. 

Principle. — K2PtCl6  is  practically  insoluble  in  absolute  alcohol, 
whereas  the  corresponding  sodium  salt  is  soluble.  On  the  other 
hand,  sodium  chloride  is  insoluble  in  absolute  alcohol,  so  that  it 
is  absolutely  necessary  to  convert  both  the  potassium  and  the 
sodium  to  the  form  of  their  chloroplatinates,  as  otherwise  the 
K2PtCl6  obtained  will  be  contaminated  with  sodium  chloride  and 
too  high  a  value  will  be  found  for  the  amount  of  potassium  present. 

Procedure:   1.   Transformation  of  the  Chlorides  into  Chloroplati- 


44  GRAVIMETRIC  ANALYSIS. 

nates.  —  The  assumption  made  is  that  the  weight  of  the  two  chlo- 
rides p  consisted  entirely  of  sodium  chloride,  and  from  this  the 
amount  of  chloroplatinic  acid  necessary  to  convert  the  chloride 
into  chloroplatinate  can  be  calculated  : 

2NaCl:Pt=p:z 

Pt 

x  =  <P  =  weight  of  Pt  in  H2PtCl6  required. 


Since  our  reagent  (Vol.  I,  p.  236)  contains  10  per  cent.  Pt,  we  have 


==C'C'  H2PtC1e  re(luired- 


The  solution  of  the  two  chlorides  in  water  (contained  in  a 
platinum  or  porcelain  evaporating-dish)  is  treated  with  a  few 
tenths  more  than  the  calculated  number  of  cubic  centimeters  of 
H2PtCl6  and  is  then  evaporated  almost  to  dryness  on  the  water-bath 
at  as  low  a  temperature  as  possible  (the  water  should  not  boil). 
After  cooling,  the  residue  is  treated  with  a  few  c.c.  of  absolute 
alcohol  (best  methyl  alcohol  *)  ,  after  which  the  solid  mass  is  broken 
up  into  a  fine  powder  by  means  of  a  stirring-rod  or  a  platinum 
spatula.  The  liquid  is  then  decanted  through  a  filter  moistened 
with  alcohol,  and  the  treatment  of  the  residue  with  alcohol  together 
with  the  breaking  up  into  powder,  etc.,  is  repeated  until  the  alcohol 
runs  through  the  filter  completely  colorless  and  the  salt  remaining 
assumes  a  pure,  gold-yellow  color  without  any  orange-colored  parti- 
cles being  present  (Na2PtCl6  +  6H20)  .  The  precipitate  is  then 
carefully  transferred  to  the  filter,  the  alcohol  is  allowed  to  com* 
pletely  drain  off,  and  the  precipitate  is  dried  in  the  hot  closet  at 
80°-90°C.  The  greater  part  of  the  precipitate  is  then  placed 
upon  a  clean  watch-glass,  the  filter  is  replaced  in  the  funnel,  and 
the  precipitate  which  still  adheres  to  it  (and  likewise  any  pre- 
cipitate adhering  to  the  dish  in  which  the  original  precipitation 
took  place)  is  dissolved  off  by  means  of  a  little  hot  water  into  a 
weighed  platinum  dish  or  crucible.  The  precipitate  is  evaporated 

*  Dupre",  Inaugural  Dissertation,  Halle,  1893. 


SODIUM:  MODIFICATION  OF  THE  CHLOROPLATINATE  METHOD.  45 

to  dryness  on  the  water-bath  at  as  low  a  temperature  as  possible, 
and  to  it  is  now  added  the  precipitate  from  the  watch-glass.  It  is 
dried  at  160°  C.  and  weighed.  The  calculation  of  the  amount  of 
potassium  chloride  corresponding  to  the  weight  of  the  precipitate 
is  performed  as  follows: 

The  weight  p  of  the  potassium  chloroplatinate  is  multiplied  by 
0.3056  and  this  gives  at  once  the  weight  of  the  potassium  chloride. 

Remark.  —  The  coefficient  0.3056  is  used  instead  of  the  true 
factor  (0.3068),  because  the  potassium  chloroplatinate  precipitate 
does  not  exactly  correspond  to  the  formula  K2PtCl6.*  It  con- 
tains, in  fact,  a  little  more  chlorine,  besides  oxygen  and  hydrogen, 
which  are  not  given  off  as  water  at  a  temperature  of  160°  C.  We 
must  assume  that  ;he  chloroplatinic  acid  is  decomposed  slightly 
on  evaporation,  perhaps  according  to  the  following  equation: 

H2PtCl6  +  H2O^HC1  +  H2PtCl5OH. 

By  this  hydrolysis  a  mixture  of  the  potassium  salts  (K2PtCl6 
+  KHPtCl5OH)  is  obtained,  but  fortunately  if  the  work  is  always 
done  in  the  same  way  these  compounds  are  always  formed  in  the 
same  relative  amounts.  Innumerable  determinations  have  shown 
that  correct  results  are  obtained  if  the  factor  0.3056  is  used  in  the 
calculations. 

Modification  of  Chloroplatinate  Method. 

Instead  of  weighing  the  K2PtCl6,  the  dry  precipitate  may  be 
heated  in  a  stream  of  hydrogen,  when  HC1  and  H2O  will  be  given 
off  and  a  mixture  of  platinum  and  potassium  chloride  will  remain 
behind. 

1.  If  the  amount  of  hydrochloric  acid  evolved  is  determined 
(p  gm.)  and  from  this  the  calculation  of  the  potassium  chloride 
made  according  to  the  following  equation, 

K2PtCl6+  4H  =  4HC1+  Pt  +  2KC1 

4HCl:2KCl=p:s 

KC1 


*Cf.  Fresenius,  Z.  anal.  Chem.,  1882,  p.  234.  Also  F.  Dupre,  "Die 
Bestimmung  der  Kali  urns  als  Kali  umplatinchlorid,"  Inaugural  Dissert.,  Halle, 
1893.  Also  W.  Dittmar  and  McArthur,  J.  Soc.  Chem.,  Ind.  6,  799,  and  Ber., 
1888,  Ref.  412. 


45  GRAVIMETRIC  ANALYSIS. 

the  result  will  be  too  low  because  less  HC1  is  evolved  than  corre- 
sponds to  the  above  equation. 

2.  If  the  mixture  of  platinum  and  potassium  chloride  remaining 
in  the  dish  is   weighed  (p  gm.)  and    the  amount   of  potassium 
chloride  is  calculated  according  to  the  equation 

(Pt+2KCl):2KCl=p:a? 

2KC1 
~Pt+2KCl'P 

too  low  a  result  will  be  obtained. 

3.  Finally,  if  the  mixture  of  platinum  and  potassium  chloride 
is  treated  with  water  and,  on  the  one  hand,  the  weight  of  the  plati- 
num remaining  undissolved  and,  on  the  other  hand,  the  weight  of 
the  potassium  chloride  which  goes  into  solution  (by  evaporating 
the  solution  and  weighing  the  residue)  is  determined,  then  the 
amount  of  potassium  chloride  calculated  from  the  weight  p  of  the 
platinum 

Pi:2KCl=p:x 
2KC1 
~' 


again  gives  a  result  which  is  too  low;  while  the  amount  of  potas- 
sium chloride  found  in  the  aqueous  solution  corresponds  to  the 
amount  of  potassium  chloride  originally  present. 

Inasmuch  as  the  precipitate  of  potassium  chloroplatinate 
possesses  a»  constant  composition  it  is  possible  to  determine  experi- 
mentally by  working  with  pure  materials  the  exact  ratio  which 
exists  between  (a)  the  amount  of  hydrochloric  acid  evolved,  (6) 
the  mixture  of  potassium  chloride  and  of  platinum  remaining 
after  the  ignition,  (c)  the  weight  of  platinum  remaining  undissolved 
after  treatment  of  the  residue  with  water  and  the  amount  of 
potassium  chloride  originally  present.  According  to  Dupre,  if 
the  amount  of  platinum  determined  according  to  3  is  multiplied 
by  the  factor  0.76142  the  true  amount  of  potassium  chloride 
will  be  obtained. 


MODIFICATION  OF   THE   CHLOROPLATINATE  METHOD.        47 

As  an  example  of  the  modified  chloroplatinate  method  we 
have  the 

Neubauer-Finkener  Method.* 

This  method  does  not  require  that  the  sodium  and  potassium 
shall  be  present  as  chlorides.  It  depends  upon  the  precipitation 
of  potassium  chloroplatinate  in  the  presence  of  ether-alcohol, 
igniting  the  precipitate  (K,PtCl6,  Xa2SO4,  etc.)  in 'hydrogen, 
washing  out  the  soluble  salts,  and  weighing  the  residual  plati- 
num. 

Procedure. — The  solution  containing  about  0.5  gm.  of  sub- 
stance is  poured  into  a  large  porcelain  casserole  and  treated 
with  a  few  drops  of  hydrochloric  acid.  Somewhat  more  than 
enough  chloroplatinic  acid  to  precipitate  the  potassium  is  added, 
and  the  solution  evaporated  on  the  water-bath  until  its  volume 
does  not  appear  to  diminish  perceptibly;  an  unnecessarily  long 
heating  is  to  be  avoided.  After  cooling  the  mass  is  moistened 
with  about  1  c.c.  of  water  and  carefully  crushed  with  the  end 
of  a  flattened  stirring-rod;  then  at  least  30  c.c.  of  alcohol  (93-96 
per  cent,  by  volume)  are  added  in  portions  of  10  c.c.,  each 
time  crushing  the  mass  with  the  rod.  If  considerable  sodium 
or  potassium  is  present,  the  mass  toward  the  end  assumes  a  soft, 
cheesy  consistency  but  eventually  becomes  hard  and  crystalline. 
The  covered  casserole  is  next  allowed  to  stand  for  half  an  hour, 
rubbing  the  precipitate  from  time  to  time.  Then  the  super- 
natant liquid  is  poured  through  a  platinum  Gooch  crucible  and 
the  precipitate  washed  by  decantation  with  alcohol.  After 
each  addition  of  alcohol  the  crystals  are  forcibly  crushed  with 
the  rod.  As  soon  as  the  filtrate  passes  colorless  through  the 
filter  the  precipitate  is  transferred  to  the  crucible  with  alcohol, 
the  alcohol  is  removed  by  washing  six  times  with  ether,  and 
the  latter  by  sucking  air  rapidly  through  the  crucible.  The 
crucible  is  covered,  and  through  an  opening  in  the  cover  hydro- 
gen (or  illuminating-gas)  f  is  passed  and  the  crucible  is  heated, 
at  first  very  gently,  to  avoid  losses  by  decreptitation.  After  five 

*  Z.  anal.  Chem.,  1900,  485. 

f  As  in  the  determination  of  copper  as  cuprous  sulphide,  cf.  p.  161. 


4  8  GRAVIMETRIC  A NA LYSIS. 

minutes  the  flame  of  the  burner  is  turned  a  little  higher  so  that 
the  bottom  of  the  crucible  just  shows  a  faint  redness  in  the 
center,*  and  this  temperature  is  maintained  for  at  least  twenty 
minutes.  It  is  then  allowed  to  cool.  The  contents  are  next 
moistened  with  cold  water,  and  hot  water  is  sucked  through 
the  crucible  for  fifteen  times  to  remove  the  soluble  salts  com- 
pletely. To  remove  calcium  sulphate,  or  other  difficultly  soluble 
salts,  the  crucible  is  filled  vrith  5  per  cent,  nitric  acid  (not  HC1), 
which  is  allowed  to  act  for  about  half  an  hour,  from  time  to  time 
replacing  with  a  little  fresh  acid.  Then  wash  with  hot  water, 
dry  and  weigh  the  platinum  from  which  the  corresponding 
amount  of  KC1  is  obtained  by  multiplying  by  0.7612,f  or  of  K20 
by  using  the  factor  0.4811. 

If  hydrochloric  acid  were  used  instead  of  nitric  acid  in  the 
above  treatment,  the  platinum  would  subsequently  run  through 
the  filter  in  a  colloidal  condition. % 

*  The  crucible  should  be  placed  upon  a  piece  of  platinum  foil  so  that 
the  flame  does  not  come  directly  in  contact  with  the  perforation  in  the  bottom 
of  the  Gooch  crucible. 

t  According  to  Neubauer  the  coefficient  0.7612  must  be  used  in  the 
presence  of  sulphates,  and  Dupre"  found  that  the  factor  0.7614  holds  in  the 
case  of  chlorides.  Similarly,  Dittmar  and  McArthur  (Z.  anal.  Chem.,  28, 
767)  state  that  the  factor  0.7611  applies  in  the  presence  of  much  magne- 
sium. 

%  Kling  and  Engels,  Z.  anal.  Chem.,  45,  315  (1906). 


DETERMINATION  OF  SMALL  AMOUNTS  OF  POTASSIUM.     49 

Determination    of    Small  Amounts  of  Potassium    in    the  Pres- 
ence of  Considerable  Sodium. 

The  solution  may  contain  sodium,  potassium,  calcium,and  mag- 
nesium in  the  form  of  their  chlorides  or  sulphates,  etc.  Hydro- 
chloric acid  gas  is  conducted  into  the 
solution,  which  has  been  concentrated  as 
much  as  possible,  until  it  has  become  sat- 
urated with  the  gas  (the  lower  end  of  the 
delivery-tube  should  be  enlarged,  as  indi- 
cated in  Fig.  23;  and  should  not  dip  into 
the  liquid).  To  every  100  c.c.  of  the  solu- 
tion 2  c.c.  of  water  are  now  added,  the 
precipitated  sodium  chloride  is  allowed  to 
settle  and  the  solution  poured  through  a  Fio.  93 

funnel  provided  with  a  platinum  filter- 
cone.     The  precipitated  salt  is  washed  three  times  by  decantation 
with  95  per  cent  alcohol,  transferred  to  the  funnel,  dried  by  suction 
and  then  washed  three  times  more  with  alcohol. 

In  the  solution  there  remains  all  of  the  potassium,  some  sodium, 
and  possibly  calcium,  magnesium,  and  sulphuric  acid. 

The  solution  is  evaporated  to  dryness  on  the  water-bath  if  possi- 
ble (or  if  sulphuric  acid  is  present  the  last  traces  of  the  free  acid 
are  removed  by  means  of  the  free  flame),  the  residue  is  weighed, 
for  every  decigram  of  the  salt  mixture  3  c.c.  of  double-normal 
hydrochloric  acid  are  added  with  more  than  enough  chloroplatinic 
acid  to  precipitate  all  of  the  potassium,  and  the  liquid  is  evaporated 
to  a  paste.  It  is  then  treated  with  20  c.c.  of  absolute  alcohol, 
and  well  stirred.  After  standing  five  minutes  5  c.c.  of  ether  are 
added,  the  mixture  is  allowed  to  stand  half  an  hour  under  a 
bell-jar,  and  then  filtered.  As  the  residue  often  contains  small 
amounts  of  other  chloroplatinates,  it  should  be  purified  as 
follows :  The  precipitate  is  allowed  to  dry  in  the  air,  it  is  dissolved 
in  a  little  hot  water,  a  few  drops  of  chloroplatinic  acid  are  added, 
and  the  above  operation  is  repeated.  The  precipitate  thus 
obtained  contains  all  of  the  potassium  in  the  presence  of  some 
sodium  chloride  and  possibly  sodium  sulphate.  It  is  washed 
with  a  mixture  of  ether  and  alcohol  until  the  liquid  runs  through 


S3  GRAVIMETRIC  ANALYSIS. 

the  filter  completely  colorless,  after  which  the  precipitate  is  dried, 
moistened  with  hot  water,  and  digested  on  the  water-bath  with  a 
few  drops  of  chemically  pure  mercury,*  constantly  stirring  with  a 
glass  rod,  until  the  liquid  appears  perfectly  colorless. 

By  means  of  this  treatment  the  potassium  chloroplatinate  is 
completely  decomposed  with  separation  of  platinum: 

KaPtCle+  4Hg = 2KC1+  2Hg2Cl2+  Pt 
or 

K2PtCl6+  2Hg = 2KC1+  2HgCl2+  Pt. 

The  mixture  is  thoroughly  dried  on  the  water-bath  and  gently 
ignited  until  the  mercury  is  all  volatilized ;  the  platinum  is  changed 
at  the  same  time  to  a  denser  form,  which  can  be  readily  washed  by 
decantation.  After  cooling,  the  mass  is  treated  with  water,  the 
aqueous  solution  is  decanted  through  a  filter,  and  the  residual 
metal  is  washed  with  hot  water,  dried,  and  cautiously  ignited.  The 
filter  is  ignited  in  a  platinum  spiral,  its  ash  is  added  to  the  main 
portion  of  the  platinum  in  the  crucible  which  is  now  ignited,  and 
weighed.  The  weight  obtained  p  multiplied  by  0.7612,  gives  the 
amount  of  potassium  chloride,  or  multiplied  by  0.3994,  gives  the 
corresponding  amount  of  potassium. 

The  above  method  serves  excellently  for  the  estimation  of 
small  amounts  of  potassium  in  mineral  waters. 


Separation    of    Potassium    from    Sodium    by    the    Perchlorate 
Method.     Schlossing-Wense.f 


Principle.  ^This  separation  depends  upon  the  insolubility  of 
potassium  perchlorate  and  the  solubility  of  sodium  perchlorate  in 
97  per  cent  alcohol. 

The  procedure  is  as  follows  : 

The  chlorides  of  the  two  metals  (sulphates  must  not  be  present 
on  account  of  the  insolubility  of  sodium  sulphate  in  alcohol)  are 
dissolved,  after  weighing,  in  20  c.c.  of  hot  water,  treated  with  1J 


*  Sonnstadt,  Z.  f.  anal.  Chem.,  36,  501. 

t  Z.  angew.  Chem.,  1891,  691,  and  1892,  233. 


SEPARATION   OF  POTASSIUM   FROM  SODIUM.  51 

times  as  much  of  the  perchlorate  solution  and  evaporated  with 
stirring  to  a  syrupy  consistency.  A  little  hot  water  is  added  and 
the  liquid  evaporated  with  constant  stirring  until  all  of  the  hydro- 
chloric acid  is  expelled  and  heavy  fumes  of  perchloric  acid  are  given 
off,  when  a  little  more  water  is  poured  over  the  residue  and  the  solu- 
tion is  again  evaporated  with  stirring.  The  perchloric  acid  lost  by 
volatilization  is  replaced  from  time  to  time.  After  cooling,  the 
mass  is  treated  with  about  20  c.c.of  97  per  cent,  alcohol  to  which  0.2 
per  cent,  by  weight  of  perchloric  acid  has  been  added  and  the  mix- 
ture is  vigorously  stirred.*  It  is  important,  however,  not  to  break 
up  the  crystals  of  potassium  perchlorate  into  too  fine  a  powder  as  the 
latter  would  readily  pass  through  the  asbestos  filter.  After  allow- 
ing the  precipitate  to  settle  the  alcohol  is  decanted  off  through  a 
Gooch  crucible,  the  residue  is  washed  again  with  the  wash-alcohol, 
and  the  excess  of  the  latter  is  removed  by  gentle  warming  The 
residue  is  dissolved  in  10  c.c.  of  hot  water,  to  which  a  little  per- 
chloric acid  has  been  added,  and  the  solution  is  evaporated  with 
stirring  until  fumes  of  perchloric  acid  are  given  off.  One  c.c.  of 
the  wash-alcohol  is  added;  and,  in  order  to  avoid  the  employment 
of  an  excess  of  the  latter,  the  precipitate  is  transferred  by  mean^  of 
a  rubber  "policeman"  to  a  Gooch  crucible,  washed  with  50-75  c.c. 
of  97  per  cent,  alcohol,  dried  at  130°  C.,  and  weighed.f 

In  case  sulphuric  acid  was  originally  present,  it  is  removed 
by  precipitation  with  barium  chloiide.  It  is  not  necessary  to 
remove  phosphoric  acid,  but,  in  case  this  acid  is  present,  it  is  advisa- 
ble to  allow  the  potassium  perchlorate  to  stand  for  some  time 
with  an  excess  of  perchloric  acid  before  treating  it  with  alcohol. 

Preparation  of  Perchloric  Acid  According  to  Kreider.% — From 
100-300  gms.  of  commercial  sodium  chlorate  (NaClO3)  are  placed 
in  a  round-bottomed  flask  and  gradually  heated  until  0x3- gen  begins 
to  be  evolved  slowly.  This  temperature  is  maintained  until  the 

*  100  c.c.  of  this  alcohol  dissolve  about  5  mg.  KC1O4  whereas  100  c.c. 
97.2  per  cent,  alcohol  containing  no  free  perchloric  acid  will  dissolve  0.0158  g. 
KC104  (W.  Wense). 

T  Using  this  method,  R.  Fitzenkam  obtained  in  three  experiments, 
100.11.  100.04,  100.24.  mean  100.13  per  cent,  of  the  potassium  chloride  taken. 

J  Z.  anorg.  Chem.  IX.  342. 


52  GRAVIMETRIC   ANALYSIS. 

mass  becomes  solid  (requiring  1^-2  hours),  whereby  the  chlorate  is 
almost  completely  changed  to  perchlorate  and  chloride. 

After  cooling,  the  melt  is  dissolved  in  water,  sufficient  hydro- 
chloric acid  is  added  to  decompose  any  chlorate  remaining,  and  the 
solution  is  evaporated  to  dryness  (the  liquid  being  constantly 
stirred  from  the  time  crystals  begin  to  separate  out). 

The  dry  mass  is  broken  up  with  a  stirring-rod  and  then  treated 
in  a  tall  beaker  with  an  excess  of  concentrated  hydrochloric  acid, 
by  means  of  which  sodium  chloride  separates  out  after  a  few  min- 
utes. The  solution  (it  now  contains  perchloric  and  hydrochloric 
acids  in  the  presence  of  small  amounts  of  sodium  chloride)  is  poured 
through  a  Gooch  crucible  and  the  residue  is  washed  once  or  twice 
by  decantation  with  concentrated  hydrochloric  acid.  The  nitrate 
is  evaporated  on  the  water-bath  until  the  hydrochloric  acid  is 
completely  expelled  and  heavy  white  fumes  of  perchloric  acid  are 
evolved. 

Inasmuch  as  commercial  sodium  chlorate  is  often  impure,  it  is 
necessary  to  test  the  perchloric  acid,  which  has  been  prepared,  for 
potassium.  For  this  purpose  a  small  amount  of  the  solution  is 
evaporated  on  the  water-bath  to  dryness  and  the  residue  is  treated 
with  97  per  cent,  alcohol,  which  will  dissolve  it  readily  in  the  absence 
of  potassium  perchlorate.  If  potassium  is  found  to  be  present, 
the  melt  obtained  by  heating  the  sodium  chlorate  as  above  de- 
scribed is  treated  with  HC1  and  evaporated  to  dryness  in  order  to 
decompose  any  sodium  chlorate  remaining.  The  residue  is  finely 
powdered  and  treated  with  97  per  cent,  alcohol  (1  c.c.  dissolves  0.2 
gm.  NaClO3)  and  filtered,  and  the  process  repeated  until  a  little  of 
the  alcoholic  solution  when  evaporated  to  dryness  leaves  absolute- 
ly no  residue. 

The  alcoholic  solution,  which  i?  now  free  from  potassium,  is 
distilled  from  a  spacious  flask  untiJ  the  perchlorate  begins  to  crys- 
tallize out,  when  it  is  poured  rapidly  into  an  evaporating-dish, 
evaporated  to  dryness,  and  treated  as  previously  described,  with 
hydrochloric  acid  etc. 

One  c.c.  of  a  potassium  perchlorate  solution  prepared  accord- 
ing to  the  above  directions  gave  A  residue  of  0.0369  gm.  which 
was  completely  soluble  in  97  per  cent .  alcohol,  as  it  should  be. 

In  order  to  ascertain  the  approximate  aiAount  of 


DETERMINATION  OF  LITHIUM,  POTASSIUM.  AND  SODIUM-      53 

acid  contained  in  the  solution,  1  c.c.  should  be  treated  with  an 
excess  of  KC1,  evaporated  to  dryness,  treated  with  an  excess  of 
97  per  cent,  alcohol,  filtered  through  a  Gooch  crucible,  and  washed 
until  the  filtrate  shows  no  turbidity  on  being  treated  with  silver 
nitrate  solution.  The  precipitate  is  then  dried  and  weighed. 

LITHIUM,  Li.    At.  Wt.  6.94- 
Forms:   Li2SO4  and  LiCl. 

The  determination  of  lithium  in  the  form  of  the  above  salts  is 
carried  out  in  practically  the  same  way  as  in  the  case  of  potassium. 
It  should  be  mentioned,  however,  that  on  evaporating  a  lithium 
salt  with  concentrated  sulphuric  acid  the  acid  salt,  LiHSO4,  is 
formed,  which  on  gentle  ignition  (even  without  the  addition  of 
ammonium  carbonate)  is  changed  to  difficultly  volatile  Li2SO4. 

Since  lithium  chloride  is  a  very  hygroscopic  salt,  it  is  necessary 
to  weigh  it  out  of  contact  with  moist  air.  To  accomplish  this, 
the  platinum  crucible,  after  being  gently  ignited,  is  placed  in  a 
desiccator  which  is  provided  with  a  calcium- chloride  tube,  and 
beside  the  crucible  is  placed  a  weighing  beaker  with  ground  glass 
stopper.  After  both  crucible  and  beaker  have  assumed  the  tem- 
perature of  the  room,  the  former  is  quickly  placed  within  the 
latter  which  is  then  stoppered.  It  is  allowed  to  stand  for  20  min- 
utes in  the  balance  case  and  then  weighed.  The  salt  is  then  placed 
in  the  crucible  and  the  above  process  repeated. 

"Determination    of    Lithium,    Potassium,    and    Sodium  in    the 
Presence  of  One  Another. 

After  determining  the  weight  of  the  combined  chlorides,  the 
potassium  is  determined  in  one  portion  as  K^PtC^,  and  in  a  second 
portion  the  lithium  is  determined  according  to  one  of  the  following 
methods : 

(a)  Gooch' s  Method* 

Principle  — Anhydrous  LiCl  is  soluble  in  anhydrous  amyl 
alcohol  (15  parts  of  amyl  alcohol  dissolve  in  the  cold  1  part  of 
LiCl,  or  10  c.c.  dissolve  0.66  gm.  LiCl)  while  KC1  and  NaCl  are 

*  Proceedings  of  the  Am   Acad.  of  Arts  and  Sciences.  22  [N.  3.  14],  177. 


54  GRAVIMETRIC  ANALYSIS. 

difficultly  soluble  in  this  liquid  (solubility  of  NaC'J  =  1 : 30,000    of 
KC1=  1:24,000). 

Procedure. — The  solution,  after  having  been  concentrated  as 
far  as  possible,  and  which  should  not  contain  more  than  0.2  gm. 
LiCl,  is  placed  in  a  50  c.c.  Erlenmeyer  flask,  5-6  c.c.  of  amyl  alcohol 
(boiling  point  132°  C.)  are  added  and  the  flask  is  placed  upon  an 
asbestos  plate  and  cautiously  heated.  The  aqueous  solution  at 
the  bottom  of  the  beaker  soon  begins  to  boil  and  the  water  vapor 
escapes  through  the  upper  layer  of  amyl  alcohol.*  As  soon  as 
all  the  water  has  been  boiled  off.  the  chlorides  of  sodium  and 
potassium  separate  out,  while  the  greater  part  of  the  lithium 
chloride  is  to  be  found  in  the  alcoholic  solution.  During  the 
evaporation  of  the  aquecus  LiCl  solution,  however,  some  LiOH  is 
formed  by  hydrolysis,  and  the  latter  compound  is  insoluble  in 
amyl  alcohol.  In  order  to  bring  this  completely  into  solution, 
the  clear  amyl  alcohol  solution  is  treated  with  2-3  drops  of  concen- 
trated hydrochloric  acid,  boiled  two  or  three  minutes  and  filtered 
while  still  warm  through  a  small  asbestos  filter.  The  crust  which 
remains  is  composed  of  sodium  and  potassium  chlorides  and  is 
washed  with  hot  amyl  alcohol,  which  has  been  boiled.  The  fil- 
trate is  evaporated  to  dryness;  and  the  residue  is  dissolved  in  a 
little  water  after  the  addition  of  some  dilute  sulphuric  acid  The 
solution  is  filtered  from  the  carbonaceous  residue  into  a  weighed 
platinum  crucible,  evaporated  as  far  as  possible  on  the  water- 
bath,  the  excess  of  sulphuric  acid  is  removed  by  gentle  heating 
over  a  flame  (the  crucible  being  held  in  an  inclined  position)  and 
it  is  then  weighed.  The  lithium  sulphate  thus  obtained  always 
contains  small  amounts  of  potassium  and  sodium  sulphates  in 
case  these  metals  were  present,  so  that  from  the  weight  obtained, 
0.00041  gm.  should  be  deducted  for  every  10  c.c.  of  the  filtrate 
(exclusive  of  the  alcohol  used  in  washing  the  residue)  in  case  only 
sodium  chloride  is  present,  or  0.00051  if  only  potassium  chloride  is 
present,  and  0.00092  if  both  sodium  and  potassium  chlorides  are 
present 

*  To  prevent  loss  by  bumping  at  this  point,  the  flask  should  be  fitted 
with  a  cork  stopper  through  which  two  tubes  pass.  If  air  is  drawn  through 
the  liquid  during  the  boiling,  the  water  evaporates  more  quickly  and  without 
bumping. 


DETERMINATION  OF  LITHIUM,  POTASSIUM,  AND  SODIUM.      55 

If  10-20  mgm.  of  lithium  chloride  were  present  in  the  original 
salt  mixture,  then  the  residue  obtained  after  filtering  and  wash- 
ing with  amyl  alcohol  is  dissolved  in  a  little  water  and  the  above 
treatment  is  repeated,  the  lithium  being  determined  in  the  com- 
bined nitrates. 

This  method  is  very  accurate,  and,  in  the  author's  opinion  it 
is  to  be  preferred  to  all  other  methods  for  the  determination  of 
lithium. 

(b)  Rammelsberg's  Method. 

Principle. — Anhydrous  lithium  chloride  is  soluble  in  a  mixture 
of  equal  parts  alcohol  and  ether  which  has  been  saturated  with 
hydrochloric  acid  gas,  whereas  the  chlorides  of  sodium  and  potas- 
sium are  practically  insoluble  therein. 

Procedure. — The  solution  of  the  chlorides  is  evaporated  to  dry- 
in  a  small  na_;k  made  of  Jena  glass  and  provided  with  a 
groun<  I  glass,  two-way  stopper  (p.  34,  Fig.  21a) .  During  the  evapo- 
ration a  current  of  dry  air  is  passed  into  the  flask  through  the  long 
tube  a  and  out  through  the  short  tube  b.  As  soon  as  the  resi- 
due has  become  dry  the  flask  is  placed  in  an  oil-bath  and  heated 
for  half  an  hour  at  140-150°  C. ,  during  which  time  dry  hydro- 
chloric acid  gas  is  passed  through  the  flask.  The  flask  and  its  con- 
tents are  allowed  to  cool  with  the  hydrochloric  acid  still  passing 
through  the  flask,  after  which  the  residue  is  treated  with  a  few 
cubic  centimeters  of  absolute  alcohol,  which  has  been  saturated  with 
hydrochloric  acid  gas  and  thereupon  diluted  with  an  equal  volume  of 
absolute  ether.  The  flask  is  tightly  stoppered  and  allowed  to  stand 
with  frequent  shaking  for  12  hours.  The  solution  is  then  poured 
through  a  filter,  wet  with  the  ether-alcohol  mixture,  and  the  residue 
is  washed  three  times  by  decantation  with  ether-alcohol.  A  few 
more  cubic  centimeters  of  ether-alcohol  are  added  to  the  contents 
of  the  flask  and  it  is  again  allowed  to  stand  for  12  hours ;  the  liquid 
is  then  poured  off  and  the  residue  is  washed  with  ether-alcohol 
until  a  trace  of  the  residue  tested  in  the  spectroscope  shows  the 
complete  absence  of  lithium.  The  ether-alcohol  extract  is  care- 
full}'  evaporated  to  dryness  in  a  water-bath  containing  lukewarm 
water,  the  residue  is  dissolved  (after  the  addition  of  a  little  dilute 
sulphuric  acid)  in  as  little  water  as  possible,  transferred  to  a  weighed 


56  GRAVIMETRIC  ANALYSIS. 

platinum  crucible  and  treated  with  sufficient  sulphuric  acid  to 
transform  the  lithium  chloride  present  completely  into  sulphate.* 
The  solution  is  evaporated  as  far  as  possible  on  the  water-bath, 
then  cautiously  over  the  free  flame,  after  which  it  is  gently  ignited 
and  the  residue  of  lithium  sulphate  is  weighed. 

Remark. — In  the  presence  of  considerable  sodium  and  potas- 
sium salts  it  is  advisable  to  remove  the  greater  part  of  these  by 
precipitation  with  hydrochloric  acid  gas  (cf.  p.  49),  filtering 
through  asbestos  and  washing  the  precipitate  with  concentrated 
hydrochloric  acid  until  the  residue  no  longer  gives  the  lithium 
spectrum.  The  results  obtained  by  this  method  are  satisfactory. 

Asides  the  above  methods  for  the  separation  of  lithium  from 
sodium  and  potassium  there  are  two  other  methods  to  be  men- 
tioned; that  of  W.  Mayer  f  and  that  of  A.  Carnot.J  According 
to  Mayer  the  lithium  is  precipitated  in  the  presence  of  NaOH  as 
Li3PO4,  which,  after  being  washed  with  ammonia  water,  is  ignited 
and  weighed.  Rammelsberg,  however,  claims  that  the  Li3P04 
always  contains  some  sodium,  so  that  the  method  is  inaccurate. 
A  great  many  experiments  tried  in  the  author's  laboratory  have 
led  to  the  same  conclusion. 

According  to  Carnot  the  lithium  is  separated  as  the  fluoride  and 
then  transformed  to  the  sulphate.  Walter  §  claims  that  this 
method  is  accurate  but  tedious. 

Example  for  practice:  Lepidolite  analysis.     (See  Index.) 

Indirect  Determination  of  Lithium  and  Sodium. 

The  mixture  of  the  two  chlorides  is  weighed  and  the  chlorine 
determined  either  gravimetrically  or  volumetrically.  (See  p.  3.) 

Indirect  Determination  of  Lithium  and  Potassium. 
The  method  is  the  same. 

*  The  above-described  method  has  been  modified  by  the  author.  Ram- 
melsberg evaporates  the  chlorides  in  the  water-bath,  heats  the  residue  till 
it  melts  and  then  after  cooling  extracts  with  ether-alcohol.  By  the  evapora- 
tion and  fusion  of  the  lithium  chloride  there  is  formed  some  lithium  hy- 
droxide which  is  changed  by  the  carbonic  acid  of  the  air  to  carbonate. 
Lithium  carbonate  is  insoluble  in  ether-alcohol  so  that  the  extraction  with 
ether-alcohol  is  not  complete. 

t  Ann.  Chem.  Pharm.,  98,  193,  and  Merling,  Z.  anal.  Chem.,  18,  563. 

J  Z.  anal.  Chem.,  29,  332.  §  The  Analyst,  16,  209. 


AMMONIUM.  57 


AMMONIUM  NH4.    Mol.  Wt.  18.04. 

Forms:  NHg,  NH.C1,  (NH4)2PtClfl,  Pt, 


We  have  two  cases  to  distinguish  : 

1.  The  ammonium  is  present  as  chloride  in  aqueous  solution. 

2.  The  ammonium  is  present  in  solution,  together  with  other 
cations  and  anions. 

1.  The.  solution  contains  only  XH4  and  Cl  ions.  In  this  case  the 
solution  may  be  evaporated  to  dryness  and  the  residue  of  ammo- 
nium chloride  weighed;  or  the  ammonium  can  be  precipitated  as 
(NH4)2PtCl6  and  the  precipitate  weighed;  or  the  ammonium 
chloroplatinate  can  be  ignited  and  the  residue  of  platinum  weighed. 

(a)  Determination  as  NH4C1. 

The  aqueous  solution  is  treated  with  concentrated  HC1  and 
evaporated  to  a  small  volume  on  the  water-bath  at  as  low  a  temper- 
ature as  possible,  the  solution  is  transferred  to  a  platinum  crucible 
(or  one  of  porcelain),  evaporated  on  the  water-bath  to  dryness,  and 
the  covered  crucible  is  dried  to  constant  weight  in  a  drying-oven. 
Good  results  are  obtained,  but  they  are  always  too  low.  On 
evaporating  the  aqueous  solution  some  NH4C1  is  driven  off,  and 
the  amount  lost  increases  in  proportion  to  the  amount  of  wrater 
used  and  the  temperature  at  which  the  evaporation  takes  place, 
on  account  of  the  NH4C1  being  partly  decomposed  according  to  the 
equation 


into  XH3  and  HC1,  both  of  which  are  volatile.* 

If,  on  the  other  hand,  a  little  hydrochloric  acid  is  added  to  the 
solution  the  dissociation  is  for  the  most  part  prevented  so  that 
the  loss  is  reduced  to  a  minimum.  The  ammonium  chloride  must 
be  dried  in  a  covered  crucible  as  otherwise  a  small  amount  of  the 

*  In  cold,  aqueous  solution  the  XH4C1  undergoes  electrolytic  dissociation 
simply: 

NH4C1<=±NH;+C1'. 


58  GRAVIMETRIC  ANALYSIS. 

salt  will  be  lost,  but  this  amount  is  small  in  comparison  with  the 
amount  which  it  is  possible  to  lose  during  the  evaporation. 

(6)  Determination  as  (NH4)2PtCl8. 

On  heating  (NH4)2PtCl6  to  130°  C.  the  salt  is  unchanged;  it 
suffers  no  dissociation,  therefore,  up  to  130°  C.  In  aqueous  solu- 
tion it  undergoes  only  electrolytic  dissociation  so  that  the  above 
salt  loses  no  (NH4)  during  the  evaporation  of  its  aqueous  solution. 

The  aqueous  solution  of  ammonium  chloride,  therefore,  is 
treated  with  an  excess  of  chloroplatinic  acid  and  a  little  hydro- 
chloric acid  and  evaporated  at  as  low  a  temperature  as  possible 
to  dryness.  The  residue  is  taken  up  in  absolute  alcohol  and 
filtered  through  a  Gooch  crucible,  dried  at  130°  C.,  and  weighed. 
From  this  weight,  the  amount  of  ammonium  chloride  originally 
present  can  be  correctly  calculated  by  using  the  old  atomic  weight 
of  platinum:  Pt=  196.9.  If  the  new  value  for  the  atomic  weight 
of  platinum  (Pt=  195.2)  is  used,  too  high  a  value  will  be  obtained 
for  the  amount  of  ammonium  present,  as  was  explained  in  the 
case  of  potassium. 

If  the  weight  of  the  (NH4)2PtCl6  =  p,  then 

pX  0.2400   =NH4C1 
PX  0.08095  =  NH4 
pX  0.07643  =  NH3. 

(c)  Determination  as  Platinum. 

Instead  of  weighing  the  (NH4)2PtCl(.  as  such,  it  can  be  decom- 
posed by  ignition*  and  the  weight  of  the  platinum  remaining 
determined.  If  the  old  value  for  the  atomic  weight  of  platinum 
(196.9)  is  used  in  this  determination  the  results  obtained  will  be 

*As  ammonium  chloroplatinate  decrepitates  strongly  on  being  heated, 
the  ignition  must  take  place  in  a  large  porcelain  crucible  which  is  provided 
with  a  close-fitting  cover.  The  precipitate  must  be  heated  gradually  at 
first  to  prevent  loss.  It  is  best  ignited  according  to  the  directions  of  Rose. 
The  precipitate  and  filter  are  placed  in  the  crucible  with  the  filter-paper  on 
top,  the  crucible  is  covered  and  heated  over  a  very  small  flame  until  the 
paper  is  completely  charred  without  allowing  the  vapor  to  escape  visibly 
from  the  crucible.  The  crucible  is  then  strongly  ignited  with  free  access 
of  air  until  the  charred  filter  is  completely  consumed. 


AMMONIUM  PRESENT  WITH  OTHER  CATIONS.  59 

about  0.4  per  cent  too  low,  while  if   the  new  value  (195.2)*  is 
used,  the  results  will  be  about  0.8  per  cent  too  high. 

Correct  results  can  be  obtained  by  multiplying  the  weight  of 
platinum  (p)  by  the  following  factors: 

pX  0.54527  =  XH4C1; 
pX  0.18391  =XH4; 
pX0.17364  =  X 


2.   The   Ammonium   is   Present   Together   with   Other   Cations 
and  Anions  in  Solution  or  in  Solid  Form. 

(a)  The  solution  is  distilled  after  the  addition  of  a  strong  base 
(NaOH  —  Ca(OH)2|),  the  ammonia  evolved  is  absorbed  in  hydro- 
chloric acid  and  the  resulting  solution  is  analyzed  according  to  1. 


FIG.  24. 

.  Procedure. — About  1  gm.  of  the  substance  to  be  analyzed  is  placed 
in  the  400-500  c.c.  Erlenmeyer  flask  K,  it  is  dissolved  in  200  c.c. 
of  water,  a  few  drops  of  litmus  solution  are  added,  and  in  case  the 

*  In  analyzing  chloroplatinates  of  organic  bases  (by  weighing  the  plat- 
inum) correct  results  may  be  obtained  by  using  for  the  atomic  weight  of 
platinum  the  value  196.9. 

t  MgO  is  frequently  recommended  for  expelling  the  ammonia.  Accord- 
ing to  a  private  communication  from  Herrn  Bormann,  of  Neunkirch,  this 
base  is  absolutely  unsuited  for  this  purpose. 


60  GRAVIMETRIC  ANALYSIS. 

solution  reacts  acid,  sodium  hydroxide  solution  (which  has  been 
previously  boiled  to  expel  traces  of  ammonia)  is  slowly  added  at  T 
with  constant  shaking  until  the  solution  changes  to  blue,  after 
which  ten  c.c.  more  of  the  caustic  soda  solution  are  added.*  The 
liquid  is  then  heated  to  boiling  and  100  c.c.  of  it  is  carefully  distilled 
into  the  receiver  F,  which  already  contains  20  c  c.  of  2  N.  hydro- 
chloric acid.  In  order  to  make  sure  that  no  ammonia  escapes 
from  the  receiver  it  is  well  to  connect  it  with  a  small  Peligot  tube 
containing  5  c.c.  of  2  N.  hydrochloric  acid  and  some  distilled 
water. 

After  100  c.c.  of  the  liquid  have  distilled  over,  all  the  ammo* 
nia  will  be  found  in  the  receiver  and  can  be  determined  according 
to  1  (a)  or  1  (6);  preferably  the  latter.  The  determination  can 
be  carried  out  much  more  quickly,  however,  if  the  receiver  contains 
a  measured  amount  of  standardized  acid  and  the  excess  is  deter- 
mined after  the  distillation  by  titrating  with  alkali  (cf .  Alkalimetry) . 

It  is  also  possible  to  make  an  accurate  determination  of  the 
amount  of  ammonia  present  by  measuring  the  volume  of  the  gas.f 

Colorimetric  Determination  of  Ammonium. 

For  the  determination  of  such  small  amounts  of  ammonia  as 
occur  in  drinking-water,  the  above  methods  are  not  suited.  In  this 
case  the  procedure  is  the  same  as  was  described  in  Vol.  I,  p.  46. 
(In  the  case  of  mineral  waters  it  is  necessary  to  add  more  than  one 
drop  of  the  soda  solution;  the  amount  necessary  is  determined 
by  adding  litmus  to  a  definite  volume  of  the  water  and  then  adding 
the  soda  solution  until  the  litmus  changes  to  blue.)  The  distillate 
is  received  in  50  c.c.  graduated  Nessler  tubes  (in  the  fourth 
one  there  is  usually  no  ammonia  to  be  detected)  and  these  are 
Nesslerized.  The  50  c.c.  of  distillate  is  mixed  with  2  c.c.  of  the 
Nessler  solution  and  the  yellow  color  produced  is  compared  with 
the  colors  produced  in  the  same  way  from  a  series  of  tubes  contain- 
ing known  amounts  of  ammonia.  When  a  standard  is  found  of 

*  The  separately  funnel  T  should  be  roughly  calibrated  before  setting 
up  the  apparatus,  by  pouring  water  into  it,  one  cubic  centimeter  at  a  time, 
and  marking  with  a  pencil  the  level  of  the  liquid  on  the  glass. 

f  Cf.  Part  III,  Gas  Analysis. 


COLORIMETRJC  DETERMINATION  OF  AMMONIUM.  61 

the  same  shade  as  the  solution  tested,  then  the  two  solutions  contain 
the  same  amount  of  ammonia. 

The  ammonium  chloride  solution  necessary  for  preparing  the 
standards  is  prepared  as  follows : 

3.141  *  gms.  of  ammonium  chloride  which  has  been  dried  at 
100°  C.  are  dissolved  in  1  liter  of  water  free  from  ammonia  (cf .  foot- 
note, Vol.  I,  p.  47).  The  solution  now  contains 

1  mgm.  of  ammonia  (NH3)  per  cubic  centimeter; 
this,  however,  is  too  strong  for  most  purposes,  so 
that  10  c.e.  of   it   are   taken   and   diluted  to  1 
liter.     Of  this  solution  1  c.c.  contains  0.01  mgm. 
XH3.     If    the    water    to    be    analyzed    contains 
considerable  ammonia,  a  smaller  portion  should 
be  taken  for  the  analysis  than  in  ordinary  cases 
(500  c.c.)  as  otherwise  the  first  distillate  (50  c.c.) 
would  give  too  intense  a  color  with  the  Nessler 
solution.     In   such   a   case   only   50   c.c.    of   the 
water  should  be  taken  for  the  analysis  and  this 
should  be  diluted  to  500  c.c.   with  water  free 
from  ammonia  and  then  distilled.  F 

In  order  to  ascertain  how  much  of  the  water 
to  take  for  the  analysis,  the  following  experiment  should  be  made; 

About  100  c.c.  of  the  water  to  be  tested  are  placed  in  a  narrow 
cylinder  (which  is  provided  with  a  ground-glass  stopper),  2  c.c.  of 
a  strongly  alkaline  sodium  carbonate  solution  f  are  added  to  pre- 
cipitate the  calcium  which  may  be  present,  the  mixture  is  vio- 
lently shaken  and  allowed  to  settle.  From  the  clear  supernatant 
liquid  50  c.c.  are  pipetted  off  into  a  Nessler  tube,  treated  with 

2  c.c.  of  Nessler  solution  and  mixed. I     If  a  strong  yellow  color,  or 


*  NH4C1-XH3  =  53.50 -17.03  =3.141. 

t  50  gms.  NaOH  and  50  gms.  Na^CXDj  (calcined)  are  dissolved  in  600  c.c. 
of  pure  distilled  water  and  the  solution  boiled  until  the  volume  is  only  500  c.c. 

J  In  the  case  of  mineral  waters  rich  in  magnesium  sulphate,  the  addition 
of  -he  10  c.c.  of  sodium  carbonate  solution  often  fails  to  prevent  a  turbidity 
on  adding  the  Nessler  reagent,  which  would  render  a  colorimetric  determina- 
tion impossible.  In  this  case  10  c.c.  of  a  boiled  BaCl2  solution  (120  gms. 
BaCl2  +  2H,O  in  500  c.c.  H2O)  should  be  added  before  treating  the  water 
with  the  sodium  carbonate  solution. 


62  GRAVIMETRIC  ANALYSIS. 

even  a  precipitate,  is  obtained,  then  only  50  c.c.  of  the  water  should 
be  taken  for  analysis.  If,  on  the  other  hand,  there  is  not  more  than 
a  faint  coloration  apparent,  then  500  c.c.  must  be  taken  for  the 
determination. 

For  the  Nesslerization,  the  three  cylinders  each  containing 
50  c.c.  of  the  distillate  are  placed  over  a  sheet  of  white  paper, 
treated  with  2  c.c.  of  the  Nessler  reagent,  and  mixed.  Beside 
them  are  placed  a  series  of  similar  cylinders  containing  respectively 
).0,  0.5,  1.0,  1.5,  2.0,  2.5,  3.0  c.c.  of  the  standard  ammonium 
chloride  solution  diluted  to  50  c.c.  These  are  also  treated  with 
2  c.c.  of  the  Nessler  reagent  and  by  matching  the  colors  obtained 
in  the  test  with  those  obtained  from  known  amounts  of  ammonia 
the  amount  present  in  the  water  can  be  easily  estimated. 

The  Nessler  reagent  should  give  a  distinct  coloration  with  500  c.c. 
of  water  containing  0.005  mgm.  NH3 ;  if  this  is  not  the  case,  it  must 
be  made  more  sensitive  by  the  addition  of  mercuric  chloride  solu- 
tion. 

For  mixing  the  liquid  in  the  cylinders  it  is  convenient  to  employ 
a  stirrer  such  as  is  shown  in  Fig.  25,  the  diameter  of  the  bulb  on 
the  end  being  only  slightly  less  than  that  of  the  cylinder.  By 
moving  this  stirrer  up  and  down  twice  the  liquid  becomes  thor- 
oughly mixed. 

KjeldahPs  Method  for  Determining  Nitrogen. 

The  methods  which  have  been  described  thus  far  are  suitable 
only  for  the  determination  of  nitrogen  when  it  is  in  solution  in 
the  form  ot  NH4  ions.  It  is,  however,  of  great  importance  to  be 
able  to  determine  nitrogen  when  it  is  present  other  than  as  an 
ammonium  compound  (in  protein,  coal,  etc.).  Inasmuch  as  it 
is  possible  to  determine  the  amount  of  ammonia  present  very  accu- 
rately, and  by  the  employment  of  volumetric  methods,  very  quickly, 
methods  were  sought  for  the  transformation  into  ammonia  of  the 
nitrogen  originally  present  in  some  other  form.  This  is  readily 
brought  about  by  the  method  of  Kjeldahl  and  its  modifications. 

By  the  oxidation  of  nitrogenous  organic  substances  with  con- 
centrated sulphuric  acid,  potassium  permanganate,  mercuric  oxide, 
etc.,  the  organic  matter  is  destroyed  and  the  nitrogen  is  completely 
changed  to  ammonium  and  held  as  ammonium  sulphate,  from 
which  the  ammonia  ?,an  be  readily  distilled  off. 


KJELDAHLS  METHOD  FOR  DETERMINING   NITROGEN  63 

Procedure  for  Kjeldahl's  Nitrogen  Determination  (W&farth's 
Modification*). — From  1  to  2  gms.  of  the  substance  to  be  analyzed 
are  placed  in  a  500-600-c.c.  flask,  made  of  difficultly  fusible  potash 
glass,  and  to  it  are  added  20  c.c.  of  sulphuric  acid  (3  volumes  of  con^ 
centrated  acid  mixed  with  2  volumes  of  fuming  sulphuric  acid)  ani" 
a  few  drops  of  mercury. f  The  flask  is  then  heated  in  an  iron  dish 
covered  with  asbestos  until  its  contents  gently  boil.  It  is  impor- 
tant, however,  to  be  sure  that  the  substance  should  be  thoroughly 
moistened  by  the  sulphuric  acid  before  the  heating,  especially  in 
the  case  of  mealy  substances.  In  order  to  avoid  a  loss  of 
nitrogenous  matter,  it  is  first  heated  for  half  a  minute  over  a 
very  small  flame  and  then  over  a  larger  one,  but  in  no  case 
should  the  flame  touch  the  flask  above  the  part  occupied  by  the 
liquid. 

The  heating  is  continued  until  the  solution  becomes  clear  and 
completely  colorless.  In  the  presence  of  iron  compounds,  how- 
ever, the  liquid  is  sometimes  slightly  yellow.  The  decomposition 
is  usually  complete  in  two  or  three  hours.  The  liquid  is  then  allowed 
to  cool,  the  sides  of  the  flask  are  washed  down  and  the  solution  is 
diluted  with  about  250  c.c.  of  water.  After  it  is  thoroughly  cool, 
80  c.c.  of  caustic  soda  solution,  free  from  nitrate,  are  quickly  added 
and  sufficient  potassium  sulphide  solution  (40  gms.  commercial 
potassium  sulphide  to  the  liter)  to  completely  precipitate  the  mer- 
cury and  cause  the  liquid  to  appear  black  (25  c.c.  of  the  potassium 
sulphide  solution  are  usually  sufficient).  A  few  grains  of  powdered 
zinc  are  then  added  and  the  flask  is  quickly  connected  with  the  distil- 
ling apparatus.  The  distilling-tube  dips  into  a  250-300-c.c.  Erlen- 
meyer  flask  containing  a  known  volume  of  normal  sulphuric  acid 
(10-20  c.c.)  and  sufficient  water  to  cover  the  lower  end  of  the  con- 
denser tube.  As  soon  as  a  noticeable  amount  of  water  vapor 
begins  to  come  over,  it  is  no  longer  necessary  to  have  the  condenser 
tube  dip  into  the  sulphuric  acid  solution  in  the  receiver.  After 
100  c.c.  of  the  liquid  have  distilled  over,  the  receiver  is  removed 
and  the  excess  of  sulphuric  acid  is  determined  by  titration  with 

*  Chem.  Ztg.,  9,  502. 

t  Gladding  uses  a  mixture  of  HjSO,  and  KHSO4,  which  usually  works 
very  well.     Then,  as  no  mercury  is  added,  the  subsequent  addition  of  N 
is  unnecessary. 


64  GRAVIMETRIC  ANALYSIS. 

one-tenth  normal  sodium  hydroxide  solution,  using  methyl  orange 
as  an  indicator.  * 

From  the  amount  of  sulphuric  acid  used,  the  amount  of 
nitrogen  present  may  be  calculated  as  follows:  Let  t  be  the 
number  of  cubic  centimeters  of  normal  sulphuric  acid  neutralized 
by  the  ammonia  evolved  from  a  gms.  of  the  substance,  then  this 
corresponds  to 

tX  0.01401  gms.  nitrogen, 

and  the  percentage  of  nitrogen  in  the  substance  is 


1.4Q1X* 

£  =  —         -  =  per  cent,  nitrogen. 

If  the  nitrogen  is  originally  present  to  a  considerable  extent  in 
the  form  of  nitrates,  oxides,  or  cyanides,  the  above  modification  of 
Kjeldahl's  method  will  not  serve  to  change  all  of  the  nitrogen  into 
ammonia.  In  such  cases  it  is  best  to  use  the  modification  proposed 
by  M.  Jodlbauerrf 

From  0.2-0.5  gm.  of  potassium  nitrate  (or  the  corresponding 
amount  of  another  nitrate)  are  treated  with  20  c.c.  of  concentrated 
sulphuric  acid  and  2.5  c.c.  of  phenolsulphonic  acid  (50  gms.  of 
phenol  dissolved  in  enough  concentrated  sulphuric  acid  of  66°  Be. 
to  make  100  c.c.  of  solution)  2-3  gms.  of  zinc  dust  and  5  drops  of 
chloroplatinic  acid  are  added  and  the  mixture  heated.  After 
heating  the  substance  with  this  mixture  for  four  hours,  the  liquid 
becomes  colorless  and  is  ready  to  be  distilled  with  the  caustic  soda 
solution. 


*In  the  titration  it  is  best  to  add  the  alkali  until  the  solution  turns 
yellow,  and  then  finish  by  adding  enough  acid  to  get  a  change  to  a  pale  pink. 
In  standardizing  the  acid  and  alkali,  the  volume  of  the  solution  and  the 
method  of  titrating  must  be  the  same  as  during  the  analysis  proper. 

f  Z.  anal.  Chem.,  27,  92. 


DETERMINATION  OF  MAGNESIUM.  65 

MAGNESIUM,  Mg.    At,  Wt.  24.32. 
Forms:  MgS04,  MgO,  Mg,P,O7- 
(a)  Determination  as  MgS04. 

This  method  for  the  determination  of  magnesium  can  always 
be  employed  when  the  magnesium  is  combined  with  an  acid  which 
can  be  volatilized  by  heating  with  sulphuiic  acid;  and  when  no 
other  metal  besides  ammonium  is  present.  A  weighed  amount 
of  the  substance  is  placed  in  a  platinum  crucible  and  treated  with 
a  slight  excess  of  concentrated  sulphuric  acid,  *  the  mixture  is 
evaporated  on  the  water-bath  as  far  as  possible,  and  the  excess  of 
free  sulphuric  acid  is  removed  by  cautiously  heating  the  crucible, 
held  in  an  inclined  position,  over  a  free  flame.  Finally  the  dry 
mass  is  heated  just  to  redness  in  a  covered  crucible,  and  after  cool- 
ing in  a  desiccator,  is  weighed  as  quickly  as  possible,  as  the 
anhydrous  magnesium  sulphate  is  hygroscopic. 

(6)  Determination  as  MgO. 

This  method  is  seldom  used  in  practice,  and  then  only  in  case 
the  magnesium  is  present  in  a  form  that  can  be  readily  changed  to 
the  oxide  by  ignition — i.e.,  as  carbonate,  nitrate  or  salt  of  an  organic 
acid.+  The  procedure  consists  simply  of  at  first  carefully  heating 
in  a  covered  crucible,  and  finally  with  the  full  heat  of  the  Teclu 
burner  in  a  half-covered  crucible. 

(c)  Determination  as  Magnesium  Pyrophosphate. 

This,  the  most  important  of  all  the  methods  for  the  determina- 
tion of  magnesium,  is  always  applicable  and  depends  upon  the 
following  principles :  If  the  solution  of  a  magnesium  salt  is  treated 
with  an  alkali  orthophosphate  solution  in  the  presence  of  ammonium 
chloride  and  ammonia,  the  magnesium  is  completely  precipitated 

*  Substances  which  react  violently  with  concentrated  H2SO4  should  be 
first  treated  with  water,  and  dilute  sulphuric  acid  added  little  by  little. 

f  Magnesium  chloride  can  be  changed  to  the  oxide  by  ignition  with  mer- 
curic oxide  hi  a  porcelain  evaporating-dish.  Mercuric  chloride  and  the  excess 
of  mercuric  oxide  are  volatilized.  In  this  way  magnesium  is  often  separated 
from  the  alkalies.  (Translator.) 


66  GRAVIMETRIC  ANALYSIS. 

as  magnesium  ammonium  phosphate,  which  by  ignition  is  changed 
to  magnesium  pyrophosphate  : 

2MgNH4PO4  =  2NH3+  H2O+  Mg2P2O7. 

Formerly  it  was  a  common  practice  to  precipitate  magne- 
sium ammonium  phosphate  in  the  cold.  Neubauer*  showed, 
however,  that  this  sometimes  leads  to  high  results  while  at 
other  times  the  results  are  low.  The  latter  is  the  case  when  the 
precipitation  takes  place  in  strongly  ammoniacal  solutions  con- 
taining but  little  ammonium  salts,  particularly  when  the  phos- 
phate solution  is  added  slowly.  Tribasic  magnesium  phosphate, 
Mg3(PO4)2,  contaminates  the  precipitate.  On  the  other  hand, 
the  results  are  too  high  if  the  precipitation  takes  place  in  neutral 
or  slightly  ammoniacal  solution  in  the  presence  of  considerable 
ammonium  salts.  In  this  case  more  or  less  monomagnesium 
ammonium  phosphate,  Mg(NH4)4(P04)2,  is  formed.  This  com- 
pound is  changed  to  magnesium  metaphosphate  on  gentle 
ignition. 


and  the  results  are  too  high.  When  only  a  little  of  the  meta- 
phosphate is  present,  the  temperature  of  the  blast-lamp  will 
eventually  lead  to  volatilization  of  some  phosphorus  pentoxide, 
so  that  nearly  correct  results  are  then  obtained. 

2Mg(P03)2  =  Mg2P207  +  P205. 

Neubauer  recommends  adding  an  excess  of  sodium  phosphate  to 
the  slightly  acid  solution  of  the  magnesium  salt,  then  stirring 
in  one-third  of  the  solution  volume  of  10  per  cent,  ammonia, 
filtering  after  four  or  five  hours,  washing  with  2.5  per  cent. 
ammonia,  dissolving  the  precipitate  in  a  little  dilute  hydro- 
chloric acid,  adding  a  few  drops  of  sodium  phosphate  solution, 
and  precipitating  by  the  addition  of  one-third  volume  of  10  per 
cent,  ammonia.  This  method,  however,  gives  too  high  results, 

*  Z.  angew.  Chem.,  1896,  439.    See  also  Gooch  and  Austin,  Z.  anorg. 
Chem.,  20,  121. 


DETERMINATION  OF  MAGNESIUM.  67 

e.g.,  9.97,  9.95,  and  9.98  per  cent.  Mg  in  MgSO4,  7H2O  instead 
of  9.88  per  cent.  Mg. 

Correct  results  can  be  obtained  by  the  method  of  B.  Schmitz,* 
or  of  W.  Gibbs.f 

Method  of  B.  Schmitz. 

The  acid  solution,  containing  magnesium  in  the  presence  of 
ammonium  salts, %  is  heated  to  boiling  and  then  treated  with  an  ex- 
cess of  sodium  or  ammonium  phosphate.  One-third  the  solution's 
volume  of  10  per  cent,  ammonia  is  at  once  added,  the  solution 
allowed  to  cool  and  filtered  through  a  Munroe  crucible  after 
standing  for  several  hours.  The  precipitate  is  washed  with  2.5 
per  cent,  ammonia,  dried  and  ignited  very  slowly,  gradually 
increasing  the  heat  until  the  precipitate  is  white.  After  cool- 
ing, the  Mg2P2C>7  is  weighed: 

2MgNH4P04  =  2NH3  +  H2O  +  Mg2P207. 

From  the  weight  of  the  latter,  p,  the  amount  of  magnesium  can  be 
calculated  as  follows: 


= weight  of  magnesium. 


Mg2P207 

The  precipitate  can  be  filtered  upon  an  ordinary  filter,  but 
in  all  cases  the  ignition  must  be  gradual  or  it  is  almost  impossible 
to  obtain  perfectly  white  pyrophosphate. 

*  Z.  anal.  Chem.  1906,  512;  cf.  Jorgensen,  ibid.  1906,  278. 

t  Am.  J.  Sci.  (3),  5,  114. 

J  Ammonium  salts  do  no  harm  when  the  precipitation  takes  place  in  hot 
solution,  but,  on  the  contrary,  cause  the  formation  of  a  coarsely  crystalline 
precipitate  which  is  easy  to  filter. 


68  GRAVIMETRIC  ANALYSIS. 


Method  of  W.  Gibbs. 

The  neutral,  not  too  concentrated,  solution  of  magnesium 
salt  containing  ammonium  salts  is  treated  at  the  boiling  tem- 
perature with  a  normal  solution  of  microcosmic  salt,  NaHNH4PO4 
•fH2O,  until  no  further  precipitation  takes  place.  Almost 
90  per  cent,  of  the  magnesium  present  is  at  once  thrown  down 
as  amorphous  magnesium  hydrogen  phosphate,  MgHPO4: 

NaHNH4P04  +  MgCl2  =  NaCl  +  NH4C1  +  MgHPO4. 

Then,  while  stirring  the  hot  solution,  about  one-third  volume  of 
10  per  cent,  ammonia  is  added  whereby  the  amorphous  pre- 
cipitate is  transformed  into  crystalline  magnesium  ammonium 
phosphate: 

M£HP04+  NH3 = MSNH4P04. 

At  the  same  time  the  magnesium  remaining  in  solution  is  thrown 
down  almost  completely. 

After  standing  two  or  three  hours,  the  supernatant'  liquid 
is  filtered  off,  and  the  precipitate  washed  three  times  by  decan- 
tation  with  2.5  per  cent,  ammonia,  finally  transferred  to  the 
filter,  washed  completely  with  2.5  per  cent,  ammonia  and  dried 
in  the  hot  closet.  The  dried  precipitate  is  transferred  as  com- 
pletely as  possible  to  a  weighed  platinum  crucible,  the  filter- 
paper  burned  in  a  platinum  wire  spiral,  and  the  ash  added  to  the 
main  portion  of  the  precipitate.  The  covered  crucible  is  heated 
very  gently  at  first  until  the  ammonia  is  all  dried  off,  then  more 
strongly  until  the  mass  is  snow  white.  The  crucible  is  cooled 
in  a  desiccator  and  weighed. 


Separation  of  Magnesium  from  the  Alkalies. 

The  methods  of  Gibbs  and  of  Schmitz  serve  to  separate  mag- 
nesium from  the  alkalies  in  those  cases  where  the  determination 
of  magnesium  alone  is  desired. 

If  it  is  desired  to  determine  magnesium  and  the  alkalies  in  one 


DETERMINATION  OF  MAGNESIUM.  69 

and  the  same  sample,  it  is  best  to  use  the  Gooch  and  Eddy  * 
modifications  of  the 

Schaffgotsche  Methodf 

The  method  is  based  upon  the  fact  that  magnesium  can*  be 
precipitated  quantitatively,  by  means  of  an  alcoholic  solution  of 
ammonium  carbonate,  as  crystalline  magnesium  ammonium  car- 
bonate, MgC03-(XH4)2CO3-6H2O. 

Preparation  of  the  Precipitant. — A  mixture  of  180  c.c.  con- 
centrated ammonia,  800  c.c.  water,  and  900  c.c.  absolute  alcohol 
is  saturated  with  commercial  ammonium  carbonate.  The  liquid 
is  shaken  with  the  powdered  carbonate,  and  after  several  hours 
the  excess  of  the  latter  is  removed  by  filtration. 

Procedure. — The  neutral  solution  containing  only  magnesium 
and  the  alkalies  (lithium  must  not  be  present),  preferably  in  the 
form  of  chlorides,  is  treated  with  an  equal  volume  of  absolute 
alcohol  and  then  with  an  excess  of  the  ammonium  carbonate  re- 
agent. The  liquid  is  vigorously  stirred  for  a  few  minutes  and 
allowed  to  stand  for  at  least  half  an  hour,  whereupon  it  is  filtered 
through  a  Gooch  or  Munroe  crucible.  The  precipitate  is  washed 
with  the  precipitant,  dried,  ignited  and  weighed  as  MgO. 

If  considerable  alkali  is  present  the  precipitate  always  con- 
tains a  small  quantity  of  it.  In  such  cases  the  precipitate  is  dis- 
solved in  hydrochloric  acid,  the  solution  evaporated  to  dryness,  the 
residue  taken  up  in  a  little  water,  and  the  precipitation  repeated. 

The  combined  filtrates  are  evaporated  to  dryness  and  the 
alkali  determined  as  described  on  page  43  et  seq. 

If,  however,  it  is  desired  to  separate  magnesium  from  the  alka- 
lies in  order  that  the  latter  may  be  determined,  the  magnesium 
may  be  precipitated  as  magnesium  hydroxide,  from  a  solution 
free  from  ammonium  salts,  by  the  addition  of  barium  hydroxide 
solution.  J  The  barium  is  then  removed  by  ammonium  carbonate 
and  the  alkalies  determined  in  the  filtrate.  For  the  detailed 
description  of  this  method  see  Silicate  Analysis.  Even  in  this 
case,  however,  the  use  of  the  Schaffgotsche  method  of  separating 
magnesium  from  the  alkalies  is  more  satisfactory. 

*Z.  anorg.  Chem.,  58,  427  (1908).  t  Pogg.  Ann.,  104,  482  (1858). 

%  Cf.  footnote  to  page  65. 


70  GRAVIMETRIC  ANALYSIS. 

,     METALS  OF  GROUP  IV. 
CALCIUM,  STRONTIUM,  BARIUM. 

CALCIUM,  Ca.     At.  Wt.  40.09. 
c.  Forms:  CaO,  CaC03,  CaSO4. 

I.  Determination  as  Calcium  Oxide  (Lime),  CaO. 

For  the  determination  of  calcium  as  CaO,  it  is  best  precipi- 
tated as  the  oxalate  and  converted  to  the  oxide  by  strong  ignition. 

Procedure. — The  neutral  or  slightly  ammoniacal  solution, 
which  besides  magnesium  and  the  alkalies  should  contain  no  other 
metal,*  is  treated  with  ammonium  chloride,  heated  to  boiling,  and 
precipitated  by  the  addition  of  a  boiling  solution  of  ammonium 
oxalate.  After  standing  some  time,  the  precipitate  becomes 
coarsely  crystalline  and  settles  to  the  bottom  of  the  beaker,  when 
a  little  more  ammonium  oxalate  solution  is  added  to  make  sure  that 
the  precipitation  has  been  complete.  It  is  allowed  to  stand  four 
hours,  when  the  clear  liquid  is  poured  through  a  filter,  the  pre- 
cipitate is  covered  with  boiling  water  containing  ammonium 
oxalate,  f  allowed  to  settle,  filtered,  and  the  operation  repeated 
three  times.  Finally  the  whole  precipitate  is  transferred  to  the 
filter  and  washed  with  hot  water  containing  ammonium  oxalate 
until  free  from  chloride.  The  precipitate  is  warmed  in  the  hot 
closet  until  nearly  dry,  when  it  is  placed  together  with  the  filter 
in  a  platinum  crucible  and  ignited  wet.  It  should  be  heated 
cautiously  at  first  in  order  that  the  too  rapid  evolution  of  carbonic 
oxide  will  not  cause  loss.  After  the  filter  is  burnt  the  crucible  is 
covered  and  strongly  heated  at  first  over  the  Teclu  burner  and 
finally  over  the  blast-lamp  for  twenty  minutes. 

The  crucible  while  still  quite  warm  is  placed,  in  the  desiccator 
shown  in  Fig.  7,  near  an  open  weighing-beaker  and  allowed  to  re- 
main there  for  one  hour.  During  the  cooling,  the  air  enters  the 
desiccator,  through  the  U  tube,  whose  outer  half  is  filled  with 

*  If  magnesium  is  present,  the  precipitation  should  be  carried  out  as 
described  on  page  76. 

t  T.  W.  Richards  found  that  the  precipitate  was  appreciably  soluble  in 
pure  water  but  practically  insoluble  in  a  dilute  solution  of  ammonium  oxalate 
(Z.  anorg.  Chem.,  28,  85  (1901)). 


DETERMINATION  OF  CALCIUM  AS  SULPHATE.  71 

soda-lime  and  whose  inner  half  contains  calcium  chloride,  in  a  dry 
condition  and  free  from  carbonic  acid  gas.  The  crucible  is  now 
placed  in  the  weighing-beaker,  quickly  covered  and  allowed  to 
stand  for  half  an  hour  in  the  air  near  the  balance,  after  which  it 
is  weighed.  The  crucible  is  again  heated  over  the  blast-lamp  for 
ten  minutes  and  is  cooled  in  exactly  the  same  way  and  weighed. 
Should  the  weight  not  be  found  constant,  the  process  must  be 
repeated.  The  above  directions  when  carefully  followed  usually 
enable  one  to  obtain  a  constant  weight  on  the  second  ignition. 

Example. — Calcite:  0.5  gm.  of  the  finely  powdered  material 
which  has  been  dried  at  100°  is  placed  in  a  300-c.c.  beaker  and 
moistened  with  20  c.c.  of  water.  The  beaker  is  covered  with  a 
watch-glass,  concentrated  hydrochloric  acid  is  added  drop  by  drop, 
and  the  liquid  is  finally  heated  until  all  is  dissolved.  The  solution 
is  then  boiled  for  fifteen  minutes  to  expel  all  carbon  dioxide,  neutral- 
ized carefully  with  ammonia,  diluted  with  150-200  c.c.  of  hot  water, 
and  precipitated  with  ammonium  oxalate  as  above  described. 

Remark. — If  both  solutions  are  not  boiling  hot  during  the 
precipitation,  the  calcium  oxalate  forms  very  fine  crystals;  it 
then  settles  very  slowly  and  passes  readily  through  the  filter. 

Calcium  oxalate  is  almost  insoluble  in  water  and  acetic  acid  in 
the  presence  of  ammonium  oxalate,  but  readily  soluble  in  mineral 
acids. 

2.  Determination  of  Calcium  as  Sulphate,  CaS04. 

This  method  is  chiefly  used  for  the  analysis  of  calcium  salts 
of  organic  acids.  For  this  purpose  the  calcium  salt  is  ignited  in 
a  weighed  platinum  crucible,  after  which  the  crucible  is  covered 
with  a  watch-glass,  carefully  treated  with  dilute  sulphuric  acid 
and  warmed  upon  the  water-bath  until  there  is  no  longer  any 
evolution  of  carbon  dioxide.  The  under  side  of  the  watch-glass 
is  carefully  washed  and  the  liquid  evaporated  as  far  as  possible 
on  the  bath.  The  excess  of  sulphuric  acid  is  then  carefully  driven 
off  by  inclining  the  crucible  and  heating  over  the  free  flame  (or 
in  an  air-bath)  (cf.  Fig.  11,  p.  27).  The  residue  is  gently  ignited 
and  weighed.  By  strong  ignition,  calcium  sulphate  loses  SO3.* 

*  0.2052  gm.  CaSO4  remained  unchanged  in  weight  after  heating  for  one 
hour  to  dark  redness;  but  on  heating  with  the  full  flame  of  a  Teclu  burner, 
it  lost  0.0004  gm.  in  weight.  On  heating  for  one  hour  over  the  blast  lamp 
it  lost  0.0051  gm.  (J.  Weber.) 


72  GRAVIMETRIC  ANALYSIS. 

Calcium  may  also  be  precipitated  as  calcium  sulphate.  The 
solution,  which  should  contain  as  little  free  acid  as  possible,  is 
treated  with  an  excess  of  dilute  sulphuric  acid,  four  volumes  of 
alcohol  are  added,  and  the  mixture  is  allowed  to  stand  12  hours. 
It  is  then  filtered  off,  washed  with  70  per  cent,  alcohol,  dried, 
separated  from  the  filter  as  completely  as  possible,  the  filter  burned 
in  a  platinum  spiral,  and  the  ash  added  to  the  main  part  of  the 
precipitate  in  a  platinum  crucible,  gently  ignited  and  weighed. 

3.  Determination  of  Calcium  as  Carbonate,  CaC03. 

Only  in  rare  cases  is  calcium  precipitated  as  carbonate  by 
ammonium  carbonate  in  the  presence  of  ammonia.  The  filtered 
and  washed  precipitate  is  gently  ignited  and  weighed  as  carbonate. 
After  weighing  it  is  necessary  to  moisten  the  residue  with  a 
little  ammonium  carbonate  solution,  evaporate  to  dryness  on 
the  water-bath,  and  again  ignite  gently.  This  is  done  in  order  to 
change  small  amounts  of  calcium  oxide,  which  may  have  been 
formed  during  the  burning  of  the  filter-paper,  back  to  carbonate. 

In  the  presence  of  considerable  ammonium  chloride  the  pre- 
cipitation of  calcium  by  means  of  ammonium  carbonate  is  not 
quite  complete,  whereas  the  precipitation  with  ammonium  oxalate 
always  is.  Consequently  it  is  advisable  in  all  cases  to  precipitate 
the  calcium  as  oxalate  and  weigh  it  as  the  oxide,  if  the  oxalate 
is  ignited  gently,  however,  it  can  be  weighed  as  the  carbonate. 

STRONTIUM,  Sr.    At.  Wt.  87.62. 
Forms:  SrS04,  SrC03,  SrO. 

The  determination  as  the  sulphate  is  the  most  accurate. 

Determination  of  Strontium  as  Sulphate,  SrS04. 

Procedure. — To  the  neutral  solution  containing  strontium,  a 
slight  excess  of  dilute  sulphuric  acid  is  added  and  as  much  alcohol 
as  there  is  volume  of  solution.  After  stirring  well,  the  mixture 
is  allowed  to  stand  twelve  hours,  filtered  and  washed,  at  first 
with  50  per  cent,  alcohol,  to  which  a  little  sulphuric  acid  has 
been  added,  and  finally  with  pure  alcohol  until  the  wash  water 
no  longer  gives  the  sulphuric  acid  reaction.  The  precipitate  is 


DETERMINATION  OF  STRONTIUM  AS  OXI^E  OR  CARBONATE.         73 

dried  and  ignited  as  described  under  the  determination  of  cal~ 
cium  as  sulphate. 

Solubility  of  Strontium  Sulphate  according  to  Fresenius. 

6895  parts  of  water  at  the  ordinary  temperature  (17-20°)  dis- 
solve 1  part  of  SrSO4. 

9638  parts  of  boiling  water  dissolve  1  part  SrS04. 

The  sulphate  is  less  soluble  in  water  containing  sulphuric  acid: 

12,000  parts  of  water  containing  sulphuric  acid  dissolves  1  part 
SrSC>4.  The  precipitate  is  soluble  in  concentrated  sulphuric  acid, 
so  that  the  water  should  not  contain  very  much  sulphuric  acid. 

In  cold,  dilute  hydrochloric  or  nitric  acid,  strontium  sulphate  is 
considerably  more  soluble,  and  also  in  solutions  containing  acetic 
acid,  magnesium  chloride,  or  alkali  chloride. 

If,  therefore,  considerable  free  acid  is  present,  it  should  be 
removed  by  evaporating  the  solution  to  dryness  and  dissolving 
the  residue  in  water.  The  strontium  is  then  precipitated  as  above 
described. 

Determination  of  Strontium  as  Oxide,  SrO,  or  as  Carbonate,  SrC03. 

The  strontium  is  precipitated  as  carbonate,  or  in  some  cases 
as  oxalate,  and  changed  by  ignition  to  the  oxide  as  described 
under  calcium. 

Strontium  carbonate  is  decomposed  by  heat  more  difficultly 
than  calcium  carbonate  and  the  determination  as  carbonate  is 
very  satisfactory.  It  is  advisable  to  treat  the  precipitate  as 
described  under  calcium,  although  it  is  usually  unnecessary  to 
heat  with  additional  ammonium  carbonate. 

Solubility  of  Strontium  Carbonate  in  Water  according  to  Fresenius. 

18,045  parts  of  water  dissolve  at  ordinary  temperatures  1  part 
of  SrG03. 

In  water  containing  ammonium  carbonate  the  salt  is  much 
less  soluble;  on  the  other  hand,  ammonium  chloride  and  ammo- 
nium nitrate  increase  its  solubility. 

In  case  calcium,  strontium,  magnesium  and  alkali  salts  are 
present  together,  as  in  minerals  and  in  mineral  waters,  the  calcium 


74  GRAVIMETRIC  ANALYSIS. 

and  strontium  are  both  precipitated  as  oxalates  and  transformed 
by  ignition  into  the  oxides.     Of.  pp.  78,  79.  r 

^ 

Solubility  of  Strontium  Oxalate  in  Water. 

12,000  parts  of  water  at  ordinary  temperatures  dissolve  1 
part  of  SrC2O4+2iH2a 

The  solubility  is  very  much  less  in  water  containing  ammonium 
oxalate. 

BARIUM,  Ba.    At.  Wt.  137.37. 

» 
Forms:  BaS04,  BaCr04,  BaC03. 

I.  Determination  as  Barium  Sulphate. 

The  solution,  slightly  acid  with  hydrochloric  acid,  is  heated  to 
boiling  and  precipitated  by  the  addition  of  an  excess  of  hot,  dilute 
sulphuric  acid.  It  is  allowed  to  stand  on  the  water-bath  until  the 
precipitate  has  settled,  the  solution  is  then  poured  through  a  filter, 
and  the  precipitate  is  washed  four  times  with  50  c.c.  of  water  to 
which  a  few  drops  of  sulphuric  acid  have  been  added.  The  pre- 
cipitate is  transferred  to  the  filter  and  washed  with  hot  water 
until  the  wash  water  ceases  to  give  the  sulphuric  acid  reaction. 
It  is  then  dried  somewhat,  ignited  wet  in  a  platinum  crucible,  and 
weighed  without  previous  heating  over  the  blast-lamp. 

Remark. — By  the  combustion  of  the  filter-paper  there  is  usually 
a  partial  reduction  of  the  barium  sulphate  to  sulphide,  but  the 
latter,  on  being  gently  ignited  in  an  inclined  crucible,  is  readily 
changed  back  to  sulphate,  so  that  there  is  no  loss  to  be  feared. 

The  procedure  for  the  determination  of  barium  as  carbonate  is 
the  same  as  was  described  under  calcium. 

Solubility  of  Barium  Sulphate  in  Water.— 344, 000  parts  of  water 
dissolve  1  part  of  BaS04. 

2.  Determination  of  Barium  as  Chromate. 

The  neutral  solution  of  the  barium  salt  is  diluted  to  about  200 
c.c.,  treated  with  4-6  drops  of  acetic  acid  (sp.  gr.  1.065),  heated  to 
boiling,  precipitated  with  a  slight  excess  of  ammonium  chromate 
(prepared  by  adding  ammonia  to  a  solution  of  ammonium  bichro- 
mate free  from  sulphate,  until  the  color  becomes  yellow),  and 


DETERMINATION  OF  BARIUM  AS  CHROMATE.  75 

allowed  to  cool.  The.  precipitate  is  filtered  off  through  a  Gooch 
micible  and  washed  with  hot  water  until  20  drops  of  the  filtrate 
give  scarcely  any  reddish-brown*  coloration  with  a  neutral  solution 
of  silver  nitrate.  -The  precipitate  is  dried  in  the  hot  closet,  after 
which  the  crucible  is  fastened  to  a  larger  porcelain  crucible  by 
means  of  an  asbestos  ring,  so  that  there  remains  a  space  of  about 
\  cm.  between  the  two  crucibles  (cf.  p.  27),  and  the  open  crucible 
is  ignited  over  the  free  flame  until  the  precipitate  becomes  a  bright 
yellow.* 

Solubility  of  Barium  Chromale.-f 

86,957  parts  of  water  at  ordinary  temperatures  dissolve  1  part  BaOO4. 

23,000    "      "  boiling  water  dissolve  1  part  BaCrO4. 

49,381]  "      "  a  0.75  per  cent,  ammonium  acetate  solution  (at  15°)  dissolve 

1  part  BaCrO4. 
45,152  parts  of  a  0.5  per  cent,  ammonium  nitrate  solution  (at  14°)  dissolve 

1  part  BaCrO4. 
23,555  parts  of  a  1.5  per  cent,  ammonium  acetate  solution  (at  15°)  dissolve 

1  part  BaCrO4. 
22,988  parts  of  0.5  per  cent,  ammonium  nitrate  solution   dissolve  1  part 

BaCrO4. 

3,670  parts  of    1  per  cent,  acetic  acid  solution  dissolve   1  part  BaCrO4. 
2,618     "      "     5    "      "  "        "          "  "         1     "         " 

IQOft  ((  (I      -lf\       (I  tl  tl  tl  (I  (f  -I  II  (I 

,i/oU  i\J  JL 

1,813     "      "  10    "      "       chromic  acid  solution  dissolve  1  part  BaCrO4. 

The  solubility  of  barium  chromate,  therefore,  increases  con- 
siderably with  increasing  concentrations  of  either  acetic  or  chromic 
acids ;  the  solubility  is  affected  to  a  much  less  degree  by  solutions 
containing  neutral  amraonium  salts.  By  the  additions  of  small 
amounts  of  neutral  ammonium  chromate  the  solubility  becomes 
lessened  to  nearly  zero. 

*  Oftentimes  small  amounts  of  the  precipitate  are  reduced  to  chromic 
oxide  by  traces  of  organic  matter,  whereby  it  appears  slightly  greenish. 
By  long-continued  ignition  in  an  open  crucible,  the  chromic  oxide  is  changed 
back  to  chromate,  when  the  precipitate  appears  a  homogeneous  yellow 
throughout. 

t  P.  Schweizer,  Z.  anal.  Chem.,  1890,  p.  414,  and  R.  Fresenius,  ibid.,  1890, 
p.  418. 


76  GRAVIMETRIC  ANALYSIS. 

SEPARATION  OP  THE  ALKALINE  EARTHS  FROM  MAGNESIUM  AND 
FROM  THE  ALKALIES. 

I.  Separation  of  Calcium  from  Magnesium  (and  Alkalies). 

The  separation  depends  upon  the  different  solubilities  of  the 
two  oxalates.  Calcium  oxalate  is  practically  insoluble  in  hot 
water,  whereas  magnesium  oxalate  is  fairly  soluble;  1500  parts 
of  cold  water,  or  1300  parts  of  boiling  water,  dissolve  1  part  of 
MgC2O4-2H2O.  In  an  excess  of  the  precipitant,  however,  mag- 
nesium oxalate  is  much  more  soluble  owing  to  the  formation  of 
complex  salts. 

If  calcium  is  precipitated  as  oxalate  from  a  dilute  solution  in 
the  presence  of  magnesium,  some  magnesium  oxalate  is  occluded 
by  the  calcium  oxalate  precipitate  even  when  the  solution  is  by 
no  means  saturated  with  magnesium  oxalate,  and  high  results 
are  obtained  for  calcium.  In  such  cases  the  error  is  usually 
compensated  according  to  Fresenius,  by  dissolving  the  pre- 
cipitate in  hydrochloric  acid  and  repeating  the  precipitation  with 
ammonia  and  ammonium  oxalate. 

The  work  of  T.  W.  Richards  *  has  shown,  however,  that  the 
quantity  of  magnesium  oxalate  occluded  is  dependent  upon  the 
concentration  of  the  undissociated  magnesium  oxalate  in  solution, 
and  upon  the  time  in  which  the  calcium  oxalate  is  in  contact 
with  the  solution.  Consequently  anything  that  prevents  the 
dissociation  of  magnesium  oxalate  will  tend  to  increase  the 
amount  of  occlusion  and  thereby  increase  the  result  of  the  calcium 
determination.  Anything  that  will  cause  the  ionization  of  the 
magnesium  oxalate  serves  to  reduce  the  amount  of  error. 

Concentrating  the  solution  and  increasing  the  amount  of 
oxalate  ions  in  solution  by  the  addition  of  considerable  ammo- 
nium oxalate,  both  tend  to  repress  the  dissociation  of  the  mag- 
nesium oxalate.  The  dissociation  of  this  compound  is  favored 
by  the  presence  of  hydrogen  ions  and  by  dilution. 

A  considerable  excess  of  ammonium  oxalate  is  necessary  in 
the  quantitative  precipitation  of  calcium  and,  moreover,  mag- 

*  Z.  anorg.  Chem.,  28,  71  (1901). 


ALTERNATE  METHOD.  77 

nesium  oxalate  forms  readily  soluble  complex  salts  with  undisso- 
ciated  ammonium  oxalate.  It  is  desirable,  therefore,  to  prevent 
the  dissociation  of  the  ammonium  oxalate  as  much  as  possible, 
and  this  is  accomplished  by  the  addition  of  another  ammonium 
salt,  preferably  the  chloride. 

Procedure. — Dilute  the  solution  with  hot  water  so  that  the 
magnesium  is  present  in  a  concentration  of  not  over  fiftieth 
normal,  and  add  a  considerable  quantity  of  ammonium  chloride, 
if  it  is  not  already  present.  To  precipitate  the  calcium,  add 
a  sufficient  volume  of  boiling  oxalic  acid  solution,  containing 
three  or  four  equivalents  of  hydrochloric  acid  to  lessen  the 
dissociation  of  the  oxalic  acid.  Introduce  a  little  methyl  orange 
into  the  boiling  solution,  and  add  ammonia  until  a  yellow  colora- 
tion is  produced.  The  ammonia  should  not  be  added  all  at 
once,  but  in  small  quantities  from  time  to  time  so  that  about 
half  an  hour  is  consumed  in  the  operation. 

After  the  neutralization  add  a  considerable  excess  of  hot 
ammonium  oxalate  solution,  allow  to  stand  four  hours  but  not 
longer  on  account  of  the  fact  that  the  occlusion  increases  with 
the  length  of  time,  filter,  and  wash  with  hot,  one  per  cent,  am- 
monium oxalate  until  the  filtrate  gives  no  test  for  chloride  after 
acidifying  a  sample  with  nitric  acid. 

Although  this  precipitate  contains  a  little  magnesium  (0.1- 
0.2  per  cent.)  it  is  also  true  that  an  almost  equal  amount  of 
magnesium  passes  into  the  filtrate,  so  that  it  is  not  advisable  to 
repeat  the  precipitation  when  made  in  this  way. 

Alternate  Method. 

W.  G.  Blasdale  *  has  studied  the  separation  of  calcium  from 
different  amounts  of  magnesium  and  succeeded  in  getting  excellent 
results  by  a  somewhat  simpler  method.  The  acid  solution  con- 
taining not  more  than  0.6  gm.  of  calcium  and  magnesium  oxides  is 
brought  to  a  volume  of  300  c.c.,  and  heated  to  boiling;  3.5  gms.  of 
ammonium  chloride  are  added  and  1  gm.  of  oxalic  acid,  and  the 
complete  precipitation  of  the  calcium  is  brought  about  by  neutral- 

*  J.  Am.  Chem.  Soc.,  31,  917  (1909).  *^ 


7  8  GRAVIMETRIC  ANALYSIS. 

izing  with  1  per  cent,  ammonia  added  during  five  minutes.  The 
precipitate  is  allowed  to  stand  for  an  hour  before  filtering.  If 
considerably  more  magnesium  than  calcium  is  present,  the 
precipitation  is  effected  in  two  stages.  First  only  enough  oxalic 
acid  is  added  to  combine  with  all  the  calcium  present.  The 
solution  is  slowly  neutralized  as  before  and  allowed  to  stand  ten 
minutes.  Then  the  balance  of  the  oxalic  acid  is  added,  the  solu- 
tion made  alkaline  and  allowed  to  stand  for  an  hour.  With  more 
than  ten  times  as  much  magnesium  as  calcium,  a  double  precita- 
tion  is  desirable. 

In  the  filtrate  the  magnesium  can  be  precipitated  by  the 
addition  of  ammonia  and  sodium  phosphate.  If,  however,  large 
amounts  of  ammonium  salts  are  present  it  is  preferable  to 
evaporate  to  dryness  in  a  platinum  or  porcelain  dish,  to  dry 
the  residue  at  110-130°  for  an  hour  or  longer,  and  to  expel  the 
ammonium  salts  by  gentle  ignition.  The  residue  is  taken  up 
in  a  little  warm  hydorchloric  acid,  diluted  with  water,  a  little 
carbon  residue  filtered  off  and  the  magnesium  determined  as 
pyrophosphate.  (Pages  66-69.) 

II.  Separation  of  Strontium  from  Magnesium. 

This  separation  finds  practical  application  in  the  analysis  of 
almost  all  mineral  waters  and  of  minerals  containing  strontium. 
In  all  of  these  cases,  however,  strontium  occurs  in  relatively  small 
amounts  in  the  presence  of  large  amounts  of  calcium  and  varying 
amounts  of  magnesium,  so  that  it  is  a  question,  first,  of  separating 
calcium  and  strontium  from  magnesium.  This  separation  is 
effected  by  the  precipitation  of  the  calcium  and  strontium  as 
oxalates  as  described  on  pp.  70  and  73. 

The  filtrate  containing  magnesium  may  also  contain  traces  of 
strontium;  hence,  after  the  removal  of  the  ammonium  salts  by 
ignition,  the  residue  is  dissolved  in  hydrochloric  acid,  sulphuric 
acid  and  alcohol  are  added,  and  the  solution  is  allowed  to  stand 
for  twelve  hours.  Any  resulting  precipitate,  consisting  of  strontium 
or  barium  sulphate,  is  filtered  off  and  weighed.  From  this  filtrate 
the  magnesium  is  precipitated  as  magnesium  ammonium  phosphate 
as  described  on  p.  66,  and  weighed  as  the  pyrophosphate. 


SEPARATION  OF  BARIUM  FROM  MAGNESIUM  AND  STRONTIUM.  79 

III.  Separation  of  Barium  from  Magnesium. 
In  case  it  is  desired  to  separate  only  barium  from  magnesium, 
the  solution  (which  must  be  free  from  nitric  acid)  is  acidified 
with  hydrochloric  acid,  heated  to  boiling,  and  the  barium  precipi- 
tated by  the  addition  of  boiling,  dilute  sulphuric  acid  (cf.  p.  74). 
The  magnesium  is  precipitated  from  the  filtrate  as  magnesium 
.ammonium  phosphate  in  the  usual  way.  In  most  cases,  however, 
a  separation  of  barium,  strontium,  and  calcium  from  the  magnesium 
is  involved.  For  this  purpose  the  three  alkaline  earths  are  pre- 
cipitated as  oxalates,  and  any  barium  or  strontium  remaining  in 
the  filtrate  is  precipitated  as  described  under  II.  The  magnesium 
is  determined  hi  the  final  filtrate. 


IV.  Separation  of  the  Alkaline  Earths  from  One  Another. 

Principle. — The  mixture  of  the  dry  nitrates  is  treated  with 
ether-alcohol,  which  dissolves  calcium  nitrate  alone.  The  residue 
is  taken  up  hi  water,  the  barium  is  precipitated  as  chromate,  and 
the  strontium  is  determined  in  the  filtrate  as  sulphate. 

PROCEDURE. 

(a)  Separation  of  Calcium  from  Strontium  and  Barium  accord- 
ing to  Rose-Stromeyer-Fresenius. 

The  three  metals  are  assumed  to  be  present  together  in  solution 
in  the  form  of  their  nitrates.  The  solution  is  evaporated  in  a 
small  Erlenmeyer  flask,  as  described  under  lithium,  p.  55,  hi  an 
oil-bath,  meanwhile  passing  a  stream  of  dry,  warm  air  through  the 
flask.  When  all  the  water  is  evaporated,  the  temperature  of.  the 
bath  is  raised  to  140°  C.  and  maintained  at  this  temperature  for 
one  to  two  hours,  still  passing  the  current  of  warm  air  through 
the  flask.  After  cooling,  the  dry  residue  is  treated  with  ten  tunes 
its  weight  of  absolute  alcohol,  corked  up,  and  allowed  to  stand 
with  frequent  shaking  for  one  to  two  hours.  An  equal  volume 
of  ether  is  now  added,  the  flask  closed,  shaken,  and  again  allowed 
to  stand  twelve  hours.  It  is  then  filtered  through  a  filter  moist- 
ened with  ether-alcohol  and  washed  with  ether-alcohol  until  a  few 
drops  of  the  nitrate  evaporated  on  platinum-foil  leave  no  residue. 


8o  GRAVIMETRIC  ANALYSIS. 

The  filtrate  is  evaporated  to  dryness  in  a  lukewarm  water-bath, 
the  calcium  nitrate  is  dissolved  in  water,  precipitated  as  the  oxalate, 
and  after  ignition  is  weighed  as  the  oxide. 

Remark. — In  case  only  a  small  amount  of  calcium  is  present 
(not  more  than  about  0.5  gm.)  the  above  separation  is  complete. 
With  large  amounts  of  calcium,  the  residue  of  strontium  and 
barium  nitrates  almost  always  contains  some  calcium.  In  this 
case  the  aqueous  solution  is  again  evaporated  to  dryness  in  the 
same  way  as  before  and  the  treatment  with  alcohol  and  ether  re- 
peated. The  calcium  is  then  determined  in  the  combined  filtrates. 

This  separation  finds  application  in  the  analysis  of  most  min- 
eral waters. 


(b)  Separation  of  Barium  from  Strontium  according  to 
Fresenius.  * 

Requirements. — 1.  A  solution  of  (NH4)2OO4  (1  c.c.  of  the  solu- 
tion should  c(Jntain  0.1  gm.  of  the  salt).  The  solution  is 
prepared  by  adding  ammonia  to  a  solution  of  ammonium 
bichromate  (free  from  sulphate)  until  the  color  of  the  solution  be- 
comes yellow.  The  solution  should  be  left  acid  rather  than  alkaline. 

2.  A  solution  of  ammonium  acetate  (1  c.c.  containing  0.31  gm. 
of  the  salt). 

3.  Acetic  acid  of  sp.  gr.  1.065. 

4.  Nitric  acid  of  sp.  gr.  1.20. 

Procedure. — The  residue,  consisting  of  strontium  and  barium 
nitrates,  is  dissolved  in  a  little  water  and  for  each  gram  of  salt 
mixture  the  solution  is  diluted  until  the  concentration  corre- 
sponds to  300  c.c.,  heated  to  boiling,  treated  with  six  drops  of 
acetic  acid  and  about  10  c.c.  of  ammonium  chromate  solution 
(this  should  be  an  excess  over  the  theoretical  amount  necessary) 
and  allowed  to  stand  one  hour.  The  precipitate  of  barium  chrornate 
is  washed  by  decantation  with  water  containing  ammonium  chro- 
mate until  the  wash  water  no  longer  gives  a  precipitate  with  ammo- 
nia and  ammonium  carbonate;  it  is  then  washed  with  pure  hot 
water  until  the  last  washing  gives  only  a  slight  reddish-brown 

*  Z.  anal.  Chem.,  29,  427  (1890). 


SEPARATION  OF  BARIUM  FROM  STRONTIUM.  8 1 

• 

coloration  with  neutral  silver  nitrate  solution.  The  precipitate 
on  the  filter  still  contains  a  little  strontium.  It  is  carefully  washed 
back  into  the  vessel  in  which  the  precipitation  took  place,  while 
any  precipitate  remaining  on  the  filter  is  dissolved  in  a  little  warm 
dilute  nitric  acid  and  washed  into  the  dish,  finally  adding  enough 
nitric  acid  to  the  precipitate  so  that  it  dissolves  completely  on 
warming  (about  2  c.c.  of  nitric  acid  being  usually  necessary).  The 
solution  is  then  diluted  to  200  c.c.,  heated  to  boiling,  treated  with 
5  c.c.  of  ammonium  acetate  solution,  added  little  by  little,  and 
finally  with  enough  ammonium  chromate  to  cause  the  disappear- 
ance of  the  odor  of  acetic  acid  from  the  solution  (usually  about 
10  c.c.  are  necessary).  After  standing  one  hour  the  liquid  is 
poured  through  a  Gooch  crucible,  the  residue*  is  treated  in  the  dish 
with  hot  water,  allowed  to  cool,  then  filtered  and  washed  with  cold 
water  until  the  filtrate  gives  only  a  slight  opalescence  with  neutral 
silver  nitrate.  The  precipitate  is  dried,  ignited  gently  in  an  air- 
bath  (cf.  p.  75),  and  weighed  as  BaCrO4.  The  filtrate  is  treated 
with  1  c.c.  nitric  acid,  concentrated  somewhat,  and  the  strontium 
precipitated  as  carbonate  by  the  addition  of  ammonia  and  am- 
monium carbonate.  The  precipitate,  which  always  contains  some 
chromate,  is  washed  once  with  hot  water,  dissolved  in  hydrochloric 
acid,  and  the  strontium  determined  as  sulphate,  according  to  p.  72. 

The  results  obtained  according  to  this  method  are  very  satis- 
factory. Experiments  performed  in  this  laboratory*  completely 
confirm  the  results  obtained  by  Fresenius. 

Remark. — In  the  opinion  of  the  author,  all  other  methods  for 
the  separation  of  the  alkaline  earths  give  incorrect  results;  for 
that  reason  they  will  not  be  discussed  in  this  book. 


*  The  results  of  seven  experiments  gave  (a)  for  the  percentage  of  barium 
chromate  obtained:  99.9,  99.9,,  100.3,  100.4,  100.7,  100.6;  mean,  100.3  per 
cent.;  (6)  for  strontium  sulphate,  100.9,  99.73,  99.f6,  99.84,  99.47,  99.77, 
99.6;  mean,  99.75  per  cent.  (H.  Schmidt.) 


82  GRAVIMETRIC  ANALYSIS. 


METALS   OF   GROUP   in. 

ALUMINIUM,  CHROMIUM,  TITANIUM,  IRON,  URANIUM,  NICKEL, 
COBALT,    ZINC,    AND    MANGANESE. 

A.     DIVISION  OF   THE  SESQUIOXIDES. 
ALUMINIUM,  CHROMIUM,  IRON,  TITANIUM,  AND   URANIUM. 

ALUMINIUM,  Al.    At.  Wt.  27.1. 
Form:  A1203. 

In  order  to  determine  aluminium  in  this  form,  the  metal  is  pre- 
cipitated as  its  hydroxide  and  converted  to  its  oxide  by  ignition 
of  the  precipitate.  . 

It  must  not  be  forgotten,  however,  that  aluminium  hydroxide 
exists  in  a  soluble  form  (hydrosol)  and  in  an  insoluble  form  (hydro- 
gel)  ;  and  further  that  the  hydrosol  is  not  completely  changed  by 
boiling  into  insoluble  hydrogel.  To  accomplish  this  the  presence  of 
salts  in  solution  (preferably  ammonium  salts)  is  also  necessary. 
Since,  however,  ammonium  salts  become  acid  on  long  boiling  (due 
to  the  escape  of  ammonia)  there  is  danger  of  the  aluminium  hy- 
droxide being  redissolved .  Furthermore,  it  is  true  that  the  hydrogel 
gradually  changes  over  into  hydrosol  by  standing  in  a  solution 
containing  only  a  small  amount  of  dissolved  salts,  or  by  remaining 
in  a  hot  solution  containing  only  a  small  amount  of  dissolved 
salts. 

From  these  facts  the  following  procedure  is  derived: 

The  solution  containing  the  aluminium  (but  no  phosphoric 
acid,  or  anything  but  aluminium  that  is  precipitated  by  ammonia) 
is  treated  with  considerable  ammonium  chloride,  or  ammonium 
nitrate,  heated  to  boiling  in  a  platinum  or  porcelain  vessel,  and 
a  slight  excess  of  ammonia  *  is  added.  The  precipitate  is  allowed 
to  settle,  after  which  the  clear  solution  is  poured  through  a  filter 
which  rests  on  a  platinum  cone,  but  without  applying  suction. 


*  The  ammonia  should  be  freshly  distilled.  When  it  has  been  kept  for 
any  length  of  time  in  glass  bottles,  the  ammonia  invariably  contains  silica, 
and  this  leads  to  high  results  in  the  case  of  all  preripitatee  formed  by  adding 
ammonia  to  an  acid  solution. 


DETERMINATION  OF  ALUMINIUM.  83 

The  precipitate  is  washed  three  times  by  decantation  with  hot 
water  to  which  a  drop  of  ammonia  and  a  little  ammonium  nitrate 
has  been  added,  and  finally  transferred  to  the  filter.  The  pre- 
cipitate is  now  washed  as  quickly  as  possible  with  the  hot  wash 
liquid  (so  that  the  precipitate  is  thoroughly  churned  up  each 
time)  until  the  filtrate  ceases  to  give  a  test  for  chlorine.  The  pre- 
cipitate is  then  dried  as  completely  as  possible  by  the  applica- 
tion of  suction  and  ignited  wet  in  a  platinum  crucible.  After  the 
precipitate  and  ash  have  become  white,  the  covered  crucible  is 
heated  over  the  blast-lamp  for  about  ten  minutes,  cooled  in  a 
desiccator  and  weighed.  The  process  is  repeated  until  a  constant 
weight  is  obtained. 

Determination  of  Aluminium  by  the  Method  of  Chancel.* 

In  the  determination  of  aluminium  in  a  sample  of  alum  by 
precipitation  with  ammonia,  there  are  a  number  of  difficulties  to 
overcome.  In  the  first  place  the  precipitate  always  contains  more 
or  less  basic  aluminium  sulphate,  and  it  requires  long  ignition  to 
expel  the  last  traces  of  sulphuric  anhydride.  It  is,  to  be  sure, 
possible  to  wash  out  all  the  sulphate  by  means  of  a  solution  of 
ammonia  and  ammonium  nitrate;  the  operation,  however,  is 
very  tedious  and  a  large  quantity  of  the  wash  liquid  is  required. 
Another  disadvantage  arises  from  the  fact  that  the  filtration  takes 
place  very  slowly,  even  when  the  precipitate  contains  basic  sul- 
phate, on  account  of  the  slimy  nature  of  aluminium  hydroxide. 

By  the  method  of  Chancel  and  in  the  two  following  methods 
these  difficulties  are  overcome. 

The  principle  of  this  and  the  following  methods  consists  in 
neutralizing,  by  salts  of  weak  acids,  the  mineral  acid  that  is  set 
free  in  the  hydrolysis  of  an  aluminium  salt;  the  weak  acid  is 
finally  removed  by  boiling  or  neutralization.  In  the  Ghancel 
method  the  mineral  acid  is  neutralized  by  sodium  thiosulphate, 

2A1C13  +  6H2CM2  Al  (OH)  3  +  6HG1, 
6HG1  +  3Xa2S203  =  GNaCl  4-  3H20  +  3S02 + 3S. 
Procedure. — The  dilute,  neutral  solution   (about  0.1  gm.  Al 

*  Compt.  rend.,  46,  987;   Z.  anal.  Chem.,  3,  391. 


84  GRAVIMETRIC  ANALYSIS. 

in  200  c.c.)  is  treated  with  an  excess  of  sodium  thiosulphate  and 
boiled  until  all  traces  of  S02  are  expelled.  Enough  ammonia  * 
is  then  added  so  that  the  odor  is  barely  perceptible  after  blowing 
away  the  vapors,  and  the  boiling  is  continued  a  little  longer. 
The  precipitate  of  A1(OH)3  and  S  is  filtered  off,  washed  with  hot 
water,  and  ignited  in  a  porcelain  crucible.  Such  a  precipitate 
of  A1(OH)3  is  much  denser  than  that  produced  by  direct  pre- 
cipitation with  ammonia  and  is  very  easy  to  filter  and  wash. 

Remark. — This  method  is  often  employed  for  separating  alu- 
minium from  iron.  Ferric  iron  is  reduced  to  ferrous  salt  by  the 
sodium  thiosulphate  and  is  not  precipitated.  In  this  case,  how- 
ever, the  final  neutralization  with  ammonia,  as  prescribed  in  the 
above  directions,  should  not  be  carried  out  or  a  little  iron  will 
come  down. 

Determination  of  Aluminium  by  the  Method  of  Alfred  Stock.f 

The  aqueous  solution  of  the  aluminium  salt,  which  on  account 
of  hydrolysis  always  shows  an  acid  reaction, 

A1C1, +3HOH«=*A1(OH), +3HC1. 

is  treated  in  the  cold  with  a  mixture  of  potassium  iodide  and 
potassium  iodate.  This  mixture  in  the  presence  of  acid  is  decom- 
posed in  accordance  with  the  equation 

KI03  +  SKI  +  6HC1  =  3H20  +  6KC1  +  3I2, 

whereby  the  equilibrium  which  existed  in  the  hydrolysis  of  the 
aluminium  salt  is  disturbed  and  the  first  reaction  continues  to 
take  place  until  all  the  aluminum  is  precipitated.  If  now  the 
iodine  in  the  solution  is  made  to  react  with  sodium  thiosulphate 
and  the  mixture  is  heated  on  the  water  bath  for  half  an  hour,  the 
precipitate  collects  in  such  a  condition  that  it  can  be  quickly 
filtered  and  washed. 

Procedure. — The  solution  in  which  the  aluminum  is  to  be 
determined  should  be  very  slightly  acid;  if  more  acid  is 
present  sodium  hydroxide  is  added  until  a  slight  permanent 
precipitate  is  obtained  which  is  redissolved  by  means  of  a 

*  If  the  addition  of  ammonia  is  omitted,  the  solution  will  retain  traces  of 
aluminium. 

t  Ber.,  1900,  548. 


DETERMINATION  OF  ALUMINIUM.  85 

few  drops  of  hydrochloric  acid.  Thereupon  equal  volumes  of  a 
25  per  cent,  potassium  iodide  solution  and  a  saturated  potassium 
iodate  solution  (about  7  per  cent.  KIOs)  are  added.  After  about 
five  minutes  the  solution  is  decolorized  by  the  addition  of  a  20 
per  cent,  sodium  thiosulphate  solution  and  a  little  of  the  potassium 
iodide  and  iodate  mixture  is  added  in  order  to  make  sure  that 
enough  was  added  in  the  first  place.  One  or  two  c.c.  more  of 
sodium  thiosulphate  are  added  and  the  solution  heated  half  an 
hour  on  the  water  bath.  The  pure  white  precipitate  settles  out 
well,  and  can  be  filtered  through  a  filter  with  relatively  wide  pores, 
washed  with  hot  water,  ignited  and  weighed  as  A12O3. 

Remark. — The  presence  of  calcium,  magnesium  and  boric  acid 
does  not  interfere  with  the  above  determination,  but  if  phosphoric 
acid  is  contained  in  the  solution,  the  phosphate  of  aluminum  is 
precipitated.  It  is  obvious  that  the  method  cannot  be  employed 
in  the  presence  of  organic  substances  such  as  tartaric  acid,  citric 
acid,  sugar,  etc.,  which  prevent  the  precipitation  of  aluminium 
hydroxide. 

Determination  of  Aluminium  by  the  Method  of  G.  Wynkoop  * 
and  of  E.  Schirm.f 

Principle. — If  a  neutral  solution  of  aluminium  (iron,  chro- 
mium, or  titanium)  is  boiled  with  an  excess  of  sodium  or  ammo- 
nium nitrite  until  no  more  fumes  are  evolved,  aluminium 
hydroxide  is  precipitated  in  a  form  that  can  be  as  easily  filtered 
as  that  of  the  last  methods. 

2  A1C1,  +  6HOH<=±2  Al  (OH)  3  +  6HC1 ; 

6HC1  +  6XaX02  -  GXaCl  +  6HXO2 ; 

6HXO2  =  3H2O  +3XO  +3X02. 

Procedure. — If  the  solution  is  acid,  enough  ammonia  is  added 
so  that  the  precipitate  first  formed  dissolves  only  slowly  on 


*  J.  Am.  Chem.  Soc.,  19,  434  (1897). 
f  Chem,  Ztg.,  1909,  877. 


86  GRAVIMETRIC  ANALYSIS. 

stirring.  An  excess  of  a  6  per  cent,  solution  of  pure  ammonium 
nitrite*  is  then  added,  the  solution  diluted  to  250  c.c.  and 
boiled  until  no  more  fumes  of  nitrous  oxides  are  evolved 
(about  20  minutes).  The  precipitate  is  filtered,  washed  with 
hot  water,  ignited  wet  in  a  platinum  crucible,  and  weighed  as 

A1A- 

Remark.  —  In  the  presence  of  more  than  1  per  cent,  of 
ammonium  salts,  these  are  hydrolyzed  enough  so  that  the  solution 
remains  acid  and  the  precipitation  of  the  aluminium  is  incomplete 
even  after  long  boiling.  In  such  a  case,  after  the  vapors  of 
nitrogen  peroxide  are  no  longer  visible,  ammonia  is  added  drop 
by  drop  until  the  odor  is  barely  perceptible.  The  precipitate  is 
allowed  to  settle  while  the  beaker  is  on  the  water-bath,  and  the 
analysis  is  finished  as  above. 

If  the  solution  contains  only  aluminium  in  the  form  of  its  chlo- 
ride, nitrate,  or  sulphate,  it  can  be  determined  by  evaporating  the 
solution  in  a  platinum  crucible  on  the  water-bath  with  the  addi- 
tion of  a  little  sulphuric  acid,  the  excess  of  the  latter  being  finally 
removed  by  cautious  heating  over  the  free  flame  in  an  inclined 
crucible.  The  residue  of  aluminium  sulphate  is  then  changed  to 
the  oxide  by  strong  ignition  over  the  blast -lamp. 

In  the  case  of  organic  salts  of  organic  acids,  the  oxide  is 
readily  obtained  by  careful  ignition  of  the  salt  in  a  platinum 
crucible. 


*  Sometimes  the  reagent  contains  a  little  barium  which  should  be  pre- 
cipitated with  ammonium  sulphate  before  using  it  in  an  analysis. 


IRON. 


IRON,  Fe.     At.  Wt.  55.85. 
Forms:  Ferric  Oxide,  Fe203,  and  Metallic  Iron. 

Determination  as  Fe203. 
(a)  By  Precipitation  with  Ammonia. 

This  is  the  form  chiefly  used  for  the  gravimetric  determination 
of  iron.  The  solution  containing  the  ferric  salt  in  the  presence  of 
ammonium  chloride  is  heated  to  about  70°  C. 
in  a  porcelain  dish  or  Jena  beaker  *  and  pre- 
cipitated by  means  of  a  slight  excess  of  am- 
monia. The  precipitate  is  washed  by  decan- 
tation  with  hot  water  and  finally  with  a 
strong  stream  of  hot  water  from  the  wash- 
bottle,  f  It  is  ignited  gradually  in  a  plati- 
num crucible  and  finally  in  the  half-covered 
crucible  over  the  Bunsen  burner. J  The 
ferric  oxide  obtained  varies  in  its  appear- 
ance according  to  the  temperature  to  which 
it  has  been  heated.  Gently  ignited  ferric 
oxide  is  reddish  brown,  whereas  when 
strongly  ignited  it  has  almost  the  appearance 
of  graphite.  Both  forms  are  difficultly 
soluble  in  dilute  hydrochloric  acid,  but  can 
be  readily  dissolved  by  digesting  with  con- 
centrated hydrochloric  acid  on  the  water-bath. 

(6)  By  Precipitation  with  Ammonium  Nitrite. 

The  precipitation  of  Fe(OH)3  from  neutral  solutions  of  ferric 
salts  takes  place  as  described  for  aluminium,  p.  85. 

*  Too  high  results  are  invariably  obtained  in  ordinary  glass  vessels.  The 
ammonia  should  be  freshly  distilled  or  the  results  will  be  high. 

f  For  this  purpose  a  wash-bottle  such  as  is  shown  in  Fig.  26  is  useful. 
By  blowing  through  the  long  arm  of  the  U-tube  (which  is  provided  with  a 
Bunsen  valve)  and  placing  the  thumb  over  the  short  arm  a  continuous 
stream  of  water  is  maintained  which  can  be  stopped  at  any  time  by  removing 
the  thumb. 

J  it  is  not  advisable  to  heat  the  covered  crucible  over  the  blast  on  account 
of  the  danger  of  forming  some  Fe3O4.  The  magnetism  of  such  a  precipitate 
can  be  shown  by  placing  a  magnet  outside  the  platinum  crucible  and  moving 
it  slowly  up  and  down. 


FIG.  26. 


88  GRAVIMETRIC  ANALYSIS. 

If  the  iron  is  in  solution  either  as  the  ferrous  or  ferric  salt  of  a 
volatile  acid,  it  can  be  readily  converted  into  ferric  oxide  by  evapo- 
ration with  sulphuric  acid  and  ignition  of  the  residue. 

2.  Determination  as  Metallic  Iron. 

Iron  may  be  determined  by  electrolysis,  but  this  method  offers 
no  advantages  over  the  gravimetric  method  just  described  or  the 
following  volumetric  process,  so  that  it  will  not  be  discussed  in  this 
book. 

In  the  case  of  the  analysis  of  oxide  iron  ores  or  of  mixtures  of 
considerable  iron  oxide  with  comparatively  little  alumina,  titanium 
dioxide,  or  silica,  the  following  method  is  accurate  and  rapid.  * 

The  finely  powdered  and  weighed  f  substance  contained  in  a 
porcelain  boat  is  introduced  into  a  tube  of  difficultly  fusible  glass 
and  heated  to  redness  in  a  stream  of  dry  hydrogen  until  no  more 
drops  of  water  condense  on  the  cool  front  end  of  the  tube  and  the 
contents  of  the  boat  appear  gray  and  not  black.  By  this  means 
the  ferric  oxide  is  reduced  to  metallic  iron: 

Fe2O3  +  3H2  =  3H20  +  2Fe. 

After  cooling  in  the  stream  of  hydrogen,  the  boat  and  its  contents 
are  again  weighed  after  remaining  some  time  in  a  desiccator.  The 
loss  in  weight  p  represents  the  amount  of  oxygen  originally  com- 
bined with  the  iron,  from  which  the  amount  of  iron  can  be  calcu- 
lated: 


2Fe 
'-3CTP- 

Remark.  —  In  attempting  to  reduce  ferric  oxide  to  iron  by 
means  of  hydrogen,  it  is  very  important  to  heat  the  oxide  to  bright 

*Rivot,  Ann.  Chem.  Phys.,  3.  Serie,  30,  188  (1850);  Liebig's  Ann.,  78, 
211  (185). 

t  Ferric  oxide  after  having  been  powdered  and  ignited  is  so  hygroscopic 
that  the  porcelain  boat  should  be  placed  within  a  weighing-beaker  with 
ground-glass  top  immediately  after  removing  it  from  the  desiccator,  and 
then  weighed. 


VOLUMETRIC  DETERMINATION  OF  IRON.  89 

redness.  At  a  dull  red  heat,  the  oxide  is  to  be  sure  reduced  to 
metal,  but  in  such  cases  black,  pyrophoric  iron  is  formed  and  the 
latter  cannot  be  exposed  to  the  air  and  weighed  without  becoming 
oxidized.  By  heating  to  a  bright  red  heat,  however,  the  iron 
becomes  gray,  is  no  longer  pyrophoric,  and  can,  if  allowed  to  cool 
in  the  stream  of  hydrogen,  which  is  subsequently  replaced  by 
carbon  dioxide,  be  safely  weighed  in  the  air  without  fear  of 
oxidation. 

Although  this  method  is  extremely  simple,  and  the  correspond- 
ing oxides  of  aluminium,  chromium,  titanium  and  zircon,  etc., 
are  not  reduced  under  the  same  conditions,  it  should  be  used  with 
caution  and  only  when  the  ferric  oxide  greatly  predominates  in  a 
mixture  of  oxides.  Otherwise  the  reduction  of  the  iron  is  incom- 
plete on  account  of  some  of  the  ferric  oxide  being  enclosed  within 
the  particles  of  foreign  oxide.  This  has  been  proved  by  the  work 
of  Daniel  and  Leberle  *  and  by  Treadwell  and  Wegelin.f 

It  is  still  more  accurate  to  dissolve  the  metallic  iron  produced 
in  dilute  sulphuric  acid  out  of  contact  with  the  air  and  determining 
the  amount  present  volumetrically  by  titrating  with  potassium 
permanganate  solution. 

3.  Volumetric  Determination  of  Iron,  according  to  Margueritte.J 

Although  the  volumetric  methods  are  discussed  in  the  second 
part  of  this  book,  this  determination  is  so  important  and  is  so 
often  used  to  test  the  purity  of  the  iron  oxide  produced  by  a  gravi- 
metric analysis  that  it  seems  proper  to  discuss  it  at  this  place. 

Principle  of  the  Method. 

Ferrous  salts  are  oxidized  by  potassium  permanganate  hi  acid 
solution  to  ferric  salts: 


If,   therefore,   a  potassium  permanganate  solution  of  known 
strength  is  slowly  added  to  the  solution  of  a  ferrous  salt,  it  will 

*  Z.  anorg.  Chem.,  34,  393  (1903). 

f  A  table  is  given,  showing  the  results  of  twelve  experiments,   in  the 
German  edition  of  this  book. 

J  Ann.  de  chim.  et  de  phys.  [3],  18  (1846),  p.  244. 


po  GRAVIMETRIC  ANALYSIS. 

be  decolorized  as  long  as  there  remains  ferrous  salt  to  react  with 
it.  As  soon  as  all  of  the  ferrous  salt  has  been  oxidized,  the  next 
drop  of  the  permanganate  will  impart  a  permanent  pink  color  to 
the  solution,  whereby  the  end-point  of  the  reaction  is  determined. 

Preparation  and  Standardization  of  the  Permanganate  Solution. 

In  most  cases  a  TV  normal  solution  of  potassium  permanganate 
is  suitable,  i.e.  one  which  contains  in  one  liter  enough  oxygen  to 
oxidize  TV  of  a  gram-atom  of  hydrogen  (1.008  gm.  of  hydrogen). 

Potassium  permanganate  in  acid  solution  reacts  according 
to  the  equation 

K2O,  Mn2O7=K2O-f  2MnO+5O, 
2KMnO4 

so  that  from  two  molecules  of  permanganate  five  atoms  of  oxygen 
(  =  10  atoms  of  hydrogen)  are  available,  thus: 

2KMnO4      KMn04      158.03 

-^  = -—  —  = — =31.61  gm.  KMnO4  =  i  gm.-atom  of 

10  5  5 

oxygen  =  1  gm.-atom  of  hydrogen. 

Consequently  it  is  necessary  to  take  -V  of  a  gram-molecule  of 
potassium  permanganate  (3.161  gms.)  for  a  liter  of  TV  normal  solu- 
tion. 

Although  it  is  possible  to  purchase  very  pure  potassium  per- 
manganate, it  is  not  advisable  to  take  the  trouble  of  weighing 
out  just  this  amount  of  the  substance  and  dissolving  it  in  exactly 
the  right  amount  of  water,  for  although  we  might  in  this  way  obtain 
the  correct  strength  of  solution,  yet  on  the  following  day  its  value 
would  be  different,  for  the  distilled  water  in  which  the  perman- 
ganate is  dissolved  almost  always  contains  traces  of  organic  matter 
oxidizable  by  the  permanganate.  Consequently  we  weigh  out 
on  a  watch-glass  approximately  the  right  amount  of  permanganate 
3.1-3.2  gms.),  dissolve  it  in  a  liter  of  water,  and  allow  it  to  stand 
eight  to  fourteen  days*  before  using  it.  After  this  time  all  of  the 
oxidizable  matter  in  the  water  will  have  been  completely  destroyed. 
The  solution  is  filtered  through  an  asbestos  filter  and  then  stand- 
ardized. 

*  Cf.  Morse,  Hopkins,  and  Walker,  Am.  Chem.  Jour.,  18  (1896),  p.  401. 


VOLUMETRIC  DETERMINATION  OF  IRON.  91 

Standardization  of  the  Potassium  Permanganate  Solution. 

It  is  possible  to  standardize  the  solution  by  a  number  of  differ- 
ent methods,  as  will  be  discussed  in  detail  under  volumetric  analysis. 
In  this  case  we  are  concerned  with  the  determination  of  iron  only, 
so  that  the  most  natural  way  for  us  to  standardize  the  solution  will 
be  by  means  of  chemically  pure  iron.  An  accurately  weighed  por- 
tion of  iron  is  dissolved  in  dilute  sulphuric  acid  out  of  contact  with 
the  air  and  permanganate  solution  is  added  from  a  glass-stoppered 
burette  until  the  solution  remains  pink  for  one-half  minute  after 
thoroughly  stirring  or  shaking. 

If  for  the  oxidation  of  a  grams  of  iron  t  cubic  centimeters  of  the 
permanganate  solution  were  necessary,  then 

a 
1  c.c.  =  —  gm.  iron. 

The  value  —  represents  the  titration  value  of  the  solution. 

Remarks. —  The  chief  difficulty  lies  in  the  procuring  of  a 
suitable  standard.  It  is  difficult  to  prepare  iron  which  is  exactly 
100  per  cent.  pure.  Moreover,  the  purity  of  a  sample  of  iron  wire, 
such  as  is  ordinarily  used  for  standardization,  cannot  be  deter- 
mined satisfactorily  by  means  of  a  gravimetric  analysis  because 
the  analytical  errors  are  greater  than  is  usually  supposed.  The 
ferric  hydroxide  precipitate  is  bulky  so  that  it  is  customary  to  take 
only  about  0.2  gm.  of  wire  for  analysis  which  yields  approximately. 
0.283  gm.  of  Fe203  so  that  an  error  of  0.0003  gm.  in  the  final 
weight  corresponds  to  0.1  per  cent,  of  iron  in  the  sample.  The 
precipitate  of  ferric  hydroxide,  moreover,  often  contains  silica 
and  alumina  when  the  analysis  is  carried  out  in  glass  beakers,  or 
when  the  reagents  used  have  been  in  contact  with  glass  for  some 
time.  This  leads  to  high  results  and  the  error  may  amount  to 
0.003  gm.  Again,  in  igniting  a  precipitate  of  hydrated  ferric 
oxide  great  care  must  be  taken  or  some  of  it  will  be  reduced  to 
magnetite  as  can  be  shown  by  a  magnet  held  outside  the  crucible.* 

*The  carbon  from  the  filter-paper  causes  the  reduction  when  the  pre- 
cipitate and  filter  are  heated  together  and  too  much  heat  is  used  at  the  start. 
Heatingthe  covered  crucible  over  the  blast  lamp  also  converts  Fe2O3  into  Fe3O4. 


92  GRAVIMETRIC  ANALYSIS. 

When  magnetite  is  thus  once  formed  it  is  practically  impossible 
to  change  it  back  to  ferric  oxide  by  further  heating  or  by  treating 
the  precipitate  with  concentrated  nitric  acid.  This  leads  to  low 
results.  When  thrown  down  from  hydrochloric  acid  solutions 
by  the  addition  of  ammonia,  the  precipitate  often  contains  some 
chloride  which  it  is  hard  to  remove  by  washing,  and  on  igniting 
such  a  precipitate  there  is  danger  of  losing  a  little  ferric  chloride 
by  volatilization.  On  account  of  these  sources  of  error  it  is 
difficult  to  carry  out  a  perfect  gravimetric  estimation  of  iron 
with  an  accuracy  sufficient  for  standardization  purposes.  The 
most  satisfactory  method  is  to  determine  the  impurities  present, 
but  even  although  this  may  give  the  correct  percentage  of  iron, 
if  there  is  any  carbon,  phosphorus,  silicon  or  sulphur  present,  as 
is  usually  the  case,  compounds  may  be  left  in  solution  which  are 
oxidizable  by  potassium  permanganate,  so  that  a  sample  of  iron 
wire  may  show  against  permanganate  an  apparent  iron  value 
of  from  100  to  101  per  cent.,  in  spite  of  the  fact  that  it  contains 
only  99.7  per  cent,  of  pure  iron. 

A.  Classen  *  has  proposed,  therefore,  to  standardize  potassium 
permanganate  against  pure  electrolytic  iron  and  in  the  author's 
laboratory  this  has  been  found  to  be  very  satisfactory.  There  is 
a  possibility,  however,  of  such  electrolytic  iron  containing  occluded 
hydrogen,  carbon,  etc.,  which  may  exert  some  effect  upon  the 
titration,  although  when  the  electrolysis  is  properly  carried  out, 
these  errors  cannot  be  large.  G.  Lunge,  in  his  report  to  the  Sixth 
International  Congress  of  Applied  Chemists  f  (Rome,  1907), 
recommended  that  permanganate  should  be  standardized  against 
one  of  four  substances : 

(1)  Pure  oxalic  acid,  the  exact  value  of  which  has  been  deter- 
mined by  titration  against  standard  barium  hydroxide  solution. 
The  barium  hydroxide  is  standardized  against  hydrochloric  acid, 
which  is  in  turn  titrated  against  sodium  carbonate.  (See 
Acidimetry.) 


*  Mohr-Classen,  Lehrbuch  der  chem.-analyt.  Titriermethode  (1896). 
f  See  also  Z.  Angew.  Chem.  17,  195  (1904). 


PREPARATION  OF  ELECTROLYTIC  IRON. 


93 


(2)  Pure  iron  wire,  the  exact  value  of  which  is  known  by  a 
comparison  with  oxalic  acid  (standardized  as  above). 

(3)  Sodium  oxalate.    (See  Acidimetry.) 

(4)  Hydrogen  peroxide  by  the  Nitrometer  Method. 

Preparation  of  Electrolytic  Iron. 

Some  commercial  ferrous  chloride  is  dissolved  in  water  and  a 
little  hydrochloric  acid,  the  solution  is  saturated  with  hydrogen 
sulphide,  and  filtered.  After  boiling  off  the  excess  of  hydrogen 
sulphide,  the  ferrous  chloride  is  oxidized  by  means  of  potassium 


FIG.  27. 

chlorate  and  hydrochloric  acid,  the  excess  of  chlorine  boiled  off, 
and  the  solution  neutralized  by  the  careful  addition  of  sodium 
hydroxide.  The  iron  is  then  precipitated  by  the  barium  carbonate 
method  (see  p.  149).  The  well-washed  precipitate  is  dissolved  in 
hot,  dilute  hydrochloric  acid  and  freed  from  barium  by  a  double 
precipitation  of  the  iron  with  ammonia.  The  hydrated  ferric 
oxide  thus  obtained  is  dried,  ignited,  and  reduced  to  metal  in  a 
stream  of  hydrogen  as  described  on  p.  88.  The  metallic  iron  is 
dissolved  in  the  calculated  amount  of  dilute  sulphuric  acid  out  of 
contact  with  the  air  (passing  C02  through  the  solution*).  The 

*  For  the  generation  of  carbon  dioxide,  an  apparatus  similar  to  that  shown 
in  Fig.  30  is  used,  only  the  wash- bottle  A  is  filled  with  permanganate  solu- 
tion, and  the  tower  C  contains  pumice  soaked  with  copper  sulphate  solution, 
above  which  is  a  plug  of  cotton. 

The  potassium  permanganate  and  copper  sulphate  both  serve  to  remove 
H2S  from  the  CO2. 


94 


GRAVIMETRIC  ANALYSIS. 


solution  is  then  diluted  with  water  until  20  c.c.  contain  about 
0.35  gm.  iron. 

In  addition  it  is  also  necessary  to  provide  a  solution  of  am- 
monium oxalate,  saturated  at  the  room  temperature. 

For  the  electrolysis,  two  electrodes  K  (Fig.  27)  are  prepared 
by  taking  two  pieces  of  platinum-foil  about  25  sq.  cm.  surface  and 


FIG.  28. 


fastening  a  piece  of  fairly  heavy  platinum  wire  to  each;  they 
are  bent  so  that  they  will  conveniently  pass  through  the  neck 
of  a  liter-flask.  The  electrodes  are  cleaned  by  boiling  in  con- 
centrated hydrochloric  acid  and  finally  igniting  them  over  the 
free  flame.  To  accomplish  the  latter  purpose,  it  is  convenient 
to  hang  them  upon  a  heavy  platinum  wire  which  is  itself  placed 


PREPARATION  OF  ELECTROLYTIC  IRON.  95 

on  an  iron  ring;  they  are  then  heated  over  the  non-luminous 
flame  of  the  Teclu  burner  (Fig.  28).* 

After  the  ignition  the  electrodes  are  allowed  to  cool  in  a  desic- 
cator and  weighed  accurately  by  the  method  of  swings  (cf.  p. 
10).  About  350  c.c.  of  the  ammonium  oxalate  solution  are  now 
placed  in  a  400-c.c.  beaker  and  20  c.c.  of  the  iron  solution 
(about  0.35  gm.  Fe)  are  added.  The  beaker  is  covered  with  a 
glass  plate  containing  three  holes  (Fig.  29).  At  the  ends  of 
the  plate  are  fastened  two  corks  which  serve  to  support  the 
two  heavy  platinum  wires  a  and  b.  Through  the  two  side 
holes  are  passed  from  below  the  bent  platinum  wires  of  the 


(L 


FIG.  29. 

cathodes  K,  leaving  them  suspended  from  a;  while  through 
the  middle  hole  the  end  of  the  spiral  anode  passes  and  is 
suspended  from  the  cross-wire  b.  The  wire  a  is  now  con- 
nected with  the  negative,  and  wire  b  with  the  positive  pole  of  a 
battery,  and  the  solution  is  electrolyzed  for  from  one  and  one-half 
to  two  hours  at  about  60°  with  a  current  of  0.5-0.7  ampere.  At 
the  end  of  this  time  there  will  be  firmly  attached  to  each  of  the 
cathodes  about  0.15-0.17  gm.  of  a  bright,  steel-gray  deposit. 
The  circuit  is  broken,  one  of  the  electrodes  is  removed,  and 
the  circuit  again  closed.  The  electrode  which  has  been  removed  is 

*  The  electrodes  must  be  above  the  inner  flame  mantle. 


96  GRAVIMETRIC  ANALYSIS. 

at  once  plunged  into  a  beaker  of  distilled  water,  taken  out,  the 
bottom  edge  touched  with  a  piece  of  filter-paper  to  remove 
the  greater  part  of  the  adhering  water,  and  then  washed  with  a 
liberal  quantity  of  absolute  alcohol  that  has  been  distilled  over  lime. 
The  lower  edge  is  again  touched  with  filter-paper,  then  washed 
with  ether  which  has  been  distilled  over  potash,  after  which  it  is 
dried  in  the  hot  closet  until  the  ether  has  evaporated  (this  takes 
about  half  a  minute).  It  is  then  placed  in  a  desiccator.  The  sec- 
ond electrode  is  now  removed  from  the  circuit  and  subjected  to 
precisely  the  same  treatment.  After  the  electrodes  have  been  in 
the  desiccator  for  fifteen  minutes  they  are  weighed. 

While  the  solution  is  being  electrolyzed  the  solvent  for  the  iron 
should  be  prepared.  In  the  liter-flask  K  (Fig.  30)  are  placed 
500  c.c.  of  water  and  50  c.c.  of  chemically  pure  concentrated  sul- 
phuric acid.  The  contents  of  the  flask  are  heated  to  boiling, 
meanwhile  passing  a  stream  of  carbon  dioxide  through  the  liquid. 
After  the  latter  has  boiled  vigorously  for  ten  minutes  the  flask 
is  closed  at  b,  removed  from  the  flame,  placed  in  cold  water,  and 
allowed  to  cool  in  an  atmosphere  of  carbon  dioxide. 

In  this  manner  a  solution  of  sulphuric  acid  is  obtained  com- 
pletely free  from  air,  so  that  there  is  no  danger  of  its  oxidizing  any 
of  the  ferrous  salt. 

One  of  the  weighed  electrodes,  on  which  the  iron  has  been  de- 
posited, is  thrown  into  the  flask  containing  the  sulphuric  acid ;  the 
flask  is  immediately  closed  and  gently  heated  on  the  water-bath, 
or  better  to  boiling,  still  passing  carbon  dioxide  through  the 
apparatus.  The  iron  dissolves  very  quickly,  leaving  no  residue.* 
The  flask  is  then  closed  at  b,  placed  in  cold  water,  and  titrated 
with  permanganate  solution  added  from  a  glass-stoppered  burette. 
After  noting  the  burette  reading,  the  permanganate  is  added  drop 
by  drop  and  the  flask  is  constantly  rotated  to  insure  thorough  mix- 
ing of  the  permanganate  with  the  ferrous  solution.  When  the  solu- 
tion possesses  a  slight  pink  color,  permanent  for  half  a  minute,  the 


*  Sometimes  a  minute  trace  of  carbon  remains  undissolved,  but  it  is  so 
small  in  amount  that  it  can  safely  be  disregarded, 
t  See  under  Volumetric  Analysis. 


PREPARATION  OF  ELECTROLYTIC  IRON. 


97 


end-point  is  reached,  and  after  the  burette  has  drained  a  second 
reading  is  taken.     A  blank  test  is  made  with  another  portion  of 


FIG.  30. 

500  c.c,  of  water  and  50  c.c.  sulphuric  acid  solution  (boiled  free  from 
air  in  the  same  way  and  allowed  to  cool  in  a  stream  of  carbon 
dioxide),  to  see  how  much  permanganate  is  necessary  to  impart 
this  pink  color  in  the  absence  of  iron.  This  amount  should  be 
subtracted  from  the  total  number  of  cubic  centimeters  of  the  per- 
manganate solution  used  in  titrating  the  iron. 

The  results  obtained  by  this  procedure  are  excellent. 

After  the  strength  of  the  permanganate  solution  has  been 
accurately  determined  by  the  above  method  the  apparent  iron 
value  of  a  sample  of  iron  wire  may  be  determined.  When  this  is 
known  it  is  possible  to  determine  accurately  the  strength  of  a  new 
permanganate  solution,  or  of  the  same  solution  at  a  future  date, 
by  titrating  against  a  solution  of  the  same  wire. 


Thus  Dr.  Schudl  obtained  the  following  values  by  using  three  methods: 


1  c.c.  KMnO4  =  0.005485  g.  Fe  with  electrolytic  iron; 
1  c.c.       "      =  0.005470  g.  "      "     iodine; 
Ic.c.      "      =0.005468  g."      "    oxalic  acid. 


98  GRAVIMETRIC  ANALYSIS. 


Determination  of  the  Apparent  Iron  Value  of  Iron  Wire. 

The  wire  is  cleaned  by  rubbing  with  a  piece  of  emery  paper  until 
it  is  perfectly  bright.  It  is  then  passed  through  filter-paper  until 
it  no  longer  leaves  a  gray  mark  on  the  paper.  The  wire  is  wound 
round  a  dry  glass  rod,  making  a  spiral,  and  a  portion  of  0.15-0.2  gm. 
is  weighed  out.  This  is  dissolved,  as  described  on  p.  601,  in  a 
flask  which  is  fitted  with  a  Bunsen  valve  *  and  contains  55  c.c. 
of  dilute  sulphuric  acid  (50  c.c.  water +  5  c.c.  cone.  H2SO4) ;  the 
solution  is  boiled  f  a  few  minutes  after  the  iron  has  all  dissolved. 
It  is  allowed  to  cool,  and  is  then  titrated  with  permanganate 
which  has  been  standardized  against  electrolytic  iron  or  sodium 
oxalate.  The  apparent  iron  value  of  the  wire  is  then  calculated. 

Every  time  a  new  supply  of  iron  wire  is  obtained  its  apparent 
iron  value  should  be  determined. 

The  following  results  of  determinations  made  with  great  care 
by  W.  A.  K.  Christie  in  the  author's  laboratory  illustrate  the 
process : 

The  permanganate  solution  was  standardized  against  iron 
wire  and  it  was  found  that  1  c.c.  =  0.005600  gm.  iron.  The  purity 
of  the  sample  of  commercial  iron  wire  was  found  in  three  titra- 
tions  to  be  99.93,  100.0  and  99.92  per  cent.,  the  average  of 
which  is  99.94  per  cent.  The  actual  purity  of  the  wire  was  deter- 
mined to  be  99.7  per  cent.  The  apparent  purity,  therefore,  is 
greater  than  the  actual  purity,  and  if  the  latter  were  to  be  used 
in  the  computation  the  permanganate  solution  would  be  found  a 
little  too  strong. 

The  author  recommends  the  standardization  against  elec- 
trolytic iron  and  compares  the  value  obtained  with  that  found 
with  iron  wire,  the  work  all  being  done  on  the  same  day.  Then 
the  apparent  iron  value  of  the  wire  will  be  known  for  future 

*  Still  better  is  the  use  of  the  glass  valve  as  recommended  by  Contat 
(Chem.  Ztg.,  1898,  298)  and  improved  by  Gockel  (Z.  Angew.  Chem.,  1899, 
620).  When  the  boiling  is  stopped,  sodium  bicarbonate  is  sucked  back 
into  the  solution,  and  there  is  no  overpressure  on  the  outside.  Cf.  p.  601. 

f  The  boiling  of  the  solution  is  necessary,  as  otherwise  hydrocarbons  or 
other  reducing  substances  remain  in  solution  so  that  too  much  perman- 
ganate is  used 


ANALYSIS  OF  FERRIC  COMPOUNDS.  99 

standardizations  and  it  will  be  necessary  to  standardize  against 
electrolytic  iron  only  when  a  new  supply  of  iron  is  purchased. 
On  the  other  hand,  it  must  be  admitted  that  the  standardization 
against  sodium  oxalate  is  full  as  accurate  and  is  easier  to  carry 
out.  See  Volumetric  Analysis,  p.  597. 


Analysis  of  Ferric  Compounds  according  to  the  Method  of 
Margueritte. 

From  what  has  already  been  said,  it  is  evident  that  in  order  to 
determine  the  amount  of  iron  present  in  a  solution  by  titration 
with  potassium  permanganate,  it  is  necessary  for  the  iron  to  be 
present  entirely  in  the  ferrous  condition.  In  order,  therefore, 
to  apply  this  method  to  the  analysis  of  ferric  compounds,  it  is 
first  necessary  to  reduce  them  completely. 

To  effect  the  reduction  of  a  ferric  sulphate  solution  we  can 
proceed  as  follows :  The  solution  is  placed  in  a  200-c.c.  flask,  acidified 
with  one-tenth  its  volume  of  pure,  concentrated  sulphuric  acid, 
the  flask  is  closed  with  a  stopper  provided  with  two  tubes  through 
which  gas  can  enter  and  leave  the  flask,  the  contents  of  the  flask 
are  heated  to  boiling  and  hydrogen  sulphide  is  passed  through  the 
solution  until  it  is  perfectly  colorless.  The  boiling  is  continued 
and  carbon  dioxide  is  now  passed  through  the  solution  until  the 
excess  of  hydrogen  sulphide  is  completely  removed.  The  solu- 
tion is  then  allowed  to  cool  in  an  atmosphere  of  carbon  dioxide 
and  titrated  exactly  as  in  the  standardization  of  the  solution  of 
permanganate. 

If  t  c.c.  of  permanganate  were  necessary  to  completely  oxidize 
the  solution  and  1  c.c.  of  the  permanganate  corresponds  to  a  gm. 
of  iron,  then  the  titrated  solution  evidently  contains  a  .  t  gm.  of 
iron. 

Besides  hydrogen  sulphide,  a  great  many  other  substances 
can  be  used  to  reduce  the  ferric  salt,  e.g.,  zinc,  sulphurous  acid, 
stannous  chloride.  The  use  of  these  substances  will  be  discussed 
in  the  portion  of  this  book  devoted  to  Volumetric  Analysis. 

Remark. — The  titration  of  a  solution  by  means  of  potassium 
permanganate  takes  place  preferably  in  a  sulphuric  acid  solution; 
in  the  case  of  hydrochloric  acid  too  high  results  will  be  obtained 


100  GRAVIMETRIC  ANALYSIS. 

(due  to  the  fact  that  the  permanganate  oxidizes  some  of  the  acid), 
unless  the  oxidation  takes  place  in  a  dilute  solution  in  the  presence 
of  a  large  excess  of  manganous  sulphate.  See  Volumetric  Analysis. 

TITANIUM,  Ti.    At.  Wt,  48.1. 

Titanium,  when  present  in  large  amounts,  is  determined  as  its 
dioxide,  TiO2;  but  if  only  small  amounts  are  to  be  determined, 
as  in  the  case  of  many  rocks  and  iron  ores,  the  colorimetric 
method  is  preferable. 

(a)  Determination  as  Titanium  Dioxide. 

The  titanium  is  precipitated  from  solution  either  by  means  of 
ammonia,  or  by  boiling  a  solution  strongly  acid  with  acetic  acid 
and  containing  considerable  ammonium  acetate;  or,  finally,  by 
boiling  the  slightly  acid  solution  of  the  sulphate.  In  all  these 
cases  it  is  precipitated  as  titanic  acid,  from  which  it  is  changed  by 
ignition  into  Ti02. 

The  two  former  methods  are  preferable  to  the  third.  See 
separation  of  titanium  from  aluminium. 

(6)  Determination  of  Titanium  Colorimetrically ;  Method  of 
A.  Weller.* 

(Suitable  for  small  amounts  of  titanium.) 

This  determination  depends  upon  the  fact  that  acid  solutions 
of  titanium  sulphate  are  colored  intensely  yellow  when  treated 
with  hydrogen  peroxide;  the  yellow  color  increases  with  the 
amount  of  titanium  present  and  is  not  altered  by  an  excess  of 
hydrogen  peroxide.  On  the  other  hand,  inaccurate  results  are 
obtained  in  the  presence  of  hydrofluoric  acid  (Hillebrand) ;  con- 
sequently it  is  not  permissible  to  use  hydrogen  peroxide  for  this 
determination  which  has  been  prepared  from  barium  peroxide  by 
means  of  hydrofluosilicic  acid.  Furthermore,  chromic,  vanadic, 
and  molybdic  acids  must  not  be  present,  since  they  also  give 
colorations  with  hydrogen  peroxide.  The  presence  of  small  amounts 
of  iron  does  not  affect  the  reaction,  but  large  amounts  of  iron  cause 
trouble  on  account  of  the  color  of  the  iron  solution.  If,  however, 
phosphoric  acid  is  added  to  the  colored  ferric  solution  it  becomes 

*  Berichte,  15,  p.  2593. 


DETERMINATION  OF   TITANIUM.  IOI 

decolorized,  and  from  such  a  solution  the  determination  of  titanium 
offers  no  difficulty.  The  solution  in  which  the  titanium  is  to  be 
determined  must  contain  at  least  5  per  cent,  of  sulphuric  acid; 
an  excess  does  not  influence  the  reaction.  The  reaction  is  so 
delicate  that  0.00005  gm.  of  TiO2  present  as  sulphate  in  50  c.c. 
of  solution  give  a  distinctly  visible  yellow  coloration. 

For  this  determination  a  standard  solution  of  titanium  sulphate 
is  required.  This  can  be  prepared  by  taking  0.6000  gm.  of  po- 
tassium titanic  fluoride  which  has  been  several  times  recrystal- 
lized  and  gently  ignited  (corresponding  to  0.2  gm.  of  TiO2).  This  is 
treated  in  a  platinum  crucible  several  times  with  a  little  water 
and  concentrated  sulphuric  acid,  expelling  the  excess  of  acid  by 
gentle  ignition,  finally  dissolving  in  a  little  concentrated  sulphuric 
acid  and  diluting  with  5  per  cent,  sulphuric  acid  to  100  c.c.  One 
cubic  centimeter  of  this  solution  corresponds  to  0.002  gm.  TiO2. 

The  determination  proper  is  carried  out  in  the  same  way  as 
described  on  p.  60,  under  the  colorimetric  determination  of  am- 
monium. 

50  c.c.  of  the  solution  which  has  been  brought  to  a  definite 
and  accurately  measured  volume  is  placed  in  a  Xessler  tube  be- 
side a  series  of  other  tubes,  each  containing  a  known  amount 
of  the  standard  titantium  solution,  filled  up  to  the  mark  with 
water  and  each  treated  with  2  c.c.  of  3  per  cent,  hydrogen  per- 
oxide *  (free  from  hydrofluoric  acid) .  The  color  of  the  solution 
in  question  is  compared  with  the  standards.  This  method  is 
only  suitable  for  the  estimation  of  small  amounts  of  titanium,  as 
the  shades  of  strongly  colored  solutions  cannot  be  compared 
accurately. 

According  to  J.  H.  Walton,  Jr.,f  titanium  in  the  pr3sence  of 
iron  may  be  determined  after  fusing  the  finely  powdered  sub- 
stance with  two  or  three  times  as  much  sodium  peroxide.  On 
extracting  with  water,  sodium  pertitanate  goes  into  solution  and 
ferric  oxide  is  left  behind.  The  filtered  solution  is  acidified  with 
sulphuric  acid,  which  is  added  until  5  per  cent,  of  free  acid  is 

*  The  hydrogen  peroxide  solution  is  prepared  shortly  before  using  by 
dissolving  commercial  potassium  percarbonate  in  dilute  sulphuric  acid. 
f  J.  Am.  Chem.  Soc.,  29,  481  (1907). 


102  GRAVIMETRIC  ANALYSIS. 

present.     The  color  of  the  solution  is  then  compared  with  that 
obtained  by  fusing  a  known  weight  of  Ti02  with  Na2O2,  etc. 

CHROMIUM,  Cr.    At.  Wt.  52.1. 

Forms:  Chromic  Oxide,  Cr203;  Barium  Chromate,  BaCr04. 
(a)  Chromic  Compounds. 

Determination  as  Chromic  Oxide. 
1.  By  Precipitation  with  Ammonia  or  Ammonium  Sulphide. 

If  the  chromium  is  present  in  solution  as  chromic  compound 
it  can  be  precipitated  exactly  as  described  under  aluminium,  by 
means  of  a  slightest  possible  excess  of  ammonia  *  in  the  presence 
of  considerable  ammonium  salts  (or  better  still,  by  the  addition  of 
freshly  prepared  ammonium  sulphide  solution  to  the  boiling  solu- 
tion) .  The  precipitated  Cr(OH)  3  is  washed  with  dilute  ammonium 
nitrate  solution  and  ignited  wet  in  a  platinum  crucible,  being  there- 
by changed  to  the  oxide,  Cr2O3.  The  results  obtained  are  always 
a  few  tenths  of  a  per  cent,  too  high  on  account  of  the  formation 
of  small  amounts  of  alkali  chromate  even  though  the  entire  opera- 
tion takes  place  in  platinum  vessels.  The  alkali  comes  from  the 
reagents.  It  can  be  shown  that  the  ignited  product  contains 
a  little  chromate,  as  the  aqueous  extraction  always  possesses  a 
slight  yellow  color  and  gives  with  silver  nitrate  a  red  precipitate 
of  silver  chromate. 

If  phosphoric  acid  is  present,  it  will  be  found  in  the  precipitate. 
In  this  case  the  dried  precipitate  is  fused  in  a  platinum  crucible 
with  sodium  carbonate  and  potassium  nitrate,  whereby  sodium 
chromate  and  sodium  phosphate  are  obtained.  The  melt  is  dis- 
solved in  water,  acidified  with  nitric  acid,  and  the  phosphoric 
acid  precipitated  by  means  of  ammonia  and  magnesia  mixture, 
as  described  under  Phosphoric  Acid.  From  the  filtrate  the 
chromium  is  determined  as  barium  chromate  in  acetic  acid  solu- 
tion as  described  below. 

*  An  excess  of  ammonia  prevents  the  complete  precipitation  of  tne 
chromium  hydroxide,  the  filtrate  is  then  colored  pink.  In  such  cases  tlio 
filtrate  must  be  boiled  until  the  excess  of  ammonia  is  expelled,  and  the 
chromium  is  all  precipitated. 


CHROMIUM.  103 

2.  By    Precipitation    with    Potassium    Iodide — lodate    Solution. 
Method  of  Stock  and  Massaciu* 

The  determination  is  carried  out  as  in  the  case  of  aluminium 
(cf.  p.  83).  The  slightly  acid  f  solution,  contained  in  a  porcelain 
dish,  is  treated  with  a  mixture  of  potassium  iodide  and  iodate, 
decolorized  after  a  few  minutes  by  means  of  sodium  thiosulphate 
solution,  treated  with  a  little  more  iodide  and  iodate  and  then  again 
with  a  few  c.c.  of  sodium  thiosulphate,  and  heated  half  an  hour  on 
the  water-bath.  The  flocculent  precipitate  of  chromic  hydrox- 
ide settles  quickly,  and  is  filtered  preferably  through  a  hot  water 
filter  under  slight  suction.  The  precipitate  is  ignited  wet  in  a 
platinum  crucible. 

3.  By  Precipitation  with  Ammonium  Nitrite. % 

If  the  solution  of  the  chromic  salt  is  acid,  it  is  neutralized 
with  ammonia  until  a  slight  permanent  precipitate  is  obtained. 
This  precipitate  is  dissolved  by  the  addition  of  a  few  drops  of 
hydrochloric  acid  and  then  an  excess  of  6  per  cent,  ammonium 
nitrite  solution  is  added  and  the  liquid  boiled  until  all  nitrous 
fumes  have  been  expelled.  By  this  means  practically  all  of  the 
chromium  will  have  been  precipitated,  but  in  order  to  throw  down 
the  last  traces,  ammonia  is  added  drop  by  drop  until  the  odor 
of  free  ammonia  barely  persists  in  the  solution.  The  precipitate 
is  allowed  to  settle  while  the  beaker  remains  on  the  water-bath, 
and  is  finally  filtered  off,  washed  with  hot  water,  ignited  wet  in 
a  platinum  crucible,  and  weighed  as  Cr2O3. 

(&)  Chromates. 

If  the  chromium  is  present  in  solution  in  the  form  of  an  alkali 
chromate,  free  from  chloride  and  large  amounts  of  sulphuric  acid, 

*  Ber.,  1901,  467. 

f  If  the  solution  is  strongly  acid,  it  is  neutralized  by  the  addition  of  pure 
KOH  solution  drop  by  drop,  until  a  faint  permanent  turbidity  is  obtained. 

J  E.  Schirm,  Chem.  Ztg.,  1909,  877.  Cf.  p.  111.  According  to  Scholler 
and  Schrauth  (ibid.,  1909,  1287)  iron,  chromium,  aluminium,  and  zinc  can  be 
precipitated  by  means  of  aniline. 


104  GRAVIMETRIC  ANALYSIS. 

it  may  be  determined  very  accurately  by  precipitation  with  mer- 
curous  nitrate  solution  as  mercurous  chromate;  on  ignition  the 
latter  is  changed  to  O2O3. 

Procedure. — The  neutral  or  weakly  acid  solution  is  treated 
with  a  solution  of  pure  mercurous  nitrate  whereby  brown,  basic 
mercurous  chromate,  (4Hg2O;3CrO3),  is  formed.  On  heating  to 
boiling,  the  precipitate  becomes  a  beautiful,  fiery  red,  being  con- 
verted into  the  neutral  salt  Hg2CrO4.  This  red  salt  settles  very 
quickly,  and  if  the  precipitation  is  complete  the  solution  above  the 
precipitate  will  be  colorless.  After  cooling,  the  precipitate  is  filtered 
off,  washed  thoroughly  with  water  containing  a  little  mercurous 
nitrate,  dried  and  separated  from  the  filter  as  completely  as  possible. 
The  filter  is  burned  in  a  platinum  spiral  and  ignited  with  the  main 
portion  of  the  precipitate,  gently  at  first  and  finally  strongly,  in 
a  platinum  crucible  under  a  hood  with  a  good  draft,  afterwards 
weighing  the  residue  as  Cr203. 

The  purity  of  the  mercurous  nitrate  must  be  tested  before  using 
it.  5  gms.  of  the  salt  should  leave  no  residue  after  being  ignited. 

This  excellent  method  for  the  determination  of  chromium 
unfortunately  permits  only  a  very  limited  application.  If 
the  solution  contains  any  considerable  amount  of  chloride, 
mercurous  chloride  will  be  precipitated  with  the  mercurous  chro- 
mate, which,  although  volatile  on  ignition,  renders  the  precipitate 
too  bulky  and  the  method  inaccurate. 

If,  therefore,  it  is  necessary  to  determine  chromium  present 
as  chromate  in  a  solution  containing  chloride,  two  other  methods 
are  at  our  disposal.  The  chromate  may  be  reduced  by  boiling 
with  sulphurous  acid  (or  by  evaporating  with  concentrated  hydro- 
chloric acid  and  alcohol)  and  analyzed  according  to  (a),  or  it 
may  just  as  accurately,  and  much  more  conveniently,  be  deter- 
mined by  precipitating  as 

Barium  Chromate, 

which  is  weighed  after  gentle  ignition. 

Procedure. — The  neutral  solution,  or  one  weakly  acid  with  acetic 
acid,  is  treated  at  the  boiling  temperature  with  a  solution  of  barium 
acetate  added  drop  by  drop,*  and  after  standing  for  some  time, 

*  If  the  barium  acetate  solution  is  added  too  quickly  some  of  it  will  be 


BARIUM   CH  ROM  ATE.  105 

is  filtered  through  a  Gooch  crucible  (without  using  very  strong 
suction,  as  otherwise  the  filter  will  soon  get  stopped  up  and  the 
solution  will  filter  extremely  slowly).  The  precipitate  is  washed 
with  dilute  alcohol  and  dried  in  the  hot  closet.  The  crucible  is 
suspended  in  a  larger  one  of  porcelain  by  means  of  an  asbestos 
ring  (cf.  page  27)  and  heated,  at  first  gently,  and  finally  over  the 
full  flame  of  a  good  Bunsen  burner.  After  five  minutes  the  cover 
is  removed  and  the  heating  is  continued  until  the  precipitate 
appears  a  uniform  yellow  throughout,  when  it  is  cooled  in  a  desic- 
cator and  weighed. 

Sometimes  the  precipitate  appears  green  on  the  sides  of  the 
crucible  owing  to  a  slight  reduction  (by  means  of  dust,  traces 
of  alcohol,  etc.)  of  chromic  acid  to  chromic  oxide.  The  latter 
gradually  takes  on  oxygen  from  the  air  during  the  long-continued 
heating  of  the  open  crucible,  so  that  the  green  color  gradually 
disappears. 

If  a  grams  of  chroma  te  were  taken  for  analysis,  and  the 
barium  chromate  precipitate  weighed  p  grams,  then  the  amount 
of  chromium  present  may  be  calculated  as  follows: 

BaCrO4  :  Cr=p  :  s 

Cr 

~BaCrO/P' 
and 


100  Cr 


Example  for  practice:  Potassium  bichromate,  K2Cr2O7,  purified 
and  dried  as  described  on  pages  33  and  35. 

Chromium  present  as  chromate  may  be  very  accurately  deter- 
mined by  volumetric  methods,  as  will  be  shown  under  Volumetric 
Analysis. 

carried  down  with  the  barium  chromate,  so  that  too  high  results  will  be 
obtained. 


106  GRAVIMETRIC  ANA LYSIS. 

URANIUM,  U.    At.  wt.,  238.5. 

Forms:  U3O8  and  U02. 

(a)  Determination  as  U308. 

Uranium  is  almost  always  precipitated  by  means  of  ammonia 
as  ammonium  uranate  and  changed  to  U3O8  by  gentle  ignition 
in  a  platinum  crucible  with  free  access  of  air.  According  to 
Zimmerman  *  this  transformation  is  only  complete  when  the 
precipitate  is  ignited  in  a  stream  of  oxygen;  the  error  is,  however, 
so  small  that  for  ordinary  purposes  it  can  be  neglected. 

According  to  the  temperature  of  ignition,  the  U3O8  appears 
dirty  green  or  black,  and  is  difficultly  soluble  in  dilute  hydro- 
chloric or  sulphuric  acids;  in  nitric  acid  it  dissolves  gradually. 
By  heating  with  dilute  sulphuric  acid  (1  vol.  cone.  H2SO4  +  6  vol. 
H2O)  in  a  closed  tube  at  150°-175°  C.  for  a  long  time  (W.  F. 
Hillebrand),f  the  U3O8  is  completely  dissolved  with  the  formation 
of  uranous  and  uranyl  sulphate: 

U308+  4H2S04 = 2U02(S04)  +U  (SO4)  2f  4H2O. 

U3O8  is  also  readily  soluble  in  dilute  sulphuric  acid  in  the 
presence  of  potassium  bichromate.  These  two  last  facts  are  taken 
advantage  of  in  the  volumetric  determination  of  uranium  (which 


(6)  Determination  as  UO2. 

The  ignited  precipitate,  obtained  in  exactly  the  same  way 
as  before,  is  heated  over  a  good  Teclu  burner,  or  over  the  blast- 
lamp,  in  a  current  of  hydrogen,  until  a  constant  weight  is  obtained 
whereby  it  is  quantitatively  changed  to  UO2.  This  is  the  most 
accurate  method  for  the  determination  of  uranium. 

The  UO2  thus  obtained  is  a  brown  powder,  insoluble  in  dilute 
hydrochloric  and  sulphuric  acids,  but  soluble  in  concentrated 
sulphuric  acid  after  long  heating,  best  in  a  closed  tube.  This 
oxide  is  also  soluble  in  nitric  acid. 

*  Ann.  d.  Ch.  und  Ph.,  232  (1886),  p.  287. 
t  Bull.  U.  S.  Geol.  Survey,  78,  p.  90. 


SEPARATION  OF  IRON  FROM  ALUMINIUM.  107 

Separation  of  Iron,  Aluminium,  Chromium,  Titanium,  and 
Uranium  from  Calcium,  Strontium,  Barium,  and  Magne- 
sium. 

The  solution  containing  the  above  substances  in  the  presence 
of  considerable  ammonium  chloride  is  placed  in  an  Erlenmeyer 
flask  and  treated  with  a  slight  excess  of  freshly  prepared  ammo- 
nium sulphide  free  from  sulphate  and  carbonate.  After  stand- 
ing overnight  the  precipitate  is  filtered  off  and  washed  with 
water  containing  ammonium  sulphide.  It  contains  the  iron  and 
uranium  as  sulphides,  the  aluminium,  chromium,  and  titanium 
as  hydroxides.  In  case  large  amounts  of  magnesium  are  present, 
some  of  it  is  almost  always  present  in  the  precipitate,  so  that  it 
is  then  necessary  to  dissolve  the  precipitate,  after  filtration,  in 
hydrochloric  acid  and  to  reprecipitate  with  ammonium  sulphide. 

Instead  of  using  ammonium  sulphide,  the  separation  can  be 
accomplished  satisfactorily  with  ammonia;  the  iron  must  then 
be  in  the  ferric  condition. 

Separation  of  Iron  from  Aluminium. 

(1)  The   solution   is    treated  in   a   porcelain   dish  with   pure 
potassium   hydroxide    solution    until    strongly    alkaline,   boiled, 
diluted  with  hot  water,  and  filtered.     The  precipitate  contains 
the  iron  as  hydroxide,  while  the  solution  contains  the  aluminium 
as  aluminate.*     For  the  iron    determination  the  precipitate  is 
dissolved    in    hydrochloric    acid,  reprecipitated   with    ammonia,t 
dried,  and  weighed  as  Fe^  (see  page  83).     The  aluminium  is 
precipitated  as  hydroxide    from  the  filtrate  by  acidifying  with 
nitric  acid  and  then  adding  ammonia. 

(2)  The  acid  solution  is  treated  with  tartaric  acid  (three  parts 
of  tartaric  acid  for  each  part  of  the  mixed  oxides  (Fe^^-  A12O3)), 
hydrogen  sulphide  is  passed  into  the  solution  until  it  is  saturated, 
as  slight  an  excess  as  possible  of  ammonia  is  added,  and  the  sulphide 
of  iron  is  allowed  to  settle  in  a  closed  Erlenmeyer  flask.     It  is  then 

*  If  the  precipitate  is  large,  it  should  be  dissolved  in  hydrochloric  acid 
and  again  precipitated  with  KOH. 

t  It  is  very  hard  to  wash  the  KOH  precipitate  free  from  alkali  so  that  the 
first  precipitate  should  not  be  weighed.  ' 


108  GRAVIMETRIC  ANALYSIS. 

filtered,  washed  with  water  containing  ammonium  sulphide,  dis- 
solved in  hydrochloric  acid, oxidized  with  a  little  potassium  chlorate 
or  nitric  acid,  and  precipitated  as  ferric  hydroxide  by  the  addition 
of  ammonia.  The  aluminium  is  determined  in  the  nitrate  by 
evaporating  to  dryness  with  the  addition  of  a  little  sodium  carbo- 
nate and  potassium  nitrate.  The  residue  is  gently  ignited  in  a 
platinum  dish  in  order  to  destroy  the  tartaric  acid,  after  which 
it  is  dissolved  in  dilute  nitric  acid,  the  carbon  filtered  off,  and  the 
aluminium  precipitated  from  the  solution  by  the  addition  of 
ammonia. 

(3)  The  neutral  solution  of  the  chlorides  or  sulphates  (not 
the  nitrates)  is  treated  with  sodium  carbonate  until  a  slight 
permanent  precipitate  is  formed,  which  is  dissolved  by  the  addi- 
tion of  a  few  drops  of  hydrochloric  acid.  The  solution  is  diluted 
to  about  250  c.c.  for  each  0.1  or  0.2  gm.  of  the  metals  present,  an 
excess  of  sodium  thiosulphate  solution  is  added,  and  the  solution 
boiled  until  every  trace  of  S02  has  disappeared.  By  this  opera- 
tion the  ferric  salt  is  reduced  to  ferrous  salt : 

2Na2S203  +  2FeCl3  -  2NaCl  +  Na2S4O6  +  2FeCl2, 
and  the  aluminium  is  precipitated  as  the  hydroxide: 

2  A1C1, + 3H2O + 3Na2S2O3  =  6NaGl + 3SO2  +  3S  +  2  Al  (OH)  3. 

The  precipitate  of  aluminium  hydroxide  and  sulphur  is  filtered 
off,  washed  with  hot  water,  dried,  transferred  as  completely  as 
possible  to  a  porcelain  crucible,  the  filter  burned  in  a  platinum 
spiral  and  the  ash  added  to  the  crucible,  which  is  ignited  gently 
until  all  the  sulphur  has  been  expelled  and  then  more  strongly 
over  the  blast  or  a  Teclu  burner  until  the  weight  is  constant. 

To  determine  the  iron,  the  filtrate  may  be  acidified  with  hydro- 
chloric acid,  the  SO2  boiled  off,  the  sulphur  filtered  off,  the  solu- 
tion oxidized  with  nitric  acid  and  precipitated  by  ammonia  as 
described  on  page  87.  It  is  still  better  to  precipitate  the 
iron  with  ammonium  sulphide,  filter,  dissolve  in  hydrochloric 
acid,  oxidize  with  nitric  acid,  and  then  precipitate  with  am- 
monia. 


SEPARATION  OF  IRON  FROM  ALUMINIUM.  109 

(4)  Both  of  the  metals  are  precipitated  with  ammonia,  filtered, 
washed,  dried,  ignited  in  a  platinum  crucible,  and  the  weight  of  the 
combined  oxides  determined.  The  mixture  is  then  digested  with 
concentrated  hydrochloric  acid  to  which  a  little  water  has  been 
added  (10HC1:1H2O)  in  a  covered  crucible  until  the  iron  is  com- 
pletely dissolved.  If  ferric  oxide  predominates,  as  is  frequently 
the  case,  the  solution  is  effected  in  one  or  two  hours.  If,  on  the 
other  hand,  a  relatively  large  amount  of  alumina  is  present  (as  is 
usually  the  case  with  silicates),  and  which  can  be  detected  by  the 
color  of  the  precipitate  produced  by  ammonia,  the  precipitate 
then  dissolves  very  slowly  and  in  many  cases  only  incom- 
pletely. 

In  the  latter  case  the  ignited  oxides  are  brought  into  solution 
by  fusing  with  12-15  times  as  much  potassium  pyrosulphate, 
K2S2O7  (cf.  Vol.  I).*  The  decomposition  of  the  oxides  is  usually 
complete  in  2-4  hours.  The  crucible  together  with  its  cover 
is  placed  in  a  beaker,  water  and  a  little  sulphuric  acid  are  added, 
and  the  melt  is  dissolved  by  warming  gently,  and  passing  a  current 
of  air  through  the  solution  in  order  to  keep  the  liquid  in  motion. 
A  small  amount  of  platinum  is  always  dissolved  by  this  treatment. 
After  removing  the  crucible  and  its  cover,  the  solution  is  heated 
to  boiling  and  saturated  with  hydrogen  sulphide.  The  solution 
is  then  filtered  into  a  flask  and  carbon  dioxide  is  passed  through 
it  until  the  excess  of  hydrogen  sulphide  is  completely  removed. 
The  contents  of  the  flask  are  then  cooled  by  placing  the  flask  in 
cold  wrater,  the  carbon  dioxide  still  passing  through  the  flask. 
The  iron  is  then  titrated  with  potassium  permanganate  solution 
as  described  on  page  99.  The  aluminium  is  determined  by  differ- 
ence, from  the  weight  of  the  combined  oxides.  For  the  determina- 
tion of  iron  in  silicates  the  above  process  is  most  suitable  (Hille- 
brand).  The  reduction  of  the  ferric  salt  to  ferrous  salt  by  means  of 
hydrogen  sulphide  possesses  great  advantages  over  the  reduction 
by  means  of  zinc,  for  in  the  former  case  no  foreign  element  is  intro- 
duced, and  furthermore  zinc  serves  to  reduce  the  titanic  acid 

*  E.  Deussen  finds  that  fusion  with  KF.HF  works  better.  The  platinum 
is  not  attached  and  the  solution  is  effected  more  readily. — Z.  angew.  Chem.,. 
1905,  815. 


no  GRAVIMETRIC  ANALYSIS. 

that  is  almost  always  present  in  rocks,  and  this  will  be  again  oxidized 
by  the  permanganate,  so  that  too  high  an  iron  value  will  be  ob- 
tained. 

If  the  iron  is  all  dissolved  by  treating  the  oxides  with  hydro- 
chloric acid,  the  solution  is  evaporated  to  dryness  and  the  residue 
is  treated  with  a  few  cubic  centimeters  of  dilute  sulphuric  acid, 
evaporated  on  the  water-bath  as  far  as  possible,  and  then  heated 
over  the  free  flame  until  fumes  of  sulphuric  acid  are  evolved. 
After  cooling,  the  product  is  dissolved  in  water  and  the  ferric 
sulphate  reduced  to  ferrous  sulphate  by  introducing  a  piece  of 
zinc,  free  from  iron,  into  the  crucible  and  covering  the  latter  with  a 
watch-glass.*  The  reduction  is  complete  in  20-30  minutes.  The 
slight  residue  of  platinum  f  is  filtered  off  with  the  excess  of  zinc 
into  a  flask  already  filled  with  carbon  dioxide.  The  residue  is 
washed  with  water  that  has  been  boiled,  and  the  solution  is  titrated 
with  potassium  permanganate  solution. 

The  latter  method  is  to  be  recommended  for  the  determination 
of  small  amounts  of  iron  in  the  presence  of  still  less  aluminium,  as 
is  the  case  in  the  analysis  of  mineral  waters. 

The  following  procedure  leads  to  the  same  end,  but  the  results 
are  not  quite  so  reliable: 

The  solution  from  which  the  iron  and  aluminium  are  to  be. 
determined  is  diluted  to  a  definite  volume  (e.g.,  250  c.c.)  and  two 
aliquot  portions  are  taken  by  means   of  a  pipette   (usually  100 
c.c.). 

In  one  portion  the  weight  of  the  combined  oxides  of  iron  and 
aluminium  is  determined  by  precipitation  with  ammonia  and  ignition 
of  the  precipitate,  while  in  the  other  the  iron  is  determined  by 
titration.  If  the  solution  contains  hydrochloric  acid,  as  is  usually 
the  case,  the  iron  is  first  precipitated  with  ammonia,  filtered, 

*  If  titanium  is  present,  the  solution  is  reduced  by  means  of  hydrogen 
sulphide. 

f  Platinum  is  perceptibly  attacked  by  long  digestion  with  ferric  chloride 
solution : 

4FeCl,  +  Pt  +  2HC1=  H2PtCl«  -f  4FeCl2. 
The  chloroplatinic  acid  is  reduced  to  platinum  by  the  action  of  zinc. 


SEPARATION  OF  IRON,  ALUMINIUM,   PHOSPHORIC  ACID,    in 

washed,  and  dissolved  in  dilute  sulphuric  acid.      The  solution  is 
then  reduced  and  titrated  as  previously  described.* 


Separation  of  Iron,  Aluminium,  and  Phosphoric  Acid. 

Although  the  determination  of  phosphoric  acid  has  not  yet 
been  considered,  we  will  describe  its  determination  in  the  presence 
of  iron  and  aluminium  because  this  highly  important  separation  is 
necessary  in  the  analysis  of  almost  all  minerals  containing  iron 
and  aluminium  as  well  as  in  the  analysis  of  many  mineral  waters. 
Two  cases  are  to  be  distinguished: 

1.  The  solution  contains  only  a  small  amount  (a  few  centigrams 
or  less)  of  iron,  aluminium,  and  phosphoric  acid. 

2.  The  solution  contains  large  amounts  of  these  substances. 

1.  In  the  first  case  the  determination  of  all  three  constituents 
must  be  undertaken  in  the  same  portion,  as  otherwise  errors  would 
be  introduced  on  account  of  the  small  amounts  to  be  determined. 
The  solution  is  first  treated  with  ammonia  whereby  the  iron,  alu- 
minium and  phosphoric  acid  are  precipitated.f 

The  precipitate  is  ignited  in  a  platinum  crucible  and  weighed: 

Fe2O3+  A12O3+  P2O5  =  A. 

The  product  is  then  fused  with  six  times  its  weight  of  a  mixture 
consisting  of  four  parts  anhydrous  sodium  carbonate  and  one  part 
pure  silica.  The  mixture  is  heated  over  the  blast-lamp,  the  melt 
is  extracted  with  water,  to  which  a  little  ammonium  carbonate 
has  been  added,  and  filtered.  The  filtrate  contains  all  of  the  phos- 
phoric acid  and  a  very  little  silicic  acid,  while  the  residue  contains 
all  of  the  iron  and  aluminium  and  considerable  silica. 

For  the  determination  of  the  phosphoric  acid,  the  filtrate  is 
evaporated  with  hydrochloric  acid  on  the  water-bath  to  dryness, 

*  It  is  necessary  to  get  rid  of  the  hydrochloric  acid  on  account  of  its  action 
upon  potassium  permanganate  (cf.  Vol.  Anal.,  under  Iron). 

t  The  phosphoric  acid  is  usually  present  in  such  small  amounts  that  the 
iron  and  aluminium  are  more  than  sufficient  to  effect  the  precipitation  of  all 
the  phosphoric  acid,  on  the  addition  of  ammonia,  as  phosphates  of  these 
metals. 


H2  GRAVIMETRIC  ANALYSIS. 

in  order  to  remove  the  silica,  the  residue  is  moistened  with  hydro- 
chloric acid,  taken  up  in  a  little  water,  filtered,  and  the  phosphoric 
acid  precipitated  in  the  filtrate  by  the  addition  of  ammonia  and 
"magnesia  mixture."  The  precipitate  of  magnesium  ammonium 
phosphate  is  changed  to  magnesium  pyrophosphate  by  ignition  and 
from  its  weight  p  the  amount  of  phosphoric  anhydride,  P2O5,  is 
calculated  (=B): 

Mg2P207:P205=p:B, 


By  subtracting  B  from  A  the  combined  weight  of  the  iron  and 
aluminium  oxides  is  obtained,  in  which  the  iron  is  determined 
volumetrically  and  the  aluminium  by  difference.  For  the  deter- 
mination of  the  iron,  the  insoluble  residue,  obtained  after  treating 
the  product  of  the  fusion  with  water  and  ammonium  carbonate, 
is  digested  with  hydrochloric  acid  in  a  small  porcelain  crucible 
until  the  iron  oxide  is  completely  dissolved.  The  solution  is  treated 
with  dilute  sulphuric  acid,  evaporated  on  the  water-bath  as  far 
as  possible,  and  then  over  a  free  flame  until  fumes  of  sulphuric 
anhydride  are  evolved.  After  cooling,  water  is  added  and  after 
digesting  on  the  water-bath  for  a  long  time  the  silica  is  filtered 
off,  the  solution  reduced  by  means  of  hydrogen  sulphide  (cf.  p. 
109,  sub.  4),  and,  after  removing  the  excess  of  hydrogen  sulphide, 
the  iron  is  titrated  with  permanganate  solution.*  From  the 
amount  of  permanganate  used,  the  amount  of  ferric  oxide  (C)  can 
be  calculated,  and  by  deducting  this  amount  from  the  weight 
of  the  combined  oxides,  the  weight  of  the  A12O3  is  ascertained  : 
A-(B+C)=A1203. 

2.  In  case  the  solution  contains  large  amounts  of  iron,  alu- 
minium, and  phosphoric  acid,  it  is  divided  into  three  aliquot  por- 
tions and  in  one  the  value  of  "  A  "  is  determined  by  precipitation 
with  ammonia;  in  the  second  the  phosphoric  acid  is  determined 
by  the  molybdate  method  ;  and  in  the  third  the  iron  is  determined 
by  titration. 

*  Instead  of  reducing  the  iron,  the  ferric  salt  may  be  titrated  directly 
with  titanous  chloride  (cf.  p.  699,  or  iodimetricaily  (cf.  p.  681). 


SEPARATION  OF  IRON  FROM  CHROMIUM.  113 


Separation  of  Iron  from  Chromium. 

1.  The  chromium  is  oxidized  in  alkaline  solution  by  means  of 
chlorine  or  bromine  to  a  soluble  chromate  and  the  insoluble  ferric 
hydroxide  is  filtered  off. 

Procedure. — The  solution  of  the  chlorides,  which  should  be 
placed  in  an  Erlenmeyer  flask  of  Jena  glass  provided  with  a 
ground-glass  stopper  and  tubes  by  which  gas  may  enter  and  leave 
the  flask,  is  treated  with  potassium  hydroxide  solution  until 
strongly  alkaline,  warmed  on  the  water-bath  and  chlorine  gas  is 
conducted  through  the  liquid,  or  bromine  water  is  added,  until 
it  becomes  distinctly  yellow  and  the  ferric  hydroxide  has  as- 
sumed its  characteristic  reddish-brown  color.  When  the  oxidation 
is  performed  by  chlorine  gas,  0.5  gm.  of  the  mixed  oxides  will  be 
completely  oxidized  in  fifteen  to  twenty  minutes.  The  solution  is 
diluted  with  water  and  filtered.  The  filtrate  is  carefully  acidified 
with  acetic  acid,  the  chromium  precipitated  by  the  addition 
of  barium  acetate,  and  the  precipitate  of  barium  chromate  is 
treated  as  described  on  p.  104.  The  ferric  hydroxide  is  dissolved 
in  hydrochloric  acid,  reprecipitated  with  ammonia  and  weighed 
as  ferric  oxide. 

Remark. — If  the  chromate  is  to  be  determined  as  barium  chro- 
mate, the  solution  must  contain  no  sulphuric  acid.  If  the  latter  is 
present,  the  chromate  is  reduced  by  evaporating  with  hydrochloric 
acid  and  alcohol;  the  solution  of  chromic  chloride  thus  obtained 
is  precipitated  with  ammonia  and  the  chromium  determined  as 
chromic  oxide. 

In  the  case  of  a  precipitate  containing  iron  and  chromic  oxides, 
it  is  fused  with  sodium  carbonate  and  a  little  potassium  chlorate, 
the  melt  is  extracted  with  water,  and  the  chromium  is  determined  in 
the  solution  by  precipitating  with  barium  acetate.  The  insoluble 
residue  from  the  aqueous  extraction  of  the  fusion  is  dissolved  in 
hydrochloric  acid,  precipitated  with  ammonia,  and  the  iron  deter- 
mined as  ferric  oxide. 

If  it  is  desired  to  precipitate  the  chromium  as  mercurous 
chromate,  the  precipitate  containing  the  iron  and  chromic  oxides 
is  fused  with  sodium  carbonate  and  potassium  nitrate,  the  melt 


H4  GRA VIMETRIC  ANALYSIS. 

extracted  with  water,  the  solution  neutralized  with  nitric  acid  and 
precipitated  with  mercurous  nitrate  solution,  as  described  on  p.  104. 

2.  It  has  been  proposed  to  analyze  the  mixture  of  ferric  and 
chromic  oxides  by  strongly  igniting  them  in  a  stream  of  hydrogen 
whereby  the  ferric  oxide  is  reduced  to  metallic  iron,  while  the  t 
chromic  oxide  is  unchanged.     The  iron  could  then  be  determined  : 
by  the  loss  of  weight.     This  method,  although  theoretically  very  J 
simple,    seems   from    experiments    carried    out   in  the    author's 
laboratory  to  be  absolutely  inadequate,  for  the  ferric   oxide  is  so 
enveloped  in  chromic  oxide  that  it  is  not  even  approximately  re- 
duced even  when  heated  over  the  blast-lamp. 

3.  Iron   may  be   separated    from    chromium  by  precipitating 
the  former  with  ammonium   sulphide  from  a  solution  containing 
sufficient  ammonium  tartrate  to  prevent  the  precipitation  of  the 
chromium.     The  separation  is  the  same  as  was  described  under 
aluminium,  p.  107,  sub.  2. 


Separation  of  Aluminium  from  Chromium. 

If  the  chromium  is  present  as  chromic  salt,  it  is  oxidized  by 
means  of  chlorine  or  bromine  in  a  solution  made  strongly  alkaline 
with  potassium  hydroxide.  The  solution  is  then  acidified  with 
nitric  acid,  and  the  aluminium  precipitated  by  ammonia  as  hydrox- 
ide, being  weighed  as  the  oxide.  In  the  absence  of  sulphuric  acid 
the  chromium  may  be  determined  in  the  nitrate  as  barium  chro- 
mate  (cf.  p.  104).  If  sulphuric  acid  is  present,  the  chromate  is 
reduced  to  chromic  salt  again  by  the  action  of  concentrated  hydro- 
chloric acid  and  alcohol,  precipitated  with  ammonia,  and  weighed 
as  the  oxide 

If,  however,  the  chromium  is  already  present  as  chromate,  the 
aluminium  is  at  once  precipitated  with  ammonia  as  hydroxide. 


Separation  of  Iron  from  Titanium. 

It  is  frequently  necessary  to  determine  both  iron  and  titanium 
in  a  precipitate  produced  by  ammonia  consisting  of  a  mixture  of 
these  two  oxides  alone,  but  it  is  more  often  necessary  to  determine 


SEPARATION  OF  IRON  FROM    TITANIUM.  115 

titanium  in  the  presence  of  iron,  aluminium,  and  phosphoric  acid, . 
all   of   which   are   precipitated   by  ammonia   in  the   analysis   of 
rocks. 

For  the  separation  of  titanium  from  iron  in  the  absence  of 
alumina,  the  following  methods  are  suitable : 

1.  The  precipitate  produced  by  ammonia  is  ignited  and  then 
fused  with  15-20  times  as  much  of  previously  dehydrated  potas- 
sium pyrosulphate  over  a  small  flame  until  completely  attacked. 
After  cooling,  the  melt  is  dissolved  in  cold  water   containing  sul- 
phuric acid,  and  the  solution  is  hastened  by  keeping  the  liquid  in 
motion  by  means  of  a  current  of  air  passed  through  it. 

The  solution  thus  obtained  is  diluted  to  a  definite  volume,  and 
after  being  thoroughly  mixed  is  divided  into  two  portions,  one  being 
used  for  the  determination  of  titanium  and  the  other  for  the  deter- 
mination of  iron.  For  the  iron  determination,  the  acid  solution  is 
saturated  with  hydrogen  sulphide  in  the  cold,  heated  to  boiling,  and 
the  precipitate  of  platinum  sulphide,  sulphur,  and  a  little  titanium 
is  filtered  off  into  a  flask  filled  with  carbon  dioxide,  and  washed 
thoroughly  with  hot  water.  The  filtrate  is  heated  to  boiling  and 
carbon  dioxide  is  passed  through  the  solution  until  the  excess  of 
hydrogen  sulphide  is  completely  removed,  when  it  is  cooled  in  an 
atmosphere  of  carbon  dioxide  and  then  titrated  with  perman- 
ganate. For  the  titanium  determination,  the  other  part  of  the 
solution  is  treated  with  sodium  carbonate  solution  until  a  slight 
precipitate  is  formed ;  this  is  dissolved  in  as  little  sulphuric  acid  as 
possible,  saturated  with  hydrogen  sulphide  in  the  cold,  and  5  gms. 
of  sodium  acetate  which  has  been  neutralized  with  acetic  acid*  are 
added.  Carbon  dioxide  is  conducted  through  the  solution,  it  is 
heated  to  boiling,  filtered  hot,  washed  with  water  containing  hydro- 
gen sulphide,  ignited  wet  in  a  platinum  crucible,  and  weighed  as 
TiO2. 

Remark. — If  considerable  iron  is  present,  the  titanic  oxide  thus 
obtained  is  likely  to  contain  iron.  It  is  brought  into  solution 
again  by  fusing  with  potassium  pyrosulphate  and  the  precipitation 
is  repeated  exactly  as  before.  In  this  way  a  precipitate  free  from 
iron  is  obtained. 

2.  The  Chancel-Stromayer  method  is  also  satisfactory.     The 
solution  from  the  pyrosulphate  fusion,  in  this  case  after  being 

*  Cf.  footnote  t D  pa~e  ICO. 


II 6  GRAyiMETRIC  ANALYSIS. 

neutralized  with  sodium  carbonate,  is  treated  with  an  excess  of 
sodium  thiosulphate,  diluted  to  about  400-500  c.c.  and  boiled  for 
some  time.  In  this  way  me ta titanic  acid  and  sulphur  are  precipi- 
tated, while  iron  remains  in  solution.  During  the  filtration,  how- 
ever, the  finely  divided  sulphur  passes  through  the  filter,  so  that 
the  first  method  is  preferable.  In  the  presence  of  considerable 
iron  the  metatitanic  acid  obtained  by  this  method  is  also  contam- 
inated with  iron,  so  that  the  separation  must  be  repeated. 

Separation  of  Aluminium  from  Titanium. 

It  has  been  proposed  to  effect  this  separation  by  diluting  to  a 
considerable  volume  the  slightly  acid  solution  of  the  melt  obtained 
by  the  potassium  pyrosulphate  fusion  and  boiling  for  some  time, 
thereby  precipitating  the  titanium  and  leaving  the  aluminium  in 
solution.  This  method,  however,  is  useless,  for  alumina  is  pre- 
cipitated with  the  metatitanic  acid  unless  the  solution  contains 
enough  acid  to  prevent  this  hydrolysis,  in  which  case  a  considerable 
amount  of  titanic  acid  remains  in  solution. 

The  best  separation  is  that  of  Gooch;*  it  consists  of  boiling  a 
solution  of  the  two  elements  containing  considerable  free  acetic 
acid  and  alkali  acetate ;  by  this  means  all  of  the  titanium  and  none 
of  the  aluminium  is  precipitated.  If,  however,  the  amount  of 
aluminium  present  is  large  (as  is  usual  in  rock  analysis),  the  pre- 
cipitate will  contain  some  aluminium,  so  that  the  separation  must 
be  repeated.  In  no  case  is  there  danger  of  the  precipitation  of  the 
titanium  being  incomplete. 

In  practice  it  is  almost  always  necessary  to  separate  the  titanium 
not  from  aluminium  alone,  but  from  iron  and  aluminium,  so  that 
the  method  of  Gooch  will  be  described  for  this  more  general  case. 

The  solution  obtained  by  dissolving  the  pyrosulphate  melt  in 
cold  water  is  treated  with  three  times  as  much  tartaric  acid  as 
the  weight  of  the  oxides,  is  saturated  with  hydrogen  sulphide 
gas,  and  then  made  slightly  ammoniacal.  By  this  means  all  of  the 
iron  is  precipitated  as  ferrous  sulphide,  while  the  aluminium  and 
titanium  remain  in  solution.  The  sulphide  of  iron  is  filtered  off, 
the  filtrate  is  acidified  with  sulphuric  acid,  heated  to  boiling,  and 
the  precipitate  of  sulphur  and  platinum  sulphide  (the  latter  from 
the  platinum  crucible  in  which  the  fusion  with  pyrosulphate  was 
*  Chemical  News,  52,  pp.  55  and  68. 


SEPARATION  OF  ALUMINIUM  FROM   TITANIUM.  117 

(nade)  is  filtered  off.  The  filtrate  is  boiled  to  expel  the  last  traces 
of  hydrogen  sulphide  and  the  tartaric  acid  is  destroyed  by  adding 
2^  times  as  much  potassium  permanganate  as  the  amount  of  tar- 
taric acid  present.  Sulphurous  acid  is  then  added  until  the  precipi- 
tated manganese  dioxide  is  redissolved,  after  which  a  slight  excess 
of  ammonia  is  added  and  then  7-10  c.c.  of  glacial  acetic  acid  for 
each  100  c.c.  of  solution.  The  solution  is  boiled  for  one  minute, 
the  precipitate  is  allowed  to  settle,  and  the  filtrate  is  decanted 
through  a  filter,*  transferred  to  the  filter,  washed  with  7  per  cent, 
acetic  acid  and  finally  with  hot  water.  The  dried  precipitate  is 
ignited  over  a  Bunsen  burner  for  from  fifteen  to  twenty  minutes 
and  then  weighed. 

The  precipitate  contains  manganese  and  aluminium,  so  that  it 
is  fused  with  three  times  as  much  sodium  carbonate.  The  melt 
(colored  green  by  the  manganese)  is  leached  with  cold  water,  leaving 
sodium  metatitanatef  and  some  alumina  undissolved.  The  precipi- 
tate is  filtered  off  by  means  of  a  small  filter,  is  ignited  in  a  platinum 
crucible,  and  fused  again  with  a  little  sodium  carbonate.  After 
cooling,  the  contents  of  the  crucible  are  dissolved  in  1.9  c.c.  of 
sulphuric  acid  (1  vol.  cone.  H2SO4:1  vol.  H2O)  diluted  to  about 
150-200  c.c.  and  treated  with  5  gm.  sodium  acetate  and  one-tenth 
of  its  volume  of  glacial  acetic  acid.  After  boiling  one  minute  and 
allowing  to  stand  until  settled,  the  precipitate  is  filtered  off,  washed 
with  7  per  cent,  acetic  acid,  then  with  water,  dried,  ignited,  and 
weighed.  This  precipitate  usually  contains  aluminium,  so  that 
it  is  again  fused  with  sodium  carbonate  and  the  melt  again  treated 
with  sulphuric  acid,  etc.,  exactly  as  described  above.  This  time 
the  precipitate  is  usually  free  from  aluminium,  but  the  process 
should  be  repeated  until  a  constant  weight  is  obtained. 

This  analysis  does  not  require  much  time,  for  usually  the 
amount  of  titanium  present  is  so  small  that  the  precipitates  filter 
and  wash  quickly. 

For  the  determination  of  very  small  amounts  of  titanium,  it  is 
advisable  to  use  the  colorimetric  method  proposed  by  Weller 
(cf.  p.  100).  Under  the  analysis  of  silicates  will  be  discussed  a 
practical  example  of  this  determination. 

*Schleicher  <fc  Schiill's  filter-paper  Xo.  589  is  satisfactory  for  this  purpose. 
tThe  sodium  metatitanate    is  hydrolized  and  forms  a  precipitate   con- 
taining a  much  higher  percentage  of  TiO2. 


n8  GRAVIMETRIC  ANALYSIS. 

Determination  of  Titanium  in  Rutile  and  Iron  Ores.     Method 
of  0.  L.  Barnebey  and  R.  M.  Isham.* 

This  method  is  based  on  the  volatilization  of  the  silica  by  hydro- 
fluoric acid  in  the  presence  of  sulphuric  acid,  evaporation  to 
dryness  and  fusion  with  sodium  carbonate  and  a  little  potassium 
nitrate  (which  converts  the  iron  and  titanium  to  insoluble  ferric 
oxide  and  sodium  acid  titanate)  extraction  with  hot  water  to 
remove  the  soluble  phosphates,  sulphates  and  aluminates,  solu- 
tion of  the  ferric  oxide  and  sodium  titanate  in  hydrochloric  acid, 
extraction  of  ferric  chloride  with  ether,  reduction  of  slight  traces 
of  iron  with  sulphurous  acid,  precipitation  of  the  titanic  acid  by 
boiling  in  acetic  acid  solution,  filtration  and  ignition  to  titanium 
oxide  (or  the  titanium  may  be  determined  colorimetrically) . 
The  method  is  accurate  and  not  long. 

Procedure. — The  sample  is  weighed  into  a  platinum  crucible, 
treated  with  a  little  water,  5  to  10  drops  of  sulphuric  acid, 
and  1  c.c.  of  hydrofluoric  acid,  and  the  mixture  heated  care- 
fully until  finally  no  more  sulphuric  acid  fumes  are  evolved. 
Five  or  10  grams  of  sodium  carbonate  and  a  little  potassium 
nitrate  are  added  and  the  mixture  fused  at  least  thirty  minutes. 
The  crucible  and  cover  are  cooled,  placed  in  a  beaker,  covered 
with  hot  water,  and  heated  until  the  melt  is  disintegrated. 
Ferric  oxide  and  sodium  titanate  are  left  insoluble  in  hot  water. 
The  crucible  is  removed,  washed,  and  any  adhering  particles  of 
ferric  oxide  and  sodium  titanate  are  dissolved  in  hot  hydrochloric 
acid  (sp.  gr.  1.1).  This  solution  is  saved.  The  residue  in  the 
beaker  is  filtered  and  washed  with  hot  water.  The  filter  is  per- 
forated and  the  residue  carefully  washed  into  a  clean  beaker  with 
hydrochloric  acid  (sp.  gr.  1.1).  (No  water  is  to  be  added  from 
this  stage  of  the  analysis  until  after  the  treatment  with  ether.) 
The  hydrochloric  acid  washings  from  the  platinum  crucible  are 
transferred  to  the  beaker  and  the  whole  heated  on  the  hot  plate 
until  solution  is  complete  and  the  total  volume  reduced  to  15  to 
20  c.c.  The  solution  is  then  cooled,  and  transferred  to  a  separa- 
tory  funnel,  the  beaker  being  rinsed  with  hydrochloric  acid 

*  J.  Am.  Chem.  Soc.,  32,  957  (1910). 


DETERMINATION  OF   TITANIUM  IN  RUTILE  AND  IRON  ORES.  119 

(sp.  gr.  1.1).  An  equal  volume  01  ether,  which  has  been  saturated 
with  concentrated  hydrochloric  acid  solution,  is  added  to  the 
solution  in  the  funnel,  a  rubber  stopper  is  inserted  in  the  top, 
the  funnel  is  inverted,  the  stop-cock  opened,  and  the  whole 
thoroughly  shaken.  The  stop-cock  is  then  closed,  the  funnel 
placed  in  an  upright  position  and  allowed  to  stand  ten  minutes, 
when  the  aqueous  layer  is  drawn  off  into  a  second  separatory 
funnel.  The  ether  is  rinsed  twice  by  shaking  well  with  5  to  10 
c.c.  portions  of  hydrochloric  acid  (sp.  gr.  1.1)  and  the  washings 
are  added  to  the  aqueous  solution.  The  treatment  with  ether 
is  repeated  two  or  three  times  until  the  last  portion  of  ether 
fails  to  show  any  greenish  tinge  due  to  the  presence  of  dissolved 
ferric  chloride. 

The  aqueous  solution  containing  all  the  titanium  in  the  pres- 
ence of  little,  if  any,  iron  and  aluminium,  is  heated  to  expel  the 
dissolved  ether,  20  c.c.  of  sulphuric  acid  (1.1)  are  added,  and 
the  solution  evaporated  until  fumes  of  sulphuric  anhydride  are 
evolved.  The  cooled  solution  is  diluted  to  about  100  c.c.  and 
nearly  neutralized  with  ammonia.  One  or  two  grams  of  ammo- 
nium bisulphite  are  added  and  the  solution  heated  on  the  hot 
plate  for  half  an  hour.  Ten  to  15  grams  of  ammonium 
acetate  are  now  added  and  the  solution  boiled  for  fifteen  minutes. 
The  precipitated  titanic  acid  is  filtered  off,  washed  with  7  per 
cent,  acetic  acid,  ignited  and  weighed  as  Ti02. 


Separation  of  Uranium  from  Iron  and  Aluminium. 

The  slightly  acid  solution,  containing  considerable  quantities 
of  ammonium  salts,  is  treated  with  an  excess  of  ammonium  car- 
bonate and  then  with  ammonium  sulphide,  allowed  to  stand  for 
some  time  in  a  closed  flask,  finally  filtered  and  washed  with  water 
containing  ammonium  sulphide. 

The  precipitate  contains  the  iron  as  sulphide  and  the  aluminium 
as  hydroxide;  in  the  filtrate  is  found  all  of  the  uranium  as 
UO2(CO3)  3  (XHJ  4.  The  precipitate  is  dissolved  in  hydrochloric 
acid,  its  solution  freed  from  hydrogen  sulphide  by  boiling,  the 
ferrous  salt  oxidized  to  ferric  salt  by  the  addition  of  potassium 


120  GRAVIMETRIC  ANALYSIS. 

chlorate,  and  the  iron  and  aluminium  determined  by  one  of  the 
methods  described  on  pages  107-109. 

The  filtrate  containing  the  uranium  is  evaporated  almost  to 
dryness,  acidified  with  hydrochloric  acid,  boiled,  and  the  uranium 
precipitated,  by  the  addition  of  ammonia,  as  ammonium  uranate. 
The  precipitate  is  filtered  off,  washed  with  2  per  cent,  ammonium 
nitrate  solution  to  which  a  little  ammonia  has  been  added,  dried, 
ignited,  and  weighed  as  UaOg. 

The  result  obtained  may  be  verified  by  heating  the  residue 
repeatedly  in  a  current  of  hydrogen  in  a  Rose  crucible  (see  Copper 
Determination)  until  a  constant  weight  is  obtained;  weighing 
as  U02.  The  purity  of  the  precipitate  may  also  be  tested  volu- 
metric ally  (see  Volumetric  Analysis). 

B.  DIVISION    OF  THE   MONOXIDES. 

MANGANESE,  NICKEL,  COBALT,  ZINC. 

MANGANESE,  Mn.    At.  Wt.  54.93. 

Forms:    MnS04,  MnS,  Mn304,  Mn2P2O7. 

i.  Determination  as  Manganous  Sulphate,  MnSO4. 

This  method,  first  proposed  by  Volhard,*  has  recently  been 
tested  by  Gooch  and  Austin, t  and  has  been  found  strictly  accurate. 
Experiments  performed  by  Schudel  in  the  author's  laboratory 
completely  confirm  Gooch's  results. 

Procedure. — The  oxide  obtained  by  the  ignition  of  the  car- 
bonate, sulphide,  or  of  manganous  manganite,  is  dissolved  in  as 
slight  an  excess  of  sulphuric  acidt  as  possible  in  a  porcelain 
crucible,  evaporated  as  far  as  possible  on  the  water-bath,  after 
which  the  excess  of  acid  is  removed  by  heating  in  an  air-bath. 
A  porcelain  crucible  provided  with  an  asbestos  ring  (see  Fig.  11, 
p.  27)  serves  for  the  air-bath.  The  walls  of  the  smaller  crucible 
should  be  separated  from  those  of  the  larger  one  by  about  1  cm. 
After  the  sulphuric  acid  has  been  removed,  the  two  crucibles 

*  Ann.  d.  Chem.  u.  Pharm.,  198,  p.  328. 
t  Zeit.  f.  anorg.  Chem.  (1898),  17,  p.  264. 

%  The  manganous  manganite  (Mn3O4)  requires  the  presence  of  reducing 
agent  (best  SO2)  or  of  pure  hydrogen  peroxide. 


SEPARATION  OF  MANGANESE  AS  CARBONATE.  121 

are  covered  and  heated  to  redness  over  a  good  Bunsen  burner, 
allowed  to  cool  in  a  desiccator  and  weighed.  From  the  weight 
of  the  manganous  sulphate,  the  amount  of  manganese  present 
may  be  calculated  as  follows: 

MnSO4:Mn=p:rc 

Mn 
~ 


(a)  Separation  of  Manganese  as  Carbonate. 

This  method  for  the  separation  of  the  manganese  permits  only 
a  limited  application,  because  no  other  metal  that  is  precipi- 
tated by  alkali  carbonates  can  be  simultaneously  present.  The 
method,  therefore,  is  only  suitable  for  the  determination  of  man- 
ganese in  solutions  of  pure  manganese  salts  containing  nothing 
else  except  alkali  and  ammonium  salts. 

According  to  H.  Tamm,*the  precipitation  is  best  accomplished 
by  means  of  ammonium  carbonate.  For  this  purpose  the  neutral 
solution  (which  may  contain  other  ammonium  salts)  is  treated 
with  a  slight  excess  of  ammonium  carbonate,  warmed  gently,  and 
the  beaker  containing  the  solution  is  allowed  to  remain  in  a  luke- 
warm water-bath  until  the  precipitate  has  settled  and  the  upper 
liquid  has  become  clear. 

The  precipitate  is  filtered  off,  washed  with  hot  water,  dried, 
ignited,  and  weighed  either  as  sulphate  according  to  1  or  as  Mn3O< 
according  to  2. 

Remark.  —  If  either  sodium  or  potassium  carbonate  is  used  to 
precipitate  the  manganese,  the  precipitate  will  always  contain 
alkali  carbonate  that  cannot  be  removed  by  washing.  After  the 
precipitate  has  been  ignited,  however,  the  alkali  carbonate  can  be 
easily  extracted  by  water.  Furthermore,  the  precipitation  is  not 
quite  quantitative;  the  filtrate  always  contains  small  amounts  of 
manganese.  In  order  to  remove  this,  it  is  necessary  to  evapo- 
rate the  aqueous  solution  to  dryness,  whereby  the  manganous  car- 
bonate is  completely  decomposed  hydrolytically  into  carbonic  acid 
and  manganous  hydroxide,  and  the  latter  in  contact  with  the  air 
changes  to  brown  manganic  oxide,  Mn2O3.  The  residue  obtained 
after  the  evaporation  is  treated  with  water,  the  small  amount 

t  Ch.  News,  26  (1872),  p.  37,  and  Zeit.  f.  anal.  Chem.,  11  (1872),  p.  425. 


122  GRAVIMETRIC  ANALYSIS. 

of  brown  manganese  compound  filtered  off,  ignited,  and  added  to 
the  main  part  of  the  precipitate. 

(b)  Separation  of  Manganese  as  Sulphide. 

This  method  is  employed  when  it  is  necessary  to  separate  man- 
ganese from  calcium,  strontium,  barium,  and  magnesium. 

We  will  distinguish  between  two  different  cases : 

(a)  The  solution  contains,  besides  manganese,  large  amounts 
of  the  alkaline  earths  or  magnesium. 

(/?)  The  solution  contains  only  small  amounts  of  the  alkaline 
earths  or  magnesium. 

(a)  In  case  large  amounts  of  the  alkaline  earths  or  magnesium 
are  present,  the  manganese  sulphide  must  be  precipitated  in  the 
cold  in  the  presence  of  considerable  ammonium  salts. 

The  solution  is  placed  in  an  Erlenmeyer  flask  of  Jena  glass  and 
about  5  gm.  of  ammonium  chloride  or  ammonium  nitrate  are  added. 
In  case  the  solution  reacts  acid,  ammonia  is  added  until  it  is  slightly 
alkaline,  and  a  slight  excess  of  freshly-prepared,  colorless  ammo- 
nium sulphide  solution  is  added.  The  flask  is  now  nearly  filled 
with  cold  distilled  water  that  has  been  boiled,  corked,  and  allowed 
to  stand  twenty-four  hours,  or,  better,  still  longer.  After  this  time 
the  flesh-colored  precipitate  will  have  settled.  The  clear  upper  liquid 
is  carefully  decanted  through  a  filter,*  taking  pains  not  to  disturb 
the  precipitate  and  to  keep  the  filter  filled  with  liquid  all  the  time. 
If  the  precipitate  is  at  all  bulky,  it  is  washed  three  times  by  decan- 
tation  with  a  5  per  cent,  solution  of  ammonium  nitrate  to  which  has 
been  added  1  c.c.  of  ammonium  sulphide.  The  precipitate  is  then 
transferred  to  the  filter  and  washed  with  dilute  ammonium  sul- 
phide water  until  20  drops  of  the  filtrate  evaporated  to  dryness  on 
a  crucible-cover  leave  no  residue.  Now  for  the  first  time  the  filter 
is  allowed  to  drain  completely  and  is  dried.  As  much  of  the  pre- 
cipitate as  possible  is  transferred  to  a  small  thin-walled  porcelain 
crucible,  the  filter-paper  is  burned  in  a  platinum  spiral,  and  the  ash 
added  to  the  main  portion  of  the  precipitate  in  the  crucible.  The 
uncovered  crucible  is  heated  over  a  small  flame  until  the  greater 
part  of  the  sulphur  has  been  burned  off,  when  the  flame  is  increased 

*  Schleicher  &  Schull's  filter-paper  No.  590  can  be  used  to  advantage. 


SEPARATION  OF  MANGANESE  AS  MANGANESE  DIOXIDE.       123 

and  the  crucible  is  finally  heated  over  the  flame  of  a  Teclu  burner, 
cooled,  and  weighed  as  Mn3O4  (cf.  p.  125,  sub.  3).  The  heating  is 
repeated  until  a  constant  weight  is  obtained.  Manganous  sulphide 
is  readily  changed  to  Mn3O4  if  the  amount  of  sulphide  is  compara- 
tively small.  In  case  more  than  0.2  gm.  is  present  there  is  danger 
of  getting  a  too  high  result  on  account  of  some  manganous  sulphate 
not  being  decomposed.  In  this  case  it  is  advisable  to  dissolve  the 
washed  precipitate  of  manganous  sulphide  in  dilute  hydrochloric 
acid,  to  evaporate  the  solution  to  dryness  in  order  to  remove  all 
hydrogen  sulphide,  to  dissolve  the  residue  in  a  little  water  and  to 
precipitate  the  manganese  as  carbonate  according  to  1;  or  the 
manganous  sulphide  can  be  weighed  as  such.  (See  p.  125.) 

(/?)  In  case  only  small  amounts  of  alkaline  earths  are  present, 
the  following  procedure  can  be  used :  The  neutral  solution  is  heated 
to  boiling,  an  excess  of  ammonia  and  some  ammonium  sulphide 
is  added  and  the  boiling  is  continued  until  the  manganous  sulphide 
has  become  a  dirty  green.  The  precipitate  is  allowed  to  settle  for 
some  minutes  and  is  then  filtered  and  washed  with  water  contain- 
ing a  little  ammonium  sulphide.  From  this  point  the  procedure 
is  the  same  as  described  under  (a). 


(c)  Separation  of  Manganese  as  Manganese  Dioxide. 

If   a  dilute   solution  of   a   manganous   salt   is   treated   with 
bromine  water  and  boiled,  the  reaction 

MnCl2  +  Br2  +  2H2O<=±MnO2  +  2HC1  +  2HBr 

does  not  take  place  unless  the  halogen  acids  are  neutralized  as 
fast  as  they  are  formed.  This  neutralization  can  be  accom- 
plished by  means  of  the  salt  of  a  weak  acid,  such  as  sodium 
acetate,  even  when  the  solution  contains  free  acetic  acid,  which 
is  scarcely  ionized  at  all  in  the  presence  of  its  alkali  salt.  Thus 
in  a  solution  such  as  is  obtained  after  the  removal  of  iron  and 
aluminium  b'y  a  basic  acetate  separation  (cf.  p.  152);  the  man- 
ganese can  be  precipitated  quantitatively  by  boiling  with  an 
excess  of  bromine  water.  The  oxide  does  not  correspond  exactly 
to  Mn02,  although  most  of  the  manganese  is  in  the  quadrivalent 


124  GRAVIMETRIC  ANALYSIS. 

condition.*  When  the  precipitate  has  collected  together  in  large 
flocks,  the  boiling  is  discontinued  and  the  precipitate  allowed  to 
settle;  it  is  filtered  and  washed  with  hot  water.  Some  chemists 
ignite  this  precipitate  and  weigh  as  Mn304  but  it  is  more  accurate 
to  dissolve  the  precipitate  in  a  mixture  of  HC1  and  H2S08  and  to 
precipitate  the  manganese  finally  as  manganese  ammonium 
phosphate.  (See  4,  p.  126.) 

Chlorine,  hydrogen  peroxide,  hypochlorites,  etc.,  may  be  used 
instead  of  bromine,  but  these  reagents  have  no  especial  advan- 
tages. 

When  the  solution  of  the  manganous  salt  contains  ammonium 
salts,  the  precipitation  of  the  manganese  does  not  take  place  by 
the  above  procedure,  because  the  sodium  acetate  serves  rather 
to  neutralize  the  acid  set  free  by  the  following  reaction: 

2NH4C1  +  3Br2  =  N2  +  2HC1  +  6HBr. 

Upon  the  addition  of  ammonia,  however,  the  precipitation  of  the 
manganese  can  be  effected.  In  this  case,  it  seems  fair  to  assume 
that  the  reaction  goes  through  the  following  stages: 

MnCl2  +  2NH4OH<=±Mn  (OH)  2  +  2NH4C1, 


The  precipitation  with  bromine  and  ammonia  is  not  so  satis- 
factory as  with  bromine  alone  in  the  presence  of  acetic  acid  and 
sodium  acetate  and  in  the  absence  of  ammonia  or  ammonium 
salt,  because  when  ammonia  is  present  much  of  the  bromine  is 
used  up  in  oxidizing  the  ammonia  or  ammonium  salt.  In  that  case 
there  is  considerable  solution  of  nitrogen,  and,  moreover,  when 
an  excess  of  bromine  is  added  the  solution  may  become  acid 
enough  to  dissolve  the  precipitated  manganese: 

2NH,  +  3Br9  =  6HBr  +  N,. 


*  The  MnO2  acts  as  the  anhydride  of  metamanganous  acid,  H2MnO3,  and 
some  manganous  manganite,  MnMnO3  or  Mn2O3,  is  contained  in  the  pre- 
cipitate. 


DETERMINATION   OF  MANGANESE.  12$ 

It  is  necessary,  therefore,  when  ammonium  salts  are  present 
to  make  sure  that  the  solution  is  ammoniacal  at  the  end  of  the 
operation. 

This  method  of  precipitating  manganese  from  solutions  pos- 
sesses disadvantages  which  make  it  useless  in  many  cases.  If, 
besides  manganese,  the  solution  contains  calcium,  zinc,  etc.,  man- 
ganites  of  these  metals  are  precipitated  with  the  manganese.  In 
this  case  the  precipitate  must  be  dissolved  in  hydrochloric  acid 
and  the  precipitation  repeated  several  times,  but  even  then  it  is 
not  possible  to  obtain  a  precipitate  altogether  free  from  these 
metals.  If  the  other  metals  are  present  only  in  small  amounts,  the 
results  obtained  by  this  method  are  sufficiently  accurate.  The 
precipitation  of  manganese  as  sulphide  in  the  presence  of  other 
metals  is  always  satisfactory  and  should  be  used  in  almost  all  cases. 

2.   Determination  of  Manganese  as  Sulphide. 

If  the  manganese  has  been  precipitated,  as  described  on  p. 
122  as  sulphide,  the  precipitate  is  separated  from  the  filter  as 
completely  as  possible,  placed  in  a  Rose  crucible  (of  unglazed 
porcelain),  the  filter  is  burned  in  a  platinum  spiral,  and  the  ash 
added  to  the  main  portion  of  the  precipitate.  Some  pure  sulphur 
which  has  been  crystallized  from  CS2  is  added,  after  which  the 
crucible  and  its  contents  are  heated  in  a  current  of  hydrogen  by 
means  of  a  Bunsen  burner  exactly  as  described  under  the  Deter- 
mination of  Copper  as  Sulphide.  After  the  excess  of  sulphur  has 
distilled  off  and  been  burned,  the  crucible  is  cooled  in  a  stream 
of  hydrogen  and  the  precipitate  is  weighed  as  MnS. 


3.  Determination  of  Manganese  as 

Inasmuch  as  all  the  oxides  of  manganese,  as  well  as  those 
compounds  which  are  converted  into  oxide  on  ignition  (manga- 
nous  salts  of  volatile  organic  and  inorganic  acids,  with  the  ex- 
ception of  the  halogen  salts),  are  converted  into  Mn3O4*  on  being 
ignited  in  the  air,  at  temperatures  between  940°  and  1100°,  it 

*  Cf.  R.  J.  Meyer  and  K.  Retgers,  Z.  anorg.  Chem.,  57,  104  (1908),  at 
530°  the  oxides  of  manganese  are  slowly  but  quantitatively  changed  into 
Mn2O3. 


126  GRAVIMETRIC  ANALYSIS. 

follows  that  this  method  for  the  determination  of  manganese  is 
quite  generally  applicable.  It  is  nearly  as  accurate  as  the 
methods  described  under  1  and  2,  if  the  ignition  of  the  precipitate 
takes  place  in  an  electric  furnace  at  about  1000°,  but  very  good 
results  are  obtained  if,  as  recommended  by  Gooch,  *  the  porcelain 
crucible  (containing  the  carbonate,  manganous  manganite,  or 
sulphide)  is  entirely  surrounded  by  the  oxidation  flame  of  a 
Teclu  burner,  whereby  a  moderately  high  heat  is  obtained  with- 
out too  much  free  access  of  air.f 

After  the  ignition,  the  crucible  and  its  contents  are  cooled  in 
a  desiccator  and  then  weighed.  From  the  weight  p  of  the 
oxide,  the  amount  of  manganese  can  be  calculated  according  to 
the  equation 


3Mn 


4.  Determination  of  Manganese  as  Manganese  Pyrophosphate, 

Mn2P207. 

This  excellent  method  was  recommended  by  W.  GibbsJ  and 
subsequently  studied  by  Gooch  and  Austin.  § 

The  slightly  acid  solution,  containing  an  amount  of  manganese 
corresponding  to  not  over  0.5  gm.  Mn2P2O7,  and  no  other  metals 
except  alkalies,  is  treated  with  20  gm.  ammonium  chloride,  5  to  10 
c.c.  of  a  cold  saturated  solution  of  sodium  phosphate,  and  ammonia, 
drop  by  drop,  until  a  slight  excess  is  present.  The  solution  is  heated 

*Zeit.  f.  anorg.  Chem.,  XVII  (1898),  p.  268. 

f  To  illustrate  the  accuracy  of  the  three  methods  just  described  for  the 
determination  of  manganese,  the  following  results  obtained  by  H.  Weitnauer 
are  given.  He  obtained  after  making  six  determinations  by  each  method 
the  following  mean  values:  50  c.c.  of  a  pure  manganese  sulphate  solution 
treated  with  ammonium  carbonate  and  changing  the  precipitate  to  sulphate 
gave  0.1025  gm.  Mn;  by  precipitating  as  sulphide  and  weighing  as  such, 
0.1027  gm.  Mn;  and  by  changing  the  sulphate  to  Mn3O4,  0.1029  gm.  Mn. 

J  Am.  J.  Science,  46,  216;  Z.  anal.  Chem.,  7,  101  (1868). 

§  Z.  anorg.  Chem.,  18,  339  (1898). 


COLOR/METRIC   DETERMINATION   OF  MANGANESE.          127 

to  boiling  and  kept  at  this  temperature  for  three  or  four  minutes, 
or  until  the  precipitate  assumes  a  silky,  crystalline  appearance. 
After  cooling,  the  precipitate  is  filtered  through  a  Gooch  or  Munroe 
crucible,  washed  with  cold  ammonium  nitrate  solution,  dried, 
and  ignited  within  a  larger  crucible  or  in  an  electric  furnace. 

The  results  are  excellent. 

Manganese  can  be  determined  very  accurately  by  volumetric 
methods  (see  Volumetric  Analysis). 

5.  Colorimetric  Determination  of  Manganese. 

Small  amounts  of  manganese  may  be  accurately  and  quickly 
determined  by  the  colorimetric  method.  This  is  chiefly  used  for 
the  estimation  of  the  manganese  present  in  iron  arid  steel.  If 
more  than  1.5  per  cent,  of  manganese  is  present,  the  results  are 
unreliable.  The  method  depends  upon  the  oxidation  of  the 
manganese  to  permanganic  acid,  bringing  the  solution  to  a  definite 
volume  and  comparing  its  color  with  another  solution  containing 
a  known  amount  of  manganese.  If  the  solutions  are  colored 
exactly  the  same  shade,  then  the  amounts  of  manganese  which 
they  contain  are  the  same. 

Procedure. — A  standard  solution  of  potassium  permanganate 
is  first  prepared  by  dissolving  0.072  gm.  of  the  crystallized  salt  in 
500  c.c.  of  water;  1  c.c.  of  this  solution  contains  0.05  mgm.  of 
manganese. 

Exactly  0.2  gm.  of  the  iron  or  steel  is  dissolved  in  15-20  c.c. 
of  nitric  acid  (sp.  gr.  1.2)  in  a  100-c.c.  measuring-flask.  The  acid 
is  heated  to  boiling  to  effect  complete  solution,  after  which  the 
solution  is  allowed  to  cool  and  diluted  up  to  the  mark  with  water. 
After  thoroughly  mixing,  10  c.c.  of  the  liquid  are  brought  by  means 
of  a  pipette  into  a  small  beaker,  2  c.c.  of  nitric  acid  (sp.  gr.  1.2) 
are  added,  and  the  liquid  is  heated  until  it  begins  to  boil,  when  the 
flame  is  removed,  0.5  gm.  of  lead  peroxide  is  added,  the  mixture 
is  shaken  and  then  heated  for  two  minutes  to  boiling.  After 
standing  some  time,  the  warm,  violet-colored  solution  is  filtered 
through  a  small  asbestos  filter*  into  a  glass-stoppered  test-tube 

*  The  asbestos  must  have  been  previously  ignited,  treated  with  KMnO4 
solution,  and  finally  washed  with  water. 


128  GRAVIMETRIC  ANALYSIS. 

about  20  cm.  high,  and  graduated  in  cubic  centimeters.  The 
filter  is  washed  with  as  little  water  as  possible,  the  tube  is  stoppered 
and  shaken  until  the  solution  is  thoroughly  mixed.  Into  a 
second  tube  of  the  same  size,  and  also  graduated  in  cubic  centi- 
meters, is  placed  1-5  c.c.  of  the  standard  manganese  solution, 
and  this  is  carefully  diluted  with  water  until  the  two  liquids  have 
exactly  the  same  shade  when  viewed  horizontally.  The  height 
of  the  liquid  in  each  tube  is  then  carefully  read. 

Assuming  that  1  c.c.  of  the  standard  solution  were  placed 
in  the  cylinder  and  diluted  to  T  c.c.  in  order  to  obtain  the  same 
shade  produced  by  t  c.c.  of  the  other  solution,  then  as  the  con- 
centrations of  the  two  liquids  are  directly  proportional  to  their 
heights, 

T:t  =  OM  mgm.::r 

_£1O05_mgm: 


This  amount  of  manganese  is  contained  in  0.02  gm.  of  the 
iron,  so  that  the  percentage  of  manganese  present  is 


=    7=er  cent.  Mn. 


Rather  more  accurate  results  are  obtained  if,  instead  of  using 
a  standard  solution  obtained  from  potassium  permanganate,  a 
sample  of  steel  is  used  containing  a  known  amount  of  manganese 
and  treated  in  exactly  the  same  way  as  the  sample  to  be  analyzed, 
a  fresh  standard  being  prepared  for  each  analysis. 

An  even  better  colorimetric  method  has  been  devised  by  M. 
Marshall  *  and  H.  E.  Walters,  f 

Although  manganese  is  precipitated  as  manganous  acid,  from 
solutions  slightly  acid  with  nitric  or  sulphuric  acid,  by  the 
addition  of  alkali  persulphates,  the  oxidation  goes  farther  and 

*Chem.  News,  83,  76  (1904). 
t  Ibid.,  84,  239  (1904). 


DETERMINATION  OF  NICKEL   AS  NICKEL   GLYOXIME         129 

permanganic  acid  is  formed  within  a  short  tune  if  a  catalytic  agent 
is  present,  °nch  as  silver  nitrate. 

2Mn(XO3)  2  +  5  (XH4)  2S2O8  +  8H2O  = 

5  (NH4)  2SO4  +  5H2SO4 + 4HNO3 + 2HMn04. 

Procedure. — 0.2  gm.  of  steel  is  placed  in  a  100-c.c.  flask  and 
dissolved  in  20  c.c.  of  cold  nitric  acid  (sp.  gr.  1.2).  Thereupon 
10  c.c.  of  silver  nitrate  solution  (1.38  gm.  AgXOa  in  a  liter  of 
water),  are  added,  the  solution  made  up  to  exactly  100  c.c.  and 
mixed.  Of  this  solution  10  c.c.  are  placed  in  glass-stoppered, 
graduated  test-tube.  After  adding  2.5  c.c.  of  ammonium  per- 
sulphate solution  (200  gm.  in  a  liter  of  water)  the  test-tube  is 
placed  in  water  at  80°  to  90°  and  allowed  to  remain  there  until 
the  bubbles  of  gas  arising  become  more  numerous  and  remain  at 
the  top  for  a  few  seconds.  The  solution  is  then  cooled  by  placing 
the  tube  in  cold  water,  and  the  color  is  compared  with  a  standard 
solution  containing  a  known  amount  of  permanganic  acid.* 


NICKEL,  Ni.    At.  Wt.  58.68. 

Forms:   Nickel  Dimethyl  Glyoxime,  NiC8H14N404;  Nickel, Ni;  and 
Nickel  Oxide,  NiO. 

i.  Determination  as  Nickel  Glyoxime,  Ni(C4H7N2O2)2. 

Dimethyl  glyoxime,  CH3-CXOH-CXOH-CH3,  was  recom- 
mended by  L.  Tschugaeff  f  as  a  reagent  for  nickel  and  used  by 
K.  Kraut  J  for  detecting  the  presence  of  traces  of  nickel  in  ashes. 
O.  Brunck  §  and  others  have  also  studied  the  reaction  and  found 
it  to  furnish  a  most  rapid  and  accurate  method  for  the  quanti- 
tative estimation  of  nickel  either  by  itself  or  in  the  presence  of 
cobalt,  zinc  and  manganese.  If  the  solution  contains  tartaric 
acid  enough  to  prevent  the  precipitation  of  iron  by  ammonia,  the 

*  Or  better  a  solution  of  a  steel  of  known  manganese  content. 
t  Z.  anorg.  Chem.,  46,  144  (1905);   Ber.,  38,  2520  (1905). 
J  Z.  angew.  Chem.,  19,  1793  (1906);  ibid.,  20,  1844  (1907). 
§  Ibid.,  20,  834  (1907). 


130  GRAVIMETRIC  ANALYSIS. 

nickel  in  a  sample  of  nickel  steel  can  be  determined  accurately 
within  two  hours  and  without  the  removal  of  any  other  metal. 
When  a  dilute,  neutral  solution  of  a  nickel  salt  is  treated  with 
an  alcoholic  solution  of  dimethyl  glyoxime,  a  red,  crystalline 
precipitate  of  nickel  dimethyl  glyoxime  is  formed. 

2(CH3)2C2(NOH)2  =  (CH3)2C2(NO)2Ni.  (CH3)2C2(NOH)2 

Dimethyl  glyoxime.  Nickel  dim3thyl  glyoxime. 

The  salt  is  soluble  in  mineral  acids  so  that  precipitation  is 
incomplete  because  of  the  acid  set  free  in  the  reaction.  It  be- 
comes quantitative,  however,  if  the  mineral  acid  is  neutralized 
by  ammonia  or  if  sodium  acetate  is  added,  whereby  the  mineral 
acid  is  replaced  by  acetic  acid  in  which  the  precipitate  is  prac- 
tically insoluble.  Large  quantities  of  ammonium  salts  or  of 
alkali  acetate  do  no  harm,  but  an  excess  of  ammonia  tends  to 
prevent  the  formation  of  the  precipitate.  The  precipitate  is 
distinctly  soluble  in  absolute  alcohol,  but  only  traces  dissolve 
in  50  per  cent,  alcohol,  and  in  more  dilute  alcohol  it  is  even  leas 
soluble.  When  thrown  down  in  the  cold  or  in  the  presence  of 
much  free  ammonia  the  precipitate  is  very  voluminous  and  hard 
to  filter. 

Procedure. — The  neutral  or  slightly  acid*  solution  is  diluted 
so  that  not  more  than  0.1  gm.  of  cobalt  is  present  in  100  c.c., 
heated  nearly  to  boiling  and  treated  with  somewhat  more  than 
the  theoretical  amount  of  an  alcoholic  1  per  cent,  solution  of 
dimethyl  glyoxime. f  Ammonia  is  then  cautiously  added  until 
the  solution  smells  slightly.  While  still  hot,  the  precipitate  is 
filtered  into  a  Gooch  or  Munroe  crucible,  washed  with  hot  water 
and  dried  at  110°  to  120°  for  45  minutes.  It  contains  20.31  per 
cent.  Ni. 

The  nickel  salt  of  dimethyl  glyoxime  is  red  and  crystalline. 
It  contains  no  water  of  crystallization  and  sublimes  at  250° 
without  decomposition. 

*  If  strongly  acid,  the  solution  is  nearly  neutralized  with  caustic  potash, 
then  the  reagent  is  added,  etc. 

t  The  volume  of  the  alcoholic  solution  should  in  no  case  be  more  than 
half  that  of  the  nickel  solution,  as  the  precipitate  is  soluble  in  alcohol. 


DETERMINATION  OF  NICKEL  AS  METAL  BY  ELECTROLYSIS.      131 

2.  Determination  of  Nickel  as  Metal  by  Electrolysis. 

From  strongly  acid  solutions  nickel  is  not  deposited  upon 
stationary  electrodes  by  a  current  of  1-3  amperes.  From  slightly 
acid  solutions  the  deposition  is  not  quantitative. 

From  ammoniacal  solutions  nickel  is  readily  deposited,  and  on 
account  of  its  simplicity  and  accuracy  this  method  is  to  be 
strongly  recommended  for  the  determination  of  nickel. 

(a)  Method  of  Gibbs.  * 

Xickel  sulphate  or  chloride  (but  not  the  nitrate)  is  dissolved  in 
an  ammoniacal  solution  of  ammonium  sulphate  and  electrolyzed. 

The  nickel  is  deposited  upon  a  weighed  cathode  and,  at  the 
end  of  the  electrolysis,  the  gain  in  weight  represents  the  quantity 
of  nickel. 

Requirements  and  Procedure. — For  this,  as  well  as  for  all  other 
electrolytic  determinations,  the  apparatus  shown  in  Fig.  31  may 
be  used. 

B  represents  a  storage-battery,  which  is  provided  with  the 
binding  posts  MM.  The  current  is  led  first  through  the  variable 
resistance  TT,  then  through  the  known  resistance  W  (a  resistance 
of  1  ohm  is  most  convenient  to  use  f),  and  from  here  to  the 
decomposition  cell  finally  back  to  the  battery. 

If  at  any  time  it  is  desired  to  measure  the  voltage  between  the 
electrodes  of  the  cell,  the  voltmeter  V  is  connected  with  the 
binding  posts  of  the  decomposition  cell,  by  throwing  the  switch 
Q,%  Fig.  32,  so  that  c  is  connected  with  6  and  c'  with  6'.  The 

*  Z.  anal.  Chem.,  3,  334  (1864).  Cf.  Fresenius  and  Bergmann,  Z.  anal. 
Chem.,  19,  320  (1880). 

t  The  resistance  w'=lQ  can  be  made  from  nickelin  wire.  The  resistance 
of  the  wire  is  measured  with  the  aid  of  the  Wheat  stone  bridge  and  a  length 
cut  off  corresponding  to  one  ohm.  This  wire  is  wound  round  a  wooden  block 
and  the  ends  fastened  to  binding  posts. 

+  If  a  commutating  switch  is  not  available,  one  can  be  prepared  by 
taking  the  cover  of  a  pasteboard  box,  about  2  cm.  deep,  fining  it  with  melted 
paraffin,  and  then,  after  cooling,  making  little  cavities  at  aa'Wce'  by 
pressing  a  test-tube,  which  is  filled  with  hot  water,  against  the  cold  wax. 
These  cavities  are  filled  with  mercury  and  the  switch  finished  with  copper 
wire.  Cf.  Fig.  36,  p.  178. 


132 


GRAVIMETRIC  ANALYSIS. 


FIG.  31. 


DETERMINATION  OF  NICKEL  AS  METAL  BY  ELECTROLYSIS.     133 

strength  of  the  current,  on  the  other  hand,  is  obtained  by  placing 
the  switch  in  the  opposite  position  with  a  united  to  6  and  a'  to 
&',  as  shown  in  Figs.  31,  32. 


FIG.  32. 
Since,  according  to  Ohm's  law,  the 

Electromotive  force 

Strength  of  current  =  -  ^  —  -.  -  , 

Resistance 

then  if  the  strength  of  the  current  is  expressed  in  amperes,  the 
electromotive  force  in  volts,  and  the  resistance  in  ohms,  we  have 


In  case  W  =  1  ohm,  then 

A.-E, 

and  the  voltmeter  will  show  directly  the  strength  of  the  current 
(amperes)  .  * 

It  is  arranged  so  that  the  current  may  be  taken  from  different 
points  along  MM,  and  it  is  thus  possible  to  carry  out  a  number 
of  electrolytic  determinations  at  the  same  time,  and  the  volt- 
meter V  serves  as  measuring  instrument  for  all  the  analyses  that 
are  in  progress.  By  means  of  the  SS  it  is  possible  to  connect 
easily  the  voltmeter  with  the  different  cells.  While  a  measure- 

*  With  weaker  currents,  the  known  resistance  can  be  made  TF'=  10  Q 
ao  that  the  voltmeter  will  show  ten  times  the  actual  current. 


134 


GRAVIMETRIC  ANALYSIS. 


ment  is  being  made  at  any  cell,  all  other  switches  must  be  cut 
out  of  circuit. 

The  decomposition  cell  consists  of  a  glass  beaker  in  which  is 
placed  as  cathode  a  wire  gauze  electrode  (first  recommended  by 
Cl.  Winkler)  and  as  anode  a  platinum  spiral.  The  electrodes 


FIG.  33. 

must  always  reach  to  the  bottom  of  the  beaker  and  the  top  of 
the  gauze  electrode  should  be  nearly  covered  by  solution.  In 
some  cases  it  is  desirable  to  use  a  platinum  dish  as  cathode,  as 
recommended  by  Classen.  (See  Fig.  36,  p.  178.) 

The  electrodes  are  usually  connected  with  two  electrode 
stands  on  which  metal  arms  are  attached  to  an  upright  glass  rod 
(Fig.  33).  To  prevent  serious  loss  of  electrolyte  by  spattering, 


DETERMINATION  OF  NICKEL  AS  METAL  BY  ELECTROLYSIS.     135 

the  beaker  is  covered  with  two  halves  of  a  watch-glass.  This  is 
not  entirely  satisfactory,  as  when  much  gas  is  evolved  a  little  of 
the  liquid  is  still  carried  off  mechanically.  This  method  of 
fastening  the  electrodes,  moreover,  has  the  disadvantage  that 
when  the  electrolysis  is  carried  out  in  a  hot  solution,  acid  or 
ammoniacal  vapors,  as  the  case  may  be.  condense  on  the  brass 


FIG.  34. 

arms  of  the  electrode  support  and  in  some  cases  the  liquids  thus 
condensed  dissolve  some  brass  and  the  resulting  solution  may 
drop  into  the  beaker,  and  spoil  the  analysis.  To  prevent  this 
misfortune,  the  author  bends  the  ends  of  the  electrodes  to  a 
right  angle  and  connects  them  with  an  electrode  stand  designed 
as  shown  in  Fig.  34. 

This  electrode  holder  consists  of  two  brass  rods  insulated 
from  one  another  by  means  of  an  intervening  layer  of  mica,  and 
the  rods  are  fastened  to  the  ring  r  through  a  piece  of  ebonite,  e. 


136  GRAVIMETRIC  ANALYSIS. 

The  openings  to  hold  the  wires  are  cut  wedge-shaped,  so  that  any 
shape  of  wire  can  be  inserted. 

Since  the  ends  of  the  electrodes  leave  the  beaker  in  a  horizontal 
direction,  the  beaker  can  be  covered  tightly  by  means  of  a  whole 
watch-glass,  and  not  only  are  losses  by  spattering  avoided,  but 
there  is  absolutely  no  danger  of  contamination  from  the  outside. 

The  Electrolysis  of  Nickel. 

For  every  0.25-0.30  gm.  nickel,  present  as  sulphate  or  chloride, 
but  not  as  nitrate,  *  5-10  gm.  of  ammonium  sulphate  and  30-40  c.c. 
of  concentrated  ammonia  are  added,  and  the  solution  diluted  with 
distilled  water  to  a  volume  of  150  c.c.  This  solution  is  electrolyzed 
at  the  room  temperature  with  a  current  of  0.5-1.5  amperes  and  an 
electrode  potential  of  2.8-3.3  volts.  The  electrolysis  is  finished 
after  three  hours,  as  can  be  shown  fairly  satisfactorily  by  adding 
a  little  water  and  allowing  the  current  to  pass  through  the  solution 
for  fifteen  or  twenty  minutes  longer.  If  at  the  end  of  this  time 
no  nickel  has  deposited  upon  the  electrode  surface  which  was 
wet  for  the  first  time  by  the  last  dilution,  the  determination  is 
finished.  If  the  solution  is  kept  at  a  temperature  of  from 
50°-60°  C.  only  about  one  hour  is  necessary  for  the  deposition. 
The  deposited  metal  adheres  firmly  to  the  electrode,  is  bright, 
and  possesses  almost  the  color  of  platinum. 

As  soon  as  the  electrotysis  is  finished,  the  watch-glass  is 
removed,  the  electrode  holder  is  raised  so  that  only  the  bottoms 
of  the  electrodes  remain  in  the  liquid,  and  the  upper  parts  of 
the  electrodes  are  washed  thoroughly  with  water  from  a  wash- 
bottle.  The  electrodes  are  then  raised  entirely  out  of  the  solution 
and  the  bottoms  washed  immediately  with  water.  The  current 
is  then  turned  off  and  the  cathode  rinsed  with  absolute  alcohol, 
after  which  it  is  dried  by  holding  it  high  above  a  gas  flame.  After 
cooling  in  a  desiccator,  it  is  weighed. 

To  clean  the  cathode,  place  it  in  a  small  beaker,  add  enough 
nitric  acid  (1:1)  to  wet  all  the  nickel,  and  heat  for  at  least  fifteen 
minutes.  This  treatment  is  absolutely  necessary  to  remove  the 
last  traces  of  nickel.  If  this  is  not  done,  the  electrode  on  being 

*  Page  131. 


THE  ELECTROLYSIS   OF  NICKEL.  137 

ignited  becomes  discolored,  and  it  is  then  very  difficult  to  clean 
the  electrode  by  repeated  treatment  with  acid  followed  by  ignition. 
The  discolored  electrode,  however,  can  be  used  for  another 
electrolytic  determination.  To  make  sure  that  all  the  nickel 
has  been  deposited  from  the  electrolyzed  solution,  the  ammonia 
is  almost  wholly  neutralized  with  hydrochloric  acid  and  a  few 
cubic  centimeters  are  added  of  1  per  cent,  solution  of  dimethyl 
glyoxime  in  alcohol.  When  less  than  a  tenth  of  a  milligram  of 
nickel  is  present,  it  will  take  several  minutes  for  a  yellow  coloration 
to  appear,  and  soon  afterward  the  red  crystals  of  nickel  salt 
will  be  precipitated. 

The  nickel  not  deposited  by  an  electrolysis  may  be  estimated 
accurately  by  shaking  the  solution  thoroughly  and  comparing 
the  color  produced  by  the  addition  of  dimethyl  glyoxime  with 
that  produced  with  a  dilute  nickel  solution  containing  a  known 
quantity  of  nickel.  Naturally  such  a  colorimetric  test  can  be 
used  only  with  very  small  quantities  of  nickel. 

Remark. — The  electrolysis  of  nickel  from  an  ammoniacal 
solution  should  not  be  continued  for  too  long  a  time,  because  the 
cathode  slowly  gains  in  weight  even  after  all  the  nickel  has  been 
deposited  from  the  solution.  The  anode  is  attacked,  causing 
platinum  to  go  into  solution,  which  is  deposited  upon  the  cathode, 
partially,  at  least. 

The  presence  of  too  little  ammonia  often  results  in  the  forma- 
tion of  black  Xi(OH)3  at  the  anode;  the  analysis  then  comes  out 
too  low. 

Classen's  method  for  depositing  nickel  from  a  solution  of 
ammonium  oxalate  apparently  gives  too  high  results  *  and 
cannot  be  recommended. 

3.  Determination  as  Nickelous  Oxide. 

The  nickel  solution  is  heated  in  a  porcelain  dish  with  bromine 
water  and  an  excess  of  pure  potassium  hydroxide,  whereby  the 
nickel  is  precipitated  as  brownish-black  nickelic  hydroxide, 
Xi(OH)3.  The  precipitate  is  filtered  off,  washed  by  decantation 

*  A.  Windelschmidt,  Dissertation,  Minister,  1907.  W.  D.  Treadwell,  Dis- 
sertation, Zurich,  1909. 


138  GRAVIMETRIC  ANALYSIS. 

with  hot  water,  dried,  and,  after  burning  the  filter,  ignited 
and  weighed  as  NiO.  The  grayish-green  oxide  thus  obtained 
always  contains  small  quantities  of  silicic  acid  and  alkali,* 
whereby  the  results  are  too  high.  By  treating  the  ignited  mass 
with  hot  water,  the  greater  part  of  the  alkali  can  be  removed. 
Drying  and  again  igniting  gives  the  weight  of  NiO  +  SiO2.  The 
oxide  is  treated  in  a  porcelain  crucible  with  hydrochloric  acid, 
evaporated  completely  to  dryness,  the  dry  residue  moistened 
with  concentrated  hydrochloric  acid  and  then  with  hot  water, 
filtered  through  a  small  filter,  washed  with  hot  water,  and  the 
filter  together  with  the  residue  ignited  wet  in  a  platinum 
crucible.  The  weight  of  this  silica,  SiO2,  subtracted  from  the 
former  weight  of  NiO  +  Si02,  gives  good  results. 

Remark. — It  is  possible  to  precipitate  nickel  quantitatively 
as  Ni(OH)2  by  means  of  caustic  potash  alone  and  the  precipitate 
is  changed  to  NiO  by  ignition.  This  method  is  open  to  the  same 
objections  as  the  above  and,  furthermore,  Ni(OH)2is  not  so  easily 
filtered  and  washed  as  Ni(OH)3. 

These  two  methods  are  more  tedious  to  carry  out  and  the 
results  are  not  as  accurate  as  in  the  case  of  the  first  two  methods 
described  and  will  probably  not  be  used  much  in  the  future. 

Besides  the  methods  described,  it  has  been  proposed  to  pre- 
cipitate nickel  as  the  sulphide,  and  weigh  it  as  the  oxide  by 
ignition  in  air.f  The  method  is  good  but  hardly  comparable 
with  the  dimethyl  glyoxime  method,  the  electrolytic  method,  or 
the  volumetric  titration  with  potassium  cyanide. 

COBALT,  Co.    At.  Wt.  58.97. 

Forms:  Co,  CoS04. 
i.  Determination  as  Metal. 

(a)  By  Electrolysis. 

The  most  accurate  method  for  the  estimation  of  cobalt  is  by 
electrolysis  and  the  details  of  the  process  are  precisely  the  same 
as  have  been  given  under  nickel,  i.  e.,  from  a  strongly  ammoniacal 

*  Cf.  A.  Windelschmidt,  loc.  cit.  and  W.  D.  Treadwell,  loc.  tit. 
t  H.  Cormimboef,  Ann.  chim.  appl.,  II,  6  (1906).     Cf.  A.  Windelschmidt, 
loc.  tit. 


DETERMINATION  OF  COBALT  AS  METAL.  139 

solution  containing  ammonium  salts  and  the  sulphate  or  chloride 
(preferably  the  former) ,  of  cobalt.  It  is  customary  to  use  a  little 
more  ammonia  than  in  the  determination  of  nickel,  because 
cobalt  has  a  greater  tendency  to  deposit  as  black  Co(OH)3  at  the 
anode.  The  duration  of  the  electrolysis  is  the  same  as  with 
nickel,  rather  than  somewhat  longer.  At  the  end  of  the  deter- 
mination, after  the  electrodes  have  been  removed,  the  entire 
solution  is  tested  for  cobalt  by  adding  ammonium  sulphide  or 
potassium  sulphocarbonate. 


(6)  By  Reduction  of  the  Oxide  in  a  Stream  of  Hydrogen. 

The  cobalt  solution  is  heated  to  boiling  in  a  porcelain  evaporat- 
ing-dish,  and  the  cobalt  is  precipitated  as  black  cobaltic  hydroxide 
by  the  addition  of  caustic  potash  and  bromine  water.  The  pre- 
cipitate is  filtered  off,*  dried,  and  ignited.  After  cooling  it 
is  treated  with  water  in  order  to  remove  the  small  amount  of 
alkali  which  is  always  present,  and  then  the  residue  is  ignited 
in  a  stream  of  hydrogen  and  weighed  as  metal.  After  weighing, 
the  metal  is  dissolved  in  hydrochloric  acid,  evaporated  to  dryness, 
the  dry  mass  moistened  with  hydrochloric  acid,  then  treated  with 
water,  and  the  small  residue  of  silicic  acid  filtered  off.  This  resi- 
due is  ignited  and  its  weight  subtracted  from  that  obtained  after  the 
ignition  in  hydrogen.  Cobalt  may  also  be  precipitated  as  cobaltous 
hydroxide  by  caustic  potash  alone,  but  the  resulting  precipitate 
is  not  so  easy  to  filter  and  wash  as  the  cobaltic  hydroxide.  The 
precipitation  by  means  of  sodium  carbonate  is  not  so  satisfactory. 

^he  oxides  of  cobalt  when  ignited  in  air  yield  a  mixture  of  CoO 
and  Go304  in  varying  proportions,  so  that  they  are  not  suited  for 
the  quantitative  determination  of  cobalt. 

Remark. — The  results  obtained  by  this  method  are  usually  a 
little  higher  than  by  electrolysis. 


*  Cobaltic  hydroxide,  unlike  nickelic  hydroxide,  has  the  tendency  of 
giving  a  turbid  filtrate  on  washing.  If,  however,  Schleicher  &  Schiill's 
filter-paper  No.  589  (blue  band)  is  used,  none  of  the  precipitate  passes 
through. 


I4o  GRAVIMETRIC  ANALYSIS. 

2.  Determination  as  Sulphate. 

The  method  is  the  same  as  was  described  under  Manganese 
(p.  104). 

ZINC,  Zn.     At.  Wt.  65.37. 
Forms:    ZnNH4P04,  Zn2P2O7,  ZnO,  ZnS,  Zn. 

i.  Determination  as  Zinc  Ammonium  Phosphate  or  Zinc 
Pyrophosphate. 

This  excellent  method,  first  recommended  by  H.  Tamm,  * 
has  been  studied  and  improved  by  G.  Losekann  and  T.  Meyer, f 
M.  Austin, J  and  especially  H.  D.  Dakin.§ 

Procedure. — The  cold  acid||  solution  of  the  zinc  salt  is  treated 
with  ammonia,  in  a  platinum  or  porcelain  dish,  until  it  is  left 
barely  acid.  Care  is  necessary  at  this  point,  as  zinc  ammonium 
phosphate  is  soluble  both  in  acids  and  ammonia.  It  is  then 
diluted  with  water  to  a  volume  of  150  c.c.  and  heated  on  the  water- 
bath.  To  the  hot  solution,  ten  times  as  much  ammonium  phos- 
phate is  added  as  there  is  zinc  present.  (If  the  diammonium 
phosphate  contains  some  monoammonium  phosphate,  the  salt 
should  be  dissolved  in  cold  water  and  dilute  ammonia  added  until 
the  solution  just  becomes  pink  with  phenolphthalein.)  The 
precipitate  that  first  forms  is  amorphous,  but  it  soon  changes  into 
fine  crystals  of  zinc  ammonium  phosphate.  The  transformation 
takes  place  more  rapidly  in  proportion  to  the  quantity  of  ammo- 
nium salts  present.  After  the  heating  has  continued  for  about 
fifteen  minutes,  the  dish  is  removed  from  the  water-bath  and  after 
being  allowed  to  settle  for  a  short  time  the  precipitate  is  filtered 
through  a  Gooch  or  Munroe  crucible,  washed  with  hot,  1  per 
cent,  ammonium  phosphate  solution  1f  until  free  from  chlorides 

*  Chem.  News,  24,  148. 

f  Chem.  Ztg.,  1886,  729. 

J  Am.  J.  Sci.,  1899;  Z.  anorg.  Chem.,  22,  212  (1900). 

§  Z.  anal.  Chem.,  39,  273  (1900). 

||  If  the  solution  is  neutral,  2  or  3  gins,  of  ammonium  chloride  are  added 
and  then  the  analysis  carried  out. 

^  According  to  Voigt,  Z.  angew.  Chem.,  1909,  2282,  the  precipitate  is 
washed  immediately  with  hot  water. 


DETERMINATION  OF  ZINC.  141 

etc.,  then  twice  with  cold  water,  then  with  50  per  cent,  alcohol, 
dried  at  110-120°  for  an  hour  and  weighed  as  ZnNH4PO4,  which 
contains  36.64  per  cent.  Zn. 

Or,  the  precipitate  may  be  weighed  as  the  pyrophosphate, 
Zn2P2O7,  in  which  case  the  dried  zinc  ammonium  phosphate  is 
heated  very  slowly  in  an  electric  oven  to  900°-1000°.  If  such 
an  oven  is  not  at  hand,  the  Gooch  or  Munroe  crucible  is  placed 
in  a  larger  platinum  crucible  and  heated  over  the  gas  flame. 
The  temperature  is  gradually  raised  until  finally  the  full  heat  of 
the  Teclu  burner  or  of  the  blast  lamp  is  reached.  The  crucible 
is  heated  until  its  weight  is  constant.  Zn2P2O7  contains  42.90 
per  cent.  Zn. 

The  determination  as  pyrophosphate  is  to  be  recommended 
when  the  zinc  solution  contains  a  very  large  quantity  of  ammonium 
salts  because  it  requires  long  washing  to  remove  these  and  this 
renders  the  results  a  little  low.  When  the  precipitate  is  weighed 
as  pyrophosphate,  the  ammonium  salts  are  volatilized  and  it  is 
not  necessary  to  remove  them  by  washing. 

Remark. — In  some  cases,  as  when  magnesium  or  aluminium 
is  present,  the  procedure  of  K.  Voigt  is  followed.  The  solu- 
tion of  the  zinc  salt,  containing  ammonium  salts  as  well,  is 
treated  with  an  excess  of  ammonia  and  then  with  ammonium  phos- 
phate. After  standing  some  time,  the  precipitate  of  magnesium 
ammonium  phosphate  and  aluminium  phosphate  is  filtered  off, 
the  zinc  ammonium  phosphate  being  soluble  in  ammonia. 
The  filtrate  is  received  in  a  platinum  or  porcelain  dish  and  is 
heated  on  the  water-bath  until  all  the  free  ammonia  has  been 
expelled,  whereby  zinc  ammonium  phosphate  separates  out 
quantitatively  in  the  form  of  the  crystalline  precipitate.  It  is 
treated  as  described  above.  If  some  of  the  precipitate  should 
adhere  firmly  to  the  sides  of  the  dish,  it  may  be  dissolved  in  a  few 
drops  of  hydrochloric  acid,  the  solution  immediately  neutralized 
with  ammonia,  and  heated  a  few  minutes  on  the  water-bath 
before  filtering. 


142  GRAVIMETRIC  ANALYSIS. 

2.  Determination  as  Zinc  Oxide. 

The  carbonate,  nitrate,  acetate,  and  oxalate  of  zinc  are  readily 
and  quantitatively  changed  to  zinc  oxide  by  ignition  in  the  air; 
in  the  case  of  the  sulphate,  when  present  in  relatively  large  amounts, 
the  transformation  into  oxide  is  difficult.  Small  amounts  of  the 
sulphate  may  be  changed  to  oxide  by  igniting  over  the  blast-lamp. 
It  is  advisable,  however,  in  case  the 'zinc  is  present  as  sulphate,  to 
precipitate  it  from  the  aqueous  solution  as  sulphide  and  weigh  it 
as  such  according  to  3;  or  to  dissolve  the  sulphide  on  the  filter  in 
dilute  hydrochloric  acid,  receiving  the  solution  in  a  weighed  plati- 
num dish,  evaporating  to  dryness  on  the  water-bath,  and  changing 
to  oxide  by  the  method  of  Volhard  as  described  below,  and  weigh- 
ing as  such. 

The  chloride  is  readily  changed  to  oxide,  according  to  Volhard, 
by  gentle  ignition  with  pure  mercuric  oxide.  The  process  is  as 
follows:  The  neutral  solution  of  the  chloride,  contained  in  a 
platinum  dish,  is  treated  with  a  large  excess  of  pure  yellow  mer- 
curic oxide  *,  suspended  in  water,  and  evaporated  to  dryness  on 
the  water-bath,  whereby  mercuric  chloride  and  zinc  oxide  are 
formed, 

ZnCl2  +  HgO  =  ZnO  +  HgCl2, 

both  of  which  are  white  substances.  Enough  mercuric  oxide 
should  be  used  so  that  the  residue  obtained  after  the  evaporation 
is  noticeably  yellow. 

The  dry  mass  is  ignited  under  a  hood  with  a  good  draft  (on 
account  of  the  mercury  vapors  being  poisonous),  at  first  gently  and 
finally  strongly,  and  the  residue  of  zinc  oxide  is  weighed,  both 
mercuric  chloride  and  oxide  being  volatile.  The  results  are 
excellent. 

*  The  mercuric  oxide  is  prepared  by  precipitating  a  solution  of  mercuric 
chloride  with  pure  caustic  potash.  The  precipitate  is  allowed  to  settle, 
washed  by  decantation  with  water  until  free  from  chloride,  and  kept  sus- 
pended in  water  in  a  bottle  with  a  wide  neck.  A  considerable  amount  of 
the  mercuric  oxide,  say  5-10  gin.,  should  leave  no  weighable  residue  after 
ignition. 


DETERMINATION  OF  ZINC  AS  SULPHIDE.  143 

If  the  solution  contains,  besides  zinc,  also  alkalies,  the  zinc  can 
be  precipitated  as  carbonate  and  changed  to  oxide  upon  ignition. 
The  precipitation  of  the  zinc  carbonate  should  take  place  in  a 
porcelain  dish  and  the  sodium  carbonate  solution  should  be  added 
drop  by  drop  to  the  cold,  barely  acid  solution  free  from  ammonium 
salts.  The  sodium  carbonate  is  added  until  the  zinc  solution 
becomes  turbid,  when  it  is  heated  to  boiling,  whereby  the  greater 
part  of  the  zinc  is  precipitated  as  granular  zinc  carbonate.  Two 
drops  of  phenolphthalem  solution  are  then  added  and  enough 
sodium  carbonate  solution  to  impart  a  distinct  pink  color.  In 
this  way  a  precipitate  of  zinc  carbonate  is  obtained  free  from  alkali, 
which  is  not  the  case  if  the  hot  solution  is  at  once  precipitated 
by  the  addition  of  an  excess  of  sodium  carbonate.*  The  precipitate 
is  filtered  from  the  hot  solution  and  washed  with  hot  water  until 
20  drops  of  the  filtrate  leave  no  residue  on  evaporation.  The  pre- 
cipitate is  dried,  the  greater  part  transferred  to  a  weighed  porce- 
lain crucible,  the  filter  burned  by  itself  in  a  platinum  spiral,  and 
the  ash  added  to  the  main  part  of  the  precipitate  in  the  crucible, 
which  is  ignited,  at  first  gently  and  finally  strongly,  over  a  Teclu 
burner  and  weighed  f  after  cooling  in  a  desiccator. 

3.  Determination  as  Sulphide. 

This  determination  is  chosen  when  the  zinc  is  present  in  a 
solution  containing  ammonium  salts,  or  when  it  is  necessary  to 
separate  zinc  from  alkaline  earths,  alkalies  or  metals  of  this  group. 
Zinc  sulphide  may  be  precipitated  from  ammoniacal  solutions,  or 
from  solutions  containing  free  acetic,  formic,  citric,  or  sulphocyanic 
acids. 


*In  case  considerable  amounts  of  ammonium  salts  are  present  there 
may  be  no  precipitation.  Sodium  carbonate  should  then  be  added  until 
the  solution  is  slightly  alkaline  and  the  solution  boiled  until  all  the  ammonia 
is  expelled. 

f  If  the  solution  contains  sulphate,  the  precipitate  produced  by  sodium 
carbonate  always  contains  more  or  less  basic  zinc  sulphate,  which  may 
easily  lead  to  high  results.  In  the  presence  of  sulphates,  therefore,  it  is 
advisable  to  precipitate  the  zinc  as  sulphide  and  determine  it  as  such  accord- 
ing to  3. 


144  GRAVIMETRIC  ANALYSIS. 

(a)  Precipitation  of  ZnS  from  Ammoniacal  Solutions. 

The  slightly  acid  solution  is  placed  in  an  Erlenmeyer  flask 
and  treated  with  sodium  carbonate  solution  until  a  permanent 
precipitate  is  obtained.  This  is  dissolved  by  the  addition  of  a 
few  drops  of  ammonia,  after  which  for  every  100  c.c.  of  the  solution 
5  gms.  of  ammonium  acetate  (or,  better,  ammonium  thiocyanate) 
are  added,  followed  by  a  slight  excess  of  freshly  prepared  ammo- 
nium sulphide,  the  flask  is  nearly  filled  with  boiled  water,  stoppered 
and  allowed  to  stand  twelve  to  twenty- four  hours.  Without  dis- 
turbing the  precipitate,  the  clear  upper  liquid  is  poured  through  a 
Schleicher  &  SchiilPs  filter  No.  590.  The  precipitate  is  covered 
with  a  solution  containing  in  every  100  c.c.  5  gms.  of  ammonium  ace- 
tate (or  ammonium  thiocyanate)  and  2  c.c.  of  ammonium  sulphide 
solution,  shaken,  allowed  to  settle,  and  the  turbid  upper  liquid  is 
poured  through  the  filter,  taking  care  to  receive  the  filtrate  in  a 
fresh  beaker ;  in  case  it  comes  through  turbid  it  is  poured  through 
the  filter  again.  The  decantation  is  repeated  three  times,  after 
which  the  precipitate  is  transferred  to  the  filter  and  washed  com- 
pletely with  the  above  solution,  taking  pains  to  keep  the  filter  full 
of  the  wash  liquid  during  the  entire  operation,  finally  washing  with 
water  containing  ammonium  sulphide  only.  The  precipitate 
is  then  dried,  transferred  as  completely  as  possible  to  a 
weighed  Rose  crucible,  the  filter  burned  by  itself  and  the  ash 
added  to  the  main  portion  of  the  precipitate.  The  precipitate  is 
now  mixed  with  the  aid  of  a  platinum  wire,  with  one-third  as 
much  pure  sulphur,  covered  with  a  layer  of  sulphur  and  heated, 
as  described  under  Manganese  (page  125)  in  a  current  of  hydro- 
gen. The  crucible  is  finally  allowed  to  cool  in  the  stream  of 
hydrogen  and  from  the  weight  of  the  zinc  sulphide  the  weight 
of  zinc  present  is  calculated, 

ZnS:Zn=p:s 
Zn 


ELECTROLYTIC  DETERMINATION  OF  ZINC.  H5 

and   if  a   is  the  amount    of  the   original  substance,  then  the 
per  cent,  of  zinc  is 


100  Zn 
rc= 


(b)  Precipitation  of  ZnS  from  Acid  Solutions. 

The  solution,  which  has  been  nearly  neutralized  with  ammonia, 
is  treated  with  ammonium  chloride  or  sulphate  and  a  little 
ammonium  or  sodium  acetate ;  and  is  then  saturated  with  hydro- 
gen sulphide.  After  the  precipitate  has  settled  completely, 
the  supernatant  solution,  is  poured  through  a  filter,  and  the  pre- 
cipitate washed  with  2  to  4  per  cei\t.  acetic  acid  which  has  been 
saturated  with  hydrogen  sulphide.  When  thoroughly  washed 
it  is  treated  as  described  above.  It  is  to  be  noted  that  the 
zinc  sulphide  shows  less  tendency  to  form  colloidal  solutions  when 
it  is  thrown  down  from  a  slightly  acid  solution  than  when  it  is 
precipitated  from  alkaline  solutions. 

4.  Electrolytic  Determination  of  Zinc. 

In  the  presence  of  acid,  zinc  is  not  deposited  by  an  electric 
current  of  0.5  to  1  ampere,  although  it  may  be  deposited  even 
then  by  stronger  currents. 

From  the  solution  of  potassium  or  sodium  zincate,  or  from  the 
complex  alkali  zinc  cyanides,  it  is  easy  to  deposit  the  zinc  quan- 
titatively. 

(a)  Method  of  F.  Spitzer.* 

The  solution  of  zinc  sulphate  (chlorides  and  nitrates  should 
be  absent)  is  treated  with  a  drop  of  phenolphthalein  and  with 
sodium  hydroxide  solution  until  a  permanent  coloration  is  obtained. 
Then  20  to  25  c.c.  of  normal  caustic  soda  solution  are  added,  the 

*  Z.  Elektrochem.,  11,  401  (1905). 


146  GRAVIMETRIC  ANALYSIS. 

solution  is  diluted  to  150-200  c.c.  and  electrolyzed,  using  a  plat- 
inum gauze  cathode,  with  a  current  of  0.8  to  1  ampere  and  3  to 
4  volts  electrode  potential.  At  the  end  of  three  hours  the  elec- 
trolysis is  finished,  provided  not  more  than  0.5  gm.  of  zinc  was 
present.  Without  breaking  the  current,  the  electrodes  are 
raised  nearly  out  of  the  bath,  the  upper  portions  are  washed 
quickly  with  water,  then  the  electrodes  are  taken  entirely  out  of 
the  solution  and  rinsed  with  water.  The  current  is  then  turned 
off,  the  cathode  washed  with  absolute  alcohol,  dried  above  a 
flarilfe,  cooled  in  a  desiccator,  and  weighed.  When  deposited 
in  this  way,  zinc  forms  a  bluish-gray  layer  that  adheres  firmly  to 
the  electrode.  To  make  sure  that  all  the  zinc  was  deposited,  the 
electrodes  are  cleaned  and  the  solution  electrolyzed  for  thirty 
minutes  longer.  A  slight  increase  in  weight  will  be  obtained  in 
every  case  because  the  anode  is  attacked  slightly  by  the  alkaline 
solution  so  that  the  cathode  slowly  continues  to  gain  in  weight 
from  deposited  platinum.  If  at  the  end  of  half  an  hour  the 
gain  in  weight  is  not  over  0.3  gm.  then  the  deposition  of  the 
zinc  was  complete  the  first  time,  as  can  be  shown  by  testing  with 
sodium  sulphide.  The  results  are  always  a  little  high.* 

To  clean  the  electrodes,  they  are  boiled  thoroughly  with 
hydrochloric  acid  (1:2)  washed  well  with  distilled  water,  and 
ignited.  It  is  not  necessary  to  cover  the  platinum  gauze  with  a 
thin  coating  of  copper  or  of  silver,  as  has  been  recommended 
when  a  platinum  dish  is  used  as  the  cathode. 

Remark. — If  too  little  caustic  soda  is  present,  a  spongy  deposit 

«~f  zinc  is  obtained  which  does  not  adhere  well  to  the  electrode. 
>r  this  reason  the  above  directions  should  be  followed  closely. 
In  the  presence  of  ammonia  the  determination  is  not  successful. 
T  ,  therefore,  it  is  desired  to  analyze  a  solution  containing  an 
ammonium  salt,  it  must  be  boiled  with  caustic  soda  until  all  the 
ammonia  has  been  expelled.     If,  moreover,  chlorides  or  nitrates 


*  Ellwood  B.  Spear,  J.  Am.  Chem.  Soc.,  32,  530:(1910).  The  experiments 
have  been  repeated  in  the  author's  laboratory  by  Janini,  who  obtained  as 
an  average  from  fourteen  determinations  with  50  c.c.  of  a  zinc  sulphate 
solution,  the  value  0.1014  gm.  Zn  instead  of  0.1008  gm.  Zn,'a  difference  of 
about  0.6  per  cent. 


SEPARA  TION  OF  MANGANESE,  ETC.,  FROM  ALKALINE  EARTHS.     147 

are  present  they  must  be  removed  by  evaporation  with  sulphuric 
acid.  The  solution  is  evaporated  on  the  water-bath  and  finally 
heated  over  the  free  flame  until  dense  vapors  of  sulphuric  acid 
are  expelled.  The  solution  is  then  diluted  and  analyzed  in  the 
usual  manner. 

(6)  The  Potassium  Cyanide  Method.  * 

A  drop  of  phenolphthalein  is  added  to  the  solution  of  zinc 
sulphate,  caustic  soda  solution  until  a  permanent  pink  coloration 
is  obtained,  and  then  potassium  cyanide  solution  until  a  clear 
solution  results.  This  is  diluted  to  a  volume  of  150-200  c.c.  and 
electrolyzed  with  a  current  of  0.5  ampere.  At  first  the  electrode 
potential  is  about  5.8  volts,  but  it  falls  during  the  analysis  on 
account  of  the  current  heating  the  solution.  The  electrolysis 
is  finished  in  two  or  three  hours. 

Other  methods  for  the  electrolytic  estimation  of  zinc  are 
given  in  A.  Classen's  book  Quantitative  Analysis  by  Electrolysis. 


SEPARATION   OF   MANGANESE,   NICKEL.    COBALT,   AND   ZINC 
FROM  THE  ALKALINE  EARTHS. 

The  separation  depends  upon  the  insolubility  of  the  sulphides 
of  the  metals  of  this  group  and  the  solubility  of  the  sulphides  of 
the  alkaline  earths. 

Procedure. — The  neutral  solution  of  the  chlorides,  contained 
in  an  Erlenmeyer  flask,  is  treated  with  ammonium  chloride  (hi 
case  it  is  not  already  present)  and  freshly -prepared  colorless 
ammonium  sulphide  solution  is  added  drop  by  drop  until  no 
further  precipitation  takes  place  and  the  liquid  has  a  distinct 
odor  of  ammonium  sulphide.  The  flask  is  then  almost  completely 
filled  with  boiled  water,  corked,  and  allowed  to  stand  twelve  hours. 
The  precipitate  is  filtered  and  washed  as  described  in  tne  Deter- 
mination of  Zinc  (p.  144). 

If  only  a  small  amount  of  alkaline-earth  metals  are  present  and 

t  Luckow,  Z.  anal.  Chern.,  19,  I  (ISSQ). 


148  GRAVIMETRIC  ANALYSIS. 

the  ammonium  sulphide  solution  is  entirely  free  from  ammonium 
carbonate,  the  separation  is  usually  complete  after  one  precipita- 
tion; in  the  presence  of  considerable  calcium,  strontium,  barium, 
or  magnesium  the  sulphide  precipitate  will  always  be  more  or  less 
contaminated  with  these  substances,  so  that  the  precipitation 
must  be  repeated.  For  this  purpose  the  washed  precipitate  is 
dried,  transferred  as  completely  as  possible  to  a  porcelain  crucible, 
the  filter-paper  burned  in  a  platinum  spiral  and  the  ash  added  to  the 
main  part  of  the  precipitate  in  the  crucible,  which  is  now  covered 
with  a  watch-glass,  treated  with  dilute  hydrochloric  acid,  and 
heated  to  boiling  after  the  evolution  of  hydrogen  sulphide  has 
ceased,  in  order  to  remove  all  of  the  hydrogen  sulphide.  A  very 
little  concentrated  nitric  acid  is  now  added  and  the  mixture  warmed 
until  the  precipitate  is  completely  dissolved;  the  solution  is  evapo- 
rated to  dryness,  treated  with  a  little  concentrated  hydrochloric 
acid,  and  again  evaporated  to  dryness  in  order  to  change  to  chloride 
any  nitrate  that  may  have  been  formed.  The  dry  mass  is  moistened 
with  a  few  drops  of  concentrated  hydrochloric  acid,  dissolved  in  hot 
water,  and  the  slight  residue  of  sulphur  filtered  off,  which,  in  case 
barium  is  present,  always  contains  a  small  amount  of  barium 
sulphate,  and  is  therefore  washed  with  hot  water,  dried,  ignited 
in  a  porcelain  crucible,  and  weighed.  The  nitrate  is  then 
precipitated  exactly  as  before  by  the  addition  of  ammonium 
sulphide. 

In  case  nickel  is  present,  a  too  great  excess  of  ammonium 
sulphide  must  be  carefully  avoided,  as  otherwise  the  nickel  sulphide 
will  pass  through  the  filter  (cf.  Vol.  I).  In  all  cases,  however, 
the  filtrate  should  be  tested  for  nickel  by  acidifying  with  acetic 
acid,  heating  to  boiling,  and  passing  hydrogen  sulphide  into 
the  solution.  If  a  slight  black  precipitate  is  produced  by  this 
treatment,  it  is  filtered  off  and  combined  with  the  main  precipi- 
tate (cf.  p.  156  et  seq).  The  filtrate  containing  the  alkaline-earth 
metals  is  freed  from  ammonium  salts  by  evaporating  to  dryness, 
dissolved  in  hydrochloric  acid,  and  examined  as  described  on  p. 
76  et  seq. 

Remark. — The  ammonium  sulphide  solution  used  in  the  above 
separation  must  be  free  from  ammonium  carbonate.  As,  however, 
all  commercial  ammonia  contains  this  salt,  it  must  be  freed  from 


PREPARATION  OF  AMMONIA  FREE  FROM  CARBONATE.      149 

carbonate  before  being  used  for  the  preparation  of  ammonium 
sulphide  solution. 

Preparation  of  Ammonia  Free  from  Carbonate.  * 

About  10  gms.  of  freshly  slaked  lime  are  added  to  500  c.c.  of 
concentrated  ammonia  contained  in  a  liter  flask  that  is  connected 
with  a  condenser.  The  condenser  is  closed  by  means  of  a  tube 
containing  soda-lime,  and  the  contents  of  the  flask  are  allowed 
to  stand  for  a  day  with  frequent  shaking.  After  this,  from  300- 
400  c.c.  of  water  are  placed  in  a  flask  and  boiled,  meanwhile 
passing  through  the  water  a  current  of  air  that  has  been  freed 
from  all  traces  of  carbon  dioxide  by  passing  through  concentrated 
caustic  potash  solution  and  then  through  a  tower  filled  with 
soda-lime.  The  water  is  allowed  to  cool  in  this  air-stream. 
The  flask  containing  the  ammonia  is  then  placed  on  the  water- 
bath  in  such  a  position  that  the  condenser-tube  is  inclined  slightly 
upward,  and  this  is  connected  with  the  delivery-tube,  through 
which  the  air  previously  passed  into  the  flask  of  boiling  water. 
By  warming  the  water-bath  the  ammonia  is  now  distilled  over 
into  the  flask  containing  the  boiled  water,  by  which  it  is  completely 
absorbed.  By  saturating  a  part  of  this  ammonia  with  hydrogen 
sulphide,  a  solution  of  ammonium  sulphide  is  prepared  suitable 
for  the  above-described  separation. 


SEPARATION  OF  THE  BIVALENT  FROM  THE  OTHER  METALS 
OF  THE  AMMONIUM  SULPHIDE  GROUP. 

This  separation  is  often  designated  as  that  of  the  protoxides 
from  the  sesquioxides;  this  designation  is  not  applicable  in  the 
case  of  titanium  and  uranyl  derivatives. 

The  Barium  Carbonate  Method. 

This  method  depends  upon  the  fact  that  ferric,  aluminium 
and  chromic  salts  (as  well  as  titanic  and  uranyl  salts)  are  preci- 

*  The  distillation  of  ammonia  also  serves  to  free  it  from  silica,  which  it 
always  contains  when  kept  in  glass  bottles  for  any  length  of  time. 


!5°  GRAVIMETRIC  ANALYSIS. 

pitated  in  the  cold  by  barium  carbonate,  while  manganese, 
nickel,  cobalt,  zinc,  and  ferrous  salts  are  not.  The  trivalent 
metals  are  strongly  decomposed  hydrolytically: 


(HCl)  +Fe(OH)Cl2. 


Free  acid  and  a  basic  salt  are  formed  by  this  hydrolysis,  the 
composition  of  the  latter  depending  upon  the  quantity  of 
the  water  and  the  temperature.  If  the  free  acid  is  removed  by 
the  addition  of  barium  carbonate,  the  equilibrium  is  disturbed 
and  the  hydrolysis  goes  further  until  finally  the  insoluble  hy- 
droxide is  formed: 

Fe  (OH)  C12  +  2HOH  =  2HC1  +  Fe(OH)3. 

The  barium  carbonate,  then,  serves  only  to  neutralize  the  acid 
set  free  by  the  hydrolysis,  and  the  total  reaction  is  expressed 
by  the  following  equation: 

..  ///  .    /  //  QTT  CO 

2FeCl3+  6HOH  +  3BaC03=3BaCl2+2Fe(OH)3+ 


The  salts  of  the  bivalent  metals  are  not  subject  to  this  hy- 
drolysis in  the  cold,  consequently  they  are  not  precipitated  by 
the  addition  of  barium  carbonate.  On  warming,  however,  they 
are  hydrolyzed  to  an  appreciable  extent  and  are  then  precipitated 
by  barium  carbonate. 

Procedure.  —  Sodium  carbonate  solution  is  added  drop  by  drop 
to  the  slightly  acid  solution  of  the  chlorides  or  nitrates,  but  not 
the  sulphates,*  of  the  metals,  in  an  Erlenmeyer  flask  until  a  slight, 
permanent  turbidity  is  produced,  which  is  then  redissolved  by 
the  addition  of  a  few  drops  of  dilute  hydrochloric  acid.  The 
solution  is  diluted  and  treated  with  pure  barium  carbonate  f 
(suspended  in  water)  until  after  thoroughly  shaking  an  excess  of  the 

*  Barium  carbonate  will  precipitate  the  bivalent  metals  when  sulphates 
are  present,  e.g.  : 

ZnSO4  +  BaCO3  =  ZnCO3-f  BaSO4. 
f  The  barium  carbonate  must  be  free  from  alkali  carbonate. 


THE  BARIUM  CARBONATE  METHOD.  151 

latter  remains  on  the  bottom  of  the  flask.  ,The  flask  is  closed 
and  allowed  to  stand  for  several  hours  with  frequent  shaking.  The 
clear  liquid  is  then  decanted  off,  the  residue  treated  with  cold 
water  and  again  decanted.  This  decantation  is  repeated  three 
times,  after  which  the  precipitate  is  transferred  to  the  filter  and 
completely  washed  with  cold  water.  The  precipitate  contains  all 
of  the  iron,  aluminium,  chromium,  titanium,  and  uranium  in  the 
presence  of  the  excess  of  barium  carbonate.  The  filtrate  contains 
the  bivalent  metals  and  barium  chloride. 

The  precipitate  is  dissolved  in  dilute  hydrochloric  acid,  boiled 
to  remore  the  carbon  dioxide,  and  the  iron,  aluminium,  chromium 
(titanium  and  uranium)  are  separated  from  the  barium  *  by  double 
precipitation  with  ammonium  sulphide  as  described  on  p.  147. 
The  iron,  aluminium,  chromium  (titanium  and  uranium)  are  sepa- 
rated from  one  another  as  described  on  pp.  107-120. 

The  filtrate  from  the  barium  carbonate  precipitation  is  freed 
from  barium  by  the  addition  of  sulphuric  acidf  to  the  boiling 
solution  after  it  has  been  made  acid  with  hydrochloric  acid.  The 
barium  sulphate  is  filtered  off  and  the  monoxides  are  separated 
from  one  another  as  described  on  p.  156. 

Remark. — The  above  separation  of  the  sesquioxides  from  the 
protoxides  is  not  absolutely  certain  in  the  presence  of  nickel  and 
cobalt.  In  this  case,  particularly  when  considerable  iron  is  pres- 
ent, the  precipitate  produced  by  barium  carbonate  contains  small 
amounts  of  nickel  and  cobalt.  This  difficulty  can  be  overcome, 
however,  by  adding  ammonium  chloride  to  the  solution  (3-5  gms. 
for  each  100  c.c.  of  solution)  before  precipitating  with  barium 
carbonate;  the  separation  is  then  satisfactory. 

*  Most  authorities  recommend  precipitating  the  barium  first  with  sul- 
phuric acid  and  then  separating  the  iron,  aluminium,  etc.  The  precipitate 
of  barium  sulphate  always  contains  small  amounts  of  the  heavy  metals, 
so  that  the  author  prefers  the  above  procedure. 

t  Or,  better,  by  double  precipitation  of  the  other  metals  with  ammonium 
sulphide. 


352  GRAVIMETRIC  ANALYSIS. 

SEPARATION  OF  IRON,  ALUMINIUM,  AND  TITANIUM  (BUT  NOT 
CHROMIUM  AND  URANIUM)  FROM  MANGANESE,  NICKEL, 
COBALT,  AND  ZINC. 

Basic  Acetate  Method. 

This  classic  method  depends  upon  the  fact  that  ferric,  alu- 
minium and  titanium  acetates  are  hydrolyzed  in  hot,  dilute 
solutions  much  more  readily  than  the  acetates  of  the  bivalent 
metals.  From  the  equation 

Fe  (C2H302)  3 + 2HOH<r>2HC2H302  +  Fe  (OH)  2  -  C2H302, 
it  is  evident  that  acid  is  set  free  which  tends  to  stop  the  reaction, 
due  to  the  solvent  action  of  hydrogen  ions.  The  concentration 
of  free  hydrogen  ions,  however,  is  kept  low  by  the  addition  of 
sodium  acetate.  Then,  as  a  rule,  some  manganese  is  likely  to 
be  precipitated,  so  that  it  is  advisable  to  dissolve  the  precipitate 
and  repeat  the  precipitation.  Hydrated  manganese  dioxide  is  more 
insoluble  than  manganous  hydroxide,  Mn(OH2),  and  hence  long 
boiling  in  the  air  tends  to  increase  the  quantity  of  manganese 
precipitated.  The  method  is  somewhat  tedious,  but  gives  excel- 
lent results. 

Procedure. — The  slightly  acid  solution  of  the  chlorides,  con- 
tained in  a  small  beaker,  is  treated  with  sodium  carbonate  solu- 
tion in  the  cold  until  a  slight  permanent  opalescence  is  obtained, 
which  is  then  redissolved  by  the  addition  of  a  few  drops  of  dilute 
hydrochloric  acid.  Meanwhile  a  boiling,  dilute  solution  of  sodium 
or  ammonium  acetate  is  prepared  in  a  large  round -bottomed 
flask,  containing  for  each  0.1  to  0.2  gm.  of  iron  or  aluminium, 
1.5  to  2  gm.  of  acetate  and  300  to  400  c.c.  water.  When  the 
iron  solution  is  ready,  the  lamp  is  taken  away  from  beneath  the 
flask,  the  iron  solution  is  added,  and  then  the  boiling  is  con- 
tinued for  one  minute,  the  flame  removed  (the  precipitate 
becomes  slimy  on  long  boiling),  the  precipitate  allowed  to  settle 
and  filtered  immediately  while  the  liquid  is  hot,  through  a  fluted 
filter,  washing  three  times  by  decantation  with  boiling  water 
containing  ammonium  or  sodium  acetate.  The  filter  together 
with  the  precipitate  is  spread  upon  a  glass  plate,  the  bulk  of  the 
precipitate  rinsed  into  a  porcelain  dish,  and  that  remaining  on 


SEPARATION   OF  IRON  FROM  MANGANESE.  153 

the  filter  dissolved  by  alternately  treating  with  concentrated 
hydrochloric  acid  and  hot  water.  The  resulting  solution  is 
evaporated  nearly  to  dryness  on  the  water-bath  and  the  basic 
acetate  precipitation  is  repeated  exactly  as  before.  The  filtered 
and  washed  precipitate  is  dissolved  in  hydrochloric  acid  and  the 
iron  separated  from  aluminium  according  to  page  107.  The 
combined  filtrates  containing  the  protoxides  are  acidified  with 
10-20  c.c.  of  concentrated  hydrochloric  acid,  in  order  to  prevent 
the  precipitation  of  hydrated  manganese  dioxide,  evaporated 
almost  to  dryness,  dissolved  in  a  little  water,  the  manganese, 
nickel,  cobalt  and  zinc  precipitated  by  ammonium  sulphide  as 
described  on  p.  147,  and  analyzed  according  to  p.  156. 

Remark. — This  procedure  requires  practice.  It  is  especially 
suited  for  the  separation  of  iron  and  titanium  from  the  protoxides; 
the  separation  is  usually  less  satisfactory  with  aluminium,  so  that 
in  case  considerable  amounts  of  the  latter  are  present,  the  barium 
carbonate  separation  is  to  be  preferred.  If  it  is  merely  a  case  of  the 

Separation  of  Iron  from  Manganese, 

the  following  modifications  of  the  basic  acetate  process  give 
satisfactory  separations  with  only  a  single  precipitation. 

(a)  O.  Brunck's  Method.* 

The  acid  solution,  containing  not  more  than  0.3  gm.  of  iron, 
is  treated  with  0.35  gm.  of  potassium  chloride  or  0.26  gm.  ammo- 
nium chloride  for  each  0.1  gm.  of  iron  present.  The  solution  is 
evaporated  to  dryness  on  the  water-bath,  the  residue  pressed 
with  a  glass  rod,  and  heated  five  or  ten  minutes  longer.  By  this 
time  practically  all  the  mineral  acid  is  expelled.  The  residual 
salts  are  dissolved  in  10  to  20  c.c.  of  water  and  to  the  resulting 
solution  there  is  added  1.5  gm.  of  sodium  acetate  for  each  0.1 
gm.  of  iron  present. f  The  solution  is  diluted  with  boiling  water 
to  a  volume  of  400  to  500  c.c.  for  each  0.2  gm.  of  iron  present; 
it  is  heated,  with  constant  stirring,  until  boiling  begins,  and  then 

*  Chem.  Ztg.,  1904,  I,  513.     Cf.  W.  Funk,  Z.  anal.  Chem.,  45,  181  (1906). 

t  The  sodium  acetate  crystals  often  contain  a  little  sodium  carbonate,  so 
that  they  should  be  dissolved  in  a  little  water  and  the  solution  made  bare'y 
acid  before  adding  it  to  the  iron  solution. 


154  GRAVIMETRIC  ANALYSIS. 

the  flame  is  removed  and  the  precipitate  allowed  to  settle.  The 
solution  is  decanted  through  a  fluted  filter  and  the  precipitate 
washed  with  hot  water.  The  precipitate  is  dissolved  in  as 
little  hydrochloric  acid  as  possible,  the  iron  precipitated  by 
ammonia,  filtered,  dried,  and  ignited  as  described  on  page  87. 
The  filtrate  from  the  basic  acetate  precipitation,  or  better  the 
combined  filtrates  from  both  precipitations,*  is  acidified  with 
hydrochloric  acid,  evaporated  nearly  to  dry  ness,  the  residue 
dissolved  in  a  little  water  and  the  manganese,  nickel,  cobalt 
and  zinc  precipitated  with  ammonium  sulphide  according  to 
the  directions  on  page  147,  and  separated  according  to  page  156. 

(6)  Method  of  A.  Mittasch.f 

The  slightly  acid  solution,  containing  not  more  than  0.3  gm. 
of  iron  and  having  a  volume  of  not  over  100  c.c.,  is  carefully 
neutralized,  while  stirring  constantly,  by  adding  ammonium  car- 
bonate solution  (200  gm.  of  the  commercial  salt  in  1  liter  of  water) 
from  a  pipette,  or  burette.  When  the  precipitate  that  is  first  pro- 
duced begins  to  dissolve  very  slowly,  the  neutralization  is  finished 
with  an  ammonium  carbonate  solution,  which  is  prepared  by  tak- 
ing 50  c.c.  of  the  first  solution  and  diluting  to  1  liter,  the  dilute 
reagent  being  added  until  the  precipitate  produced  will  not 
dissolve  within  one  or  two  minutes  of  stirring.  At  this  point, 
3  c.c.  of  double  normal  acetic  acid  are  added,  and  the  solution 
stirred  until  the  precipitate  disappears.  The  solution  is  diluted 
with  400  c.c.  of  hot  water  and  heated  until  it  begins  to  boil,  when 
the  greater  part  of  the  iron  will  have  been  precipitated.  Then 
20  c.c.  of  ammonium  acetate  solution  (60  gm.  of  the  commercial 
salt  in  1  liter  of  water)  {  are  added  and  the  boiling  continued  for 

*  The  ammoniacal  filtrate  from  the  Fe(OH)3  precipitate  is  acidified  with 
5  c.c.  of  concentrated  hydrochloric  acid  before  adding  it  to  the  filtrate  from 
the  basic  acetate  precipitation,  otherwise  manganese  is  likely  to  be  pre- 
cipitated when  the  two  filtrates  are  mixed. 

t  Z.  anal.  Chem.,  42,  508  (1903). 

J  Commercial  ammonium  acetate  has  the  symbol  NH4C2H3O2 •  HC2H3O2. 
If  none  of  it  is  on  hand,  500  c.c.  2N.  solution  ammonia  are  mixed  with 
50  c.c.  2N.  acetic  acid;  the  mixture  must  be  faintly  acid.  Of  this  solution 
10  c.c.-f-5  c.c.  2N.  acetic  acid  are  used  for  the  precipitation  of  the  iron, 
and  10  c.c.  of  2N.  acetic  acid  are  added  to  dissolve  the  precipitate  pro- 
duced by  ammonium  carbonate. 


SEPARATION  OF  IRON  AND  ALUMINIUM  FROM  MANGANESE.  155 

a  minute  longer.  Without  waiting  for  the  precipitate  to  settle, 
it  is  filtered  off  and  washed  with  hot  water  until  free  from 
chlorides. 

The  small  quantity  of  precipitate  adhering  to  the  sides  of  the 
vessel  in  which  the  precipitation  took  place  is  dissolved  in  a 
few  drops  of  hydrochloric  acid,  the  iron  precipitated  by  ammonia 
and  the  ferric  hydroxide  filtered  off  through  a  separate  filter. 
Both  niters  are  now  dried,  burned  and  the  iron  weighed  as  Fe2Or 


SEPARATION  OF   IROH   AND  ALUMINIUM  FROM  MANGANESE, 
NICKEL,  COBALT,  AND  ZINC. 

Sodium  Succinate  Method. 

This  method,  applicable  for  the  separation  of  large  quantities 
of  iron  from  small  quantities  of  manganese,  nickel,  etc.,  is  based 
upon  the  fact  that  ferric  iron  is  quantitatively  precipitated  from 
neutral  solutions  as  light-brown  ferric  succinate  by  the  addition  of 
neutral  alkali  succinate  solution,  while  manganese,  nickel,  etc., 
remain  in  solution. 

Procedure. — In  case  the  solution  contains  free  acid  and  all  the 
iron  is  hi  the  ferric  form,  it  is  neutralized  with  ammonia  until  a 
reddish-brown  coloration  is  formed,  when  sodium  or  ammonium 
acetate  is  added  until  the  color  becomes  a  deep  brown,  and  then  the 
solution  of  alkali  succinate,  after  which  the  mixture  is  warmed 
gently,  allowed  to  cool,  filtered,  and  washed  at  first  with  cold  water, 
then  with  warm  water  containing  ammonia,  until  20  drops  of  the 
filtrate  leave  no  residue  when  evaporated  to  dryness  on  platinum. 
By  means  of  the  washing  with  ammonia,  the  ferric  succinate  is 
changed  to  ferric  hydroxide  which  is  dried  and  weighed  as  ferric 
oxide  after  ignition  in  a  porcelain  crucible.  If  aluminium  is  present, 
the  ignited  residue  is  further  analyzed  as  described  on  p.  107.  The 
bivalent  metals  in  the  filtrate  are  best  precipitated  by  the  addition 
of  ammonium  sulphide  and  analyzed  as  follows: 


156  GRAVIMETRIC  A NA LYSIS. 


SEPARATION  OF  THE  BIVALENT  METALS  OF  THE  AMMONIUM 
SULPHIDE  GROUP  FROM  ONE  ANOTHER. 

Separation  of  Zinc  from  Nickel,  Cobalt,  and  Manganese. 

All  methods  for  this  separation  rest  upon  the  slight  solubility 
of  zinc  sulphide  and  the  ready  solubility  of  the  remaining  sulphides 
in  their  state  of  formation.*  At  this  point  it  may  be  well  to  say 
a  few  wrords  with  regard  to  the  most  recent  explanation  of  the 
process  that  takes  place  in  the  solution  of  electrolytes. 

Solubility  Product. 

Inasmuch  as  no  substance  is  absolutely  insoluble  in  water,  it 
follows  that  in  every  case  where  a  precipitate  is  produced  the  solu- 
tion is  saturated  with  the  substance  and  (according  to  Ostwald)  in 
the  case  of  difficultly  soluble  substances  the  dissolved  portion  is 
practically  completely  dissociated  electrolytically.  The  binary  sub- 
stance A,  consisting  of  the  elements  B  and  C,  is  decomposed  in 
aqueous  solution  according  to  this  scheme: 

A<=±B+C. 

If  the  concentrations  of  the  ions  B  and  C  are  designated  by 
[B]  and  [C],  and  that  of  the  undissociated  portion  by  [A],  then 
according  to  the  mass-action  law  the  following  relation  holds 
for  any  given  temperature: 

,  . .     =  constant. 

Every  increase  of  [B]  or  [C]  causes,  therefore,  an  increase  of 
[A],  and,  as  the  solution  is  already  saturated  with  A}  this  will 
produce  precipitation  of  the  substance. 

This  product  [B.C],  which  if  exceeded  causes  a  supersatura- 
tion  of  the  solution,  and  consequently  precipitation,  is  called  the 

*  Nickel  and  cobalt  sulphides  when  once  formed  are  insoluble  in  dilute 
acids.  These  substances  probably  exist  in  two  allotropic  modifications, 
of  which  one  is  soluble  and  the  other  insoluble.  The  soluble  form,  has 
never  been  isolated. 


EXPLANATION  OF  THE  SOLUTION  OF  SULPHIDES  IN  ACIDS.      i57 

solubility  product.  If,  therefore,  in  any  solution  the  solubility 
product  is  already  reached,  then  the  solution  is  saturated  with  respect 
to  the  substance  A,  and  if  the  solubility  product  is  not  reached,  then 
the  liquid  exerts  a  solvent  action  upon  the  solid  substance. 

Explanation  of  the  Solution  of  Sulphides  in  Acids. 

According  to  the  above  theory,  the  solution  of  a  sulphide  (e.g., 
zinc  sulphide)  in  acid  is  conceived  to  take  place  as  follows: 

On  treating  the  solid  sulphide  with  water,  a  part  of  the 
salt  is  dissolved  until  the  solubility  product  is  reached.  This 
almost  inappreciable  amount  is  practically  completely  dissociated 
into  ions.  On  adding  acid  to  the  solution,  the  positive  hydrogen 
ions  unite  with  the  negative  sulphur  ions  to  form  neutral  hydrogen 
sulphide,  which  being  a  very  weak  acid  is  only  dissociated  to  a 
slight  extent,  so  that  sulphur  ions  disappear  from  the  solution 
and  the  solubility  product  of  zinc  sulphide  is  no  longer  reached: 

ZnS  +  2HC1  =  H2S  +  ZnCl2. 

The  liquid,  therefore,  dissolves  more  of  the  solid  zinc  sulphide  and 
the  above  reaction  again  takes  place  and  this  process  is  repeated 
until  all  of  the  zinc  sulphide  is  brought  into  solution.  The  solu- 
bility of  a  sulphide  in  acid,  therefore,  is  proportional  to  its  solubility 
product  and  to  the  concentration  of  the  hydrogen  ions.  If  we, 
then,  desire  to  precipitate  zinc  by  means  of  hydrogen  sulphide 
from  a  neutral  solution  of  an  inorganic  compound,  the  following 
consideration  shows  us  how  this  may  be  accomplished:  If  hydro- 
gen sulphide  is  conducted  into  a  solution  containing  zinc  com- 
bined with  a  mineral  acid,  the  zinc  is  indeed  precipitated,  but  as 
the  amount  of  zinc  sulphide  formed  increases,  there  is  an  increase 
in  the  concentration  of  the  hydrogen  ions: 

/SH*        .  , 
ZnCl2  +  2HSH  =  Zn<          +  2HC1. 


The  precipitation  is,  therefore,  incomplete.     It  can  be  made 
complete,  however,  if  we  can  avoid  this  increase  in  the  concentra- 

*  The  Zn(SH)2  is  at  once  decomposed  into  ZnS  and  HjS. 


158  GRAVIMETRIC   ANALYSIS. 

tion  of  the  hydrogen  ions.    This  can  take  place  by  replacing  the 
mineral  acid  formed  by  a  weaker  acid,  i.e.  one  which  is  only  slightly 
dissociated,  so  that  the  solution  will  contain  fewer  hydrogen  ions.* 
The  following  methods  depend  upon  this  principle. 

Method  of  Smith  and  Brunner.f 

Procedure. — The  hydrochloric  acid  solution  of  the  four  metals 
is  treated  with  sodium  carbonate  until  a  permanent  precipitate  is 
formed,  which  is  redissolved  by  the  addition  of  a  few  drops  of  very 
dilute  hydrochloric  acid.  Into  this  almost  neutral  solution  hydro- 
gen sulphide  is  passed  for  five  minutes,  then  a  few  drops  of  a  -very 
dilute  solution  of  sodium  or  ammonium  acetate  are  added  and  the 
solution  is  saturated  with  hydrogen  sulphide,  allowed  to  stand 
overnight,  filtered,  and  washed  with  hydrogen  sulphide  water  which 
contains  in  every  100  c.c.  2  gms.  of  ammonium  salt  (either  the  chlo- 
ride, sulphate,  or  sulphocyanate).  The  zinc  is  then  determined 
either  as  oxide  or  sulphide  according  to  the  methods  described 
on  pp.  142  and  143. 

Remark. — Inasmuch  as  the  exact  amount  of  acid  to  be  set  free 
is  unknown,  it  is  impossible  to  tell  exactly  how  much  alkali  acetate 
is  necessary,  and  herein  lies  the  chief  difficulty.  If  too  much  alkali 
acetate  is  added,  some  nickel  or  cobalt  sulphide  may  be  precipitated 
(shown  by  the  gray  color  of  the  zinc  precipitate).  If  not  enough 
alkali  acetate  is  added,  the  zinc  will  not  be  completely  precipitated. 
The  following  separation  is  more  certain. 

Method  of  Cl.  Zimmerman.  J 

Procedure. — The  weakly  acid  solution  is  treated  with  sodium 
carbonate  solution  until  a  permanent  precipitate  is  formed,  which 
is  redissolved  by  the  addition  of  a  few  drops  of  very  dilute  hydro- 
chloric acid,  then  for  every  80  c.c.  of  the  solution  10,  or  at  the  most 
15,  drops  of  double-normal  hydrochloric  acid,§  and  10  c.c.  o: 

*  Concerning  the  equilibrium  conditions  in  the  precipitation  of  sulphides 
by  hydrogen  sulphide,  see  Bruner  and  Zawadzki,  Chem.  Zentr.,  1910,  5. 

t  Chem.  Centrabl.,  1895,  26. 

j  Ann,  d.  Chem.  u.  Phann,  199,  (1879)  p.  3;   204  (1880),  p.  226. 

§  The  addition  of  hydrochloric  acid  is  in  all  cases  necessary,  because  other- 
wise nickel  sulphide  will  be  precipitated  with  the  zinc  sulphide,  especially 
when  considerable  nickel  and  little  zinc  are  present. 


METHOD  OF  CL.  ZIMMERMAN.  159 

ammonium  sulphocyanate  (1:5)  solution  are  added,  after  which  the 
solution  is  heated  to  about  70°  C.  and  is  saturated  with  hydrogen 
sulphide.  At  first  the  solution  becomes  only  slightly  turbid.*  but 
after  some  time  pure  white  zinc  sulphide  is  thrown  down  in  clouds, 
constantly  becoming  denser.  After  the  solution  has  become  sat- 
urated with  hydrogen  sulphide,  the  beaker  is  covered  and  allowed 
to  stand  in  a  moderately  warm  place  until  the  precipitate  has  set- 
tled and  the  upper  liquid  is  clear,  after  which  the  precipitate  is 
filtered  and  washed,  as  described  in  the  method  of  Smith  an.d 
Brunner. 

From  the  filtrate  nickel,  cobalt,  and  manganese  are  precipitated 
by  means  of  ammonium  sulphide,  filtered  and  separated  according 
to  the  following  methods. 

Remark.  —  What  is  the  part  played  by  the  ammonium  sul- 
phocyanate in  this  determination?  Certainly  it  cannot  act  the 
same  as  the  ammonium  acetate  in  the  Smith-Brunner  method,  for 
sulphocyanic  acid  is  not,  like  acetic  acid,  a  weak  acid,  but  a  very 
strong  one,  almost  as  strong  as  hydrochloric  acid  itself,  and  the 
dissociation  of  strong  acids  is  only  slightly  influenced  by  the  addi- 
tion of  their  neutral  salts. 

Ammonium  sulphocyanate  probably  simply  "salts  out"  the 
zinc  sulphide  (cf.  Vol.  I). 

By  the  action  of  hydrogen  sulphide  upon  the  zinc  salt,  zinc  sul- 
phide is  produced  both  in  the  hydrogel  and  hydrosol  forms  and  the 
ammonium  sulphocyanate  changes  the  latter  into  the  insoluble 
hydrogel.  If  this  explanation  is  correct,  the  separation  of  zinc 
from  nickel,  etc.,  will  succeed  equally  well  if  the  ammonium  sul- 

*  There  are  at  the  start  but  few  zinc  ions  in  the  solution.  The  four 
metals  are  present  for  the  most  part  in  the  form  of  complex  thiocyanates 
of  the  general  formula  [R(CNS)4](NH4)2.  The  zinc  salt,  like  carnallite 
(see  Vol.  I)  is  slightly  dissociated, 


[Zn(CNS)J(NH)2^Zn 

and  the  zinc  thiocyanate  is  converted  into  insoluble  sulphide  by  the  action 
of  hydrogen  sulphide.  When  the  zinc  begins  to  precipitate  as  sulphide, 
the  equilibrium  is  disturbed  and  eventually  all  the  zinc  becomes  precip- 
itated. 


i6o 


GRAVIMETRIC  ANALYSIS. 


phocyanate  is  replaced  by  ammonium  chloride  or  ammonium  sul- 
phate.   That  this  is  the  case  is  shown  by  the  following  method. 

"Salting-out  Method.^ 

Experiments  were  performed  by  G.  H.  Kramers  in  order  to 
determine  whether  the  separation  of  zinc  from  nickel  and  cobalt 
could  be  accomplished  in  weakly  acid  solutions  by  hydrogen  sul- 
phide after  the  addition  of  any  ammonium  salt  of  a  strong  acid.* 
The  results  obtained  showed  this  to  be  possible. 

Procedure. — The  neutral  solution  f  containing  the  nickel  and 
zinc  either  in  the  form  of  sulphate  or  chloride  (the  sum  of  the 
oxides  present  amounting  to  about  J  per  cent,  of  the  weight  of  the 
solution)  is  treated  with  8-10  drops  of  double-normal  hydrochloric 
acid  and  about  2  per  cent,  of  ammonium  sulphate  (referred  to  the 
total  amount  of  liquid)  and  the  solution  is  saturated  at  50°  C.  with 
hydrogen  sulphide;  the  warm  solution  is  allowed  to  stand  until  the 
pure  white  precipitate  of  zinc  sulphide  has  settled  out  and  is  then 
treated  exactly  as  described  under  the  Method  of  Zimmerman. 

Results. — In  the  following  experiments  a  zinc  sulphate  solution 
containing  5.890  gms.  zinc  to  the  liter  and  a  solution  of  nickel  sul- 
phate containing  5.320  gms.  nickel  to  the  liter  were  used. 


2 

f 

2 

6 

0 

e 

i 

c.c.  NH4CNS 
1:5. 

c.c.(NH4)2S04 
1  :5. 

g 

o 
o 

c 

Wt.  of  Zn 
Calculated. 

P 

Wt.of  Ni 
Calculated. 

f20 
60 

20 
20 



3 
3 

5 
5 



0.1188 
0  3533 

0.1178 
0  3534 

0.1072 
0  1051 

0  .  1066 
0  1066 

O  j 

?0 

60 

10 

10 

0  1184 

0  1178 

0  3206 

0  3192 

w 

?0 

60 

15 

10 

0  1182 

0  1178 

{5 

20 

60 

30 

10 

0  1089 

0.1178 

?0 

20 

5 

5 

0  1173 

0  1178 

0 

60 

20 

6 

10 

0  3536 

0  3534 

0.1082 

0  1066 

1 

20 

60 

12 

10 

0  1184 

0.1178 

i' 

20 

ro 

60 

60 
20 
20 

"eo" 

110 

12 

8 
8 

20 
5 

10 



0.1168 
0.1184 
0  3542 

0.1178 
0.1178 
0  3534 

0.1064 

0.1066 

• 

',0 

60 

100 

24 

20 

0  1168 

0  1178 

20 

60 

20 
20 

60 
110 

8 
g 



5 
10 

0.1182 
0  3552 

0.1178 
0  3534 

0.1074 

0.1066 

* 

20 

60 

100 

24 



20 

0.1190 

0.1178 

*  Or  any  other  salt,  e.g.,  a  potassium  salt. 

t  If  the  solution  is  acid,  it  is  neutralized  by  sodium  carbonate  as  described 
under  the  preceding  methods. 


SEPARATION  OF  MANGANESE  FROM  NICKEL  AND  COBALT.     161 

Separation  of  Manganese  from  Nickel  and  Cobalt. 

The  solution  of  the  chlorides  or  sulphates  is  treated  with  an 
excess  of  sodium  carbonate,  strongly  acidified  with  acetic  acid, 
and  for  each  gram  of  nickel  or  cobalt  present  5  gms.  of  ammonium 
acetate  are  added,  the  solution  is  diluted  to  100-200  c.c.,  heated  to 
70-80°  C.,  saturated  with  hydrogen  sulphide,  filtered,  and  washed 
with  hot  water.  The  manganese  is  in  the  filtrate,  and  the  nickel 
and  cobalt  are  in  the  precipitate. 

Remark. — The  filtrate  often  contains  small  amounts  of  nickel 
and  cobalt.  In  order  to  remove  these  metals,  the  solution  should  be 
concentrated  and  colorless  ammonium  sulphide  added.  It  is  then 
made  slightly  acid  with  acetic  acid,  warmed,  and  filtered.  In 
case  a  precipitate  of  nickel  or  cobalt  sulphides  is  formed  by 
this  treatment,  the  filtrate  is  again  tested  in  the  same  way  and  the 
process  repeated  until  no  further  precipitation  is  produced. 

Separation  of  Cobalt  from  Nickel, 
(a)  Method  of  Tschugaeff-Brunck.* 

This  method  is  probably  the  quickest  and  most  accurate  for 
the  estimation  of  nickel  in  the  presence  of  cobalt.  It  depends 
upon  the  fact  that  nickel  is  quantitatively  precipitated  by 
means  of  dimethyl  glyoxime,  from  a  barely  ammoniacal  solution 
or  from  a  slightly  acid  solution  containing  sodium  acetate.  Cobalt, 
under  these  conditions,  is  not  precipitated. 

Procedure. — If  the  quantity  of  cobalt  present  does  not  exceed 
the  quantity  of  nickel,  the  procedure  is  exactly  the  same  as  when 
nickel  alone  is  present;  with  larger  quantities  of  cobalt  two  or 
three  times  as  much  of  the  dimethyl  glyoxime  reagent  is  added 
and  the  precipitation  is  accomplished  exactly  as  described  on 
page  129.  For  the  determination  of  both  nickel  and  cobalt, 
the  original  solution  is  divided  into  two  portions.  In  one  por- 
tion the  nickel  is  determined  as  outlined  above,  and  in  the  other 
the  two  elements  are  deposited  electrolytically  as  described  on 
page  136,  and  the  cobalt  found  by  difference.  If  only  a  little 
of  the  substance  is  available,  the  two  metals  are  deposited 

*  O.  Brunck,  Z.  angew.  Chem.,  1907,  1848. 


1 62  GRAVIMETRIC  ANALYSIS. 

together  by  electrolysis,  the  weighed  deposit  dissolved  in  nitric 
acid  (the  electrodes  must  be  completely  immersed  in  the  acid 
and  the  solution  boiled  for  at  least  20  minutes),  the  resulting 
solution  concentrated  to  a  small  volume,  and  the  nickel  deter- 
mined as  described  above.  The  method  can  be  recommended 
strongly. 

(6)  The  Potassium  Nitrite  Method  of  N.  W.  Fischer.* 

Brunck's  Modification.^ 

The  solution  containing  an  excess  of  acid  is  evaporated  to 
dryness  in  a  porcelain  dish  and  the  residue  treated  with  one  or 
two  drops  of  dilute  hydrochloric  acid  and  5  to  10  c.c.  of  water. 
Pure  caustic  potash  solution  is  then  added  drop  by  drop 
until  the  reaction  is  barely  alkaline.  The  resulting  precipitate 
is  dissolved  in  as  little  glacial  acetic  acid  as  possible,  half  of  the 
solution's  volume  of  50  per  cent,  potassium  nitrite  solution  is 
added,  and  10  drops  more  of  acetic  acid;  the  mixture  is  stirred 
well  and  allowed  to  stand  twenty-four  hours.  At  the  end  of 
this  time  the  precipitation  is  almost  always  complete.  It  should 
be  tested,  however,  by  removing  a  little  of  the  undiluted  solution 
with  a  pipette,,  adding  to  it  a  little  more  potassium  nitrite  solution, 
and  allowing  to  stand  a  little  longer.  If  at  the  end  of  an  hour 
no  further  precipitation  results,  then  all  the  cobalt  has  been 
precipitated.  If  a  precipitate  is  formed,  the  whole  solution  is 
treated  with  more  potassium  nitrite  and  again  allowed  to  stand. 
The  clear  liquid  is  poured  through  a  filter,  the  residue  trans- 
ferred to  the  filter  and  washed  with  a  10  per  cent,  potassium  acetate 
solution  until  1  c.c.  of  the  filtrate  on  being  acidified  with  acetic 
acid  and  boiled  with  1  c.c.  of  a  1  per  cent,  solution  of  dimethyl 
glyoxime  will  show  no  test  for  nickel.  This  is  usually  the  case 
after  washing  four  times.  As  much  of  the  precipitate  as  possible 
is  now  transferred  to  a  small  porcelain  dish,  which  is  covered 
with  a  watch-glass,  cautiously  acidified  with  sulphuric  acid,  and 
heated  on  the  water-bath  until  no  more  brown  vapors  are  evolved. 

*  Pogg.  Ann.,  71,  545  (1847). 
%  Z.  angew.  Chem.,  1907,  1847. 


SEPARATION  OF  COBALT  FROM  NICKEL.  163 

The  small  quantity  of  precipitate  remaining  on  the  filter  is  dis- 
solved by  pouring  hot,  dilute  sulphuric  acid  through  the  filter 
and  this  acid  is  added  to  the  main  solution  of  the  cobalt.  After 
evaporating  as  far  as  possible  on  the  water-bath,  the  heating 
is  continued  on  an  air-bath  until  dense  vapors  of  sulphuric  acid 
are  evolved.  After  cooling,  the  residue  is  dissolved  in  water  and 
the  cobalt  determined  electrolytically,  as  described  on  p.  138. 
If  it  is  not  convenient  to  carry  out  an  electrolysis,  the  nitrite 
precipitate  is  dissolved  in  hydrochloric  acid  and  the  cobalt 
.ietermined  according  to  p.  139  (6). 

The  nitrate  containing  the  nickel  can  be  treated  with  hydro- 
chloric acid  until  the  nitric  acid  is  completely  decomposed,  and  the 
nickel  then  precipitated  as  black  nickelic  hydroxide  by  caustic 
potash  and  bromine  water,  filtered,  washed,  and  weighed  as  the 
oxide,  according  to  p.  137. 

Remark. — This  method  gives  reliable  results  provided  the 
solution  is  free  from  alkaline  earths.  In  the  latter  case  the  nickel 
and  alkaline-earth  metals  are  precipitated  with  the  cobalt.  (Cf. 
Vol.  I.) 

(c)  Liebig's  Potassium  Cyanide  Method.  * 

This  method  is  based  upon  the  different  behavior  of  the  com- 
plex cyanogen  compounds  of  both  metals  towards  bromine  or 
chlorine  in  alkaline  solution.  (Cf.  Vol.  I.) 

Procedure. — The  neutral  solution,  which  may  contain  only 
nickel,  cobalt,  and  the  alkalies,  is  treated  with  an  excess  of  purest 
98  per  cent,  potassium  cyanide  and  5  gm.  of  pure  potassium  hy- 
droxide, after  which  bromine  water  is  added,  with  constant  stirring, 
until  no  more  nickelic  hydroxide  is  precipitated.  Care  must  be 
taken  that  the  solution  remains  strongly  alkaline  until  the  end 
of  the  process ;  upon  this  point  depends  the  success  of  this  excellent 
method.  When  the  precipitation  is  complete,  the  solution  is' 
diluted  with  cold  water  and  the  nickel  determined  as  oxide,  as 
described  on  p.  137'.  . 

The  cobalt  remains  in  the  filtrate  as  potassium  cobalticyanide.  \ 
After  the  addition  ot  dilute  sulphuric  acid,  the  solution  is  evapo- 

*  Ann.  d.  Chem.  u.  Pharm.,  65,  244;  87,  128. 


1  64  GRAVIMETRIC  ANALYSIS, 

rated  as  far  as  possible  on  the  water-bath,  a  little  concentrated 
sulphuric  acid  is  added,  and  the  residue  is  heated  over  a  free  flame 
until  dense,  white  fumes  are  evolved  and  the  effervescence  has 
ceased  : 

2K3Co(CN)6  +  12H2SO4  +  12H2O  = 

2CoS04  +  3K2S04  +  llCO+C02+6(NH4)2SO4  +  SO3. 

The  cold,  blue  mass  is  dissolved  in  water  and  the  cobalt 
deposited  electrolytically;  or,  the  cobalt  may  be  precipitated  by 
the  addition  of  bromine  water  and  potassium  hydroxide,  filtered, 
dried  and  determined  as  metal  according  to  p.  139. 

(d)  Liebig's  Mercuric  Oxide  Method. 

In  this  method  advantage  is  taken  of  the  fact  that  potassium 
nickelocyanide,  like  almost  all  other  complex  cyanogen  compounds, 
is  decomposed  by  mercuric  oxide,  whereas  potassium  cobalti' 
cyanide,  on  the  contrary,  is  unaffected  : 


Procedure.  —  A  slight  excess  of  pure  potassium  cyanide  is  added 
to  the  neutral  solution,  which  is  then  heated  on  the  water-bath 
for  at  least  one  hour  in  order  to  change  the  potassium  cobalto- 
cyanide  to  potassium  cobalticyanide  (cf  .  Vol.  I)  .  The  solution  is 
then  treated  with  a  suspension  of  mercuric  oxide  in  water  and 
heated  for  a  long  time,  with  frequent  stirring,  upon  the  water- 
bath.  The  decomposition  is  complete  after  one  or  two  hours. 
The  solution  is  diluted  somewhat  with  hot  water,  and  the  pre- 
cipitate, consisting  of  nickelous  hydroxide  and  the  excess  of 
mercuric  oxide,  is  filtered  off,  dried,  ignited  under  a  hood  with  a 
good  draft,  and  the  residue  of  nickel  oxide  weighed  as  described 
on  p.  137.  It  is  better,  however,  to  dissolve  the  nickel  oxide 
in  sulphuric  acid  and  determine  it  electrolytically  according  to 
p.  136,  or  as  the  salt  of  dimethyl  glyoxime,  according  to  p.  129. 
The  filtrate  containing  potassium  cobalticyanide  and  mercuric 
cyanide  is  treated  with  sulphuric  acid  exactly  as  described 
under  (6)  and  the  cobalt  determined  as  metal,  preferably  by 
electrolysis. 


SEPARATION  OF  NICKEL  FROM  ZINC.  165 

The  author  has  also  tested  and  found  satisfactory  the  method 
of  Ilinsky  and  Knorre;*  but  it  seems  to  have  no  advantages  over 
the  above-described  procedures. 

Recently  Rosenheirn  and  Huldschinsky  f  have  applied  VogePs 
qualitative  test  for  cobalt  (cf.  Vol.  I,  p.  137)  to  the  quantitative 
separation  of  this  metal  from  nickel,  and  have  obtained  excellent 
results. 

Separation  of  Nickel  from  Zinc.     Method  of  Tschugaeff-Brunck.J 

The  solution  is  treated  with  ammonium  chloride,  and  enough 
ammonia  to  make  it  slightly  ammoniacal;  no  precipitate  will  be 
formed  if  sufficient  ammonium  chloride  has  been  added.  The 
solution  is  then  just  acidified  with  hydrochloric  acid,  heated  to 
boiling  and  the  nickel  precipitated  with  an  alcoholic  1  per  cent, 
dimethyl  glyoxime  solution  exactly  as  outlined  on  p.  129. 

In  the  filtrate,  it  is  best  to  precipitate  the  zinc  as  sulphide 
by  acidifying  with  acetic  acid  and  saturating  the  hot  solution 
with  hydrogen  sulphide  (cf.  p.  145). 

Remark. — When  considerable  zinc  is  present  it  is  necessary  to 
add  more  dimethyl  glyoxime  to  precipitate  the  nickel. 

Separation  of  Nickel  from  Manganese.     Method  of  Tschugaeff- 

Brunck.§ 

The  analysis  is  carried  out  exactly  as  described  above  with 
the  only  difference  that  the  final  precipitation  takes  place  in  an 
acetic  acid  solution.  The  greater  part  of  any  mineral  acid 
present  is  neutralized  carefully  with  ammonia,  the  barely  acid 
solution  is  treated  with  1  per  cent,  dimethyl  glyoxime  solution 
and  then,  after  the  precipitate  has  formed,  sodium  acetate  is 
added  and  the  analysis  continued  according  to  p.  129.  If  the 
alkali  acetate  is  added  before  the  dimethyl  glyoxime,  a  very 
voluminous  precipitate  is  formed  which,  to  be  sure,  can  be  filtered 

*  Berichte,  18,  669. 

t  Ibid.,  34,  2050. 

t  Z.  angew.  Chem.,  1907,  1849. 

§  Ibid. 


1 66  GRAVIMETRIC  ANALYSIS. 

with  suction,  but  even  then  the  nitration  is  tedious.  Thus  when 
possible  it  is  best  to  add  the  sodium  acetate  after  the  dimethyl 
glyoxime.  When,  on  the  other  hand,  iron  has  been  removed  by 
a  basic  acetate  separation  and  nickel  and  manganese  are  to  be 
determined  in  the  nitrate,  the  precipitation  must  take  place  in  a 
solution  already  containing  sodium  acetate.  In  the  nitrate 
from  the  nickel  dimethyl  glyoxime  precipitation,  the  manganese 
is  precipitated  with  ammonium  sulphide  and  determined  as 
described  on  p.  125. 

Separation  of  Nickel  from  Iron. 

If  the  iron  is  present  in  the  ferrous  condition  it  is  oxidized 
by  boiling  with  nitric  acid.  Then  from  1  to  3  gm.  of  tartaric 
acid  are  added  and  the  solution  made  slightly  ammoniacal  in 
order  to  find  out  whether  enough  tartaric  acid  has  been  added 
(the  solution  must  remain  perfectly  clear).  After  making  barely 
acid  with  hydrochloric  acid,  the  nickel  is  precipitated  with 
dimethyl  glyoxime,  the  acid  just  neutralized  with  ammonia, 
and  the  analysis  continued  according  to  p.  129. 

Determination  of  Nickel  in  Steel.  * 

The  sample,  weighing  about  0.5  gm.,  is  dissolved  in  10  c.c.  of 
concentrated  hydrochloric  acid,  enough  nitric  acid  is  added  to 
completely  oxidize  the  iron,  and  if  there  is  any  separation  of 
silica  at  this  point  some  hydrofluoric  acid  is  added.  Two  or  three 
gms.  of  tartaric  acid  are  introduced,  and  the  solution  diluted  to  a 
volume  of  300  c.c.  It  is  then  carefully  tested  to  see  whether  enough 
tartaric  acid  is  present  to  prevent  any  precipitation  of  iron  when 
the  solution  is  -made  alkaline  with  ammonia,  more  tartaric  acid 
being  added  if  necessary.  The  solution,  which  is  left  slightly  acid, 
is  heated  nearly  to  boiling  and  treated  with  30  c.c.  of  a  1  per  cent, 
alcoholic  solution  of  dimethyl  glyoxime.  The  acid  is  finally  very 
carefully  neutralized  with  ammonia,  leaving  the  solution  so  that 
it  barely  smells  of  this  reagent.  The  precipitate  is  filtered  through 

*  O.  Brunck,  Stahl  und  Eisen,  28,  331. 


REMOVAL   OF  FERRIC  CHLORIDE  BY  ETHER.  167 

a  Gooch  or  Munroe  crucible,  washed  with  hot  water,  dried  at 
110°-120°  for  45  minutes  and  weighed  as  Ni(C4H7N2O2)2. 

By  this  method  the  nickel  in  a  sample  of  steel  -can  be  deter- 
mined within  about  two  hours.  The  results  are  accurate,  but 
lower  than  is  often  obtained  in  practice,  because  the  cobalt  is 
usually  determined  with  the  nickel,  which  is  not  the  case  in  this 
method. 


Removal  of  Ferric  Chloride  by  Ether,  Method  of  Rothe. 

The  fact  that  ferric  chloride  dissolved  in  hydrochloric  acid, 
sp.  gr.  1.1,  is  more  soluble  in  ether  than  in  this  acid  is  often  taken 
advantage  of  in  the  determination  of  metals  such  as  nickel,  copper, 
vanadium  and  chromium  in  samples  of  steel.  It  has  also  been 
used  for  the  determination  of  sulphur  in  steel  after  oxidation  to 
sulphuric  acid,  which  does  not  dissolve  in  the  ether.  The  under- 
lying principle  is  the  same  as  that  governing  the  distribution  of 
iodine  between  water  and  carbon  disulphide  (see  pp.  658  and  659, 
footnote).  An  example  will  be  given  of  such  a  process  in  the 
Blair  method  for  estimating  vanadium,  molybdenum,  chromium 
and  nickel  in  steel.  (See  p.  313.) 


1 68  GRAVIMETRIC  ANALYSIS. 


METALS   OF   GROUP   II. 

MERCURY,  LEAD,  BISMUTH,  COPPER,  CADMIUM,  ARSENIC, 
ANTIMONY,  TIN  (PLATINUM,  GOLD,  SELENIUM,  TELLURIUM, 
MOLYBDENUM,  GERMANIUM,  TUNGSTEN,  AND  VANADIUM). 

A.    SULPHO-BASES. 
MERCURY,  LEAD,  BISMUTH,  COPPER,  CADMIUM. 

MERCURY,  Hg.    At.  Wt.  200.0. 
Forms:  HgS,  Hg2Cl2,  and  Hg. 
Determination  as  Sulphide. 

(a)  By  Precipitation  with  Hydrogen  Sulphide. 

The  solution  containing  no  oxidizing  substances  (FeCl3,  O, 
much  HNO3,  etc.)  and  the  mercury  entirely  as  mercuric  salt  is 
saturated  with  hydrogen  sulphide  in  the  cold,  the  precipitate 
allowed  to  settle,  filtered  through  a  Gooch  crucible,  washed  with 
cold  water,  dried  at  105°-110°  C.  and  weighed. 

Remark. — This  method  affords  excellent  results  and  should  be 
used  whenever  possible.  Unfortunately,  however,  it  is  not  always 
applicable,  for  in  most  cases  the  solution  to  be  analyzed  contains 
strong  nitric  acid  (obtained  by  the  solution  of  impure  mercuric 
sulphide  in  aqua  regia,  by  the  decomposition  of  organic  mercury 
compounds  by  the  method  of  Carius,  or  by  the  oxidation  of  mercurous 
salts).  It  is  not  possible  to  expel  the  excess  of  nitric  acid  by 
evaporating  the  solution  with  hydrochloric  acid,  because  consider- 
able amounts  of  mercuric  chloride  are  thereby  volatilized  with 
the  escaping  steam.  Thus  50  c.c.  of  a  mercuric  chloride  solution 
containing  0.5235  gm.  of  the  salt,  treated  with  10  c.c.  of  nitric 
acid  and  evaporated  on  the  water-bath  five  times  almost  to  dryness, 
with  the  addition  each  time  of  50  c.c.  concentrated  hydrochloric 
acid,  yielded  in  separate  experiments  0.3972  gm.  mercuric  sulphide 
=  88.56  per  cent,  and  0.3695  gm.  mercuric  sulphide  =  82.39  per 
cent.,  or,  in  other  words,  a  loss  of  11-17  per  cent.  In  such  a  case  the 
following  procedure  suggested  by  Volhard  should  be  used: 


DETERMINATION  OF  MERCURY  AS  SULPHIDE.  169 

(b)  By  Precipitation  with  Ammonium  Sulphide. 

The  acid  solution  of  the  mercuric  salt  is  almost  neutralized 
with  pure  sodium  carbonate  and  is  treated  with  a  slight  excess  of 
freshly-prepared  ammonium  sulphide.  Pure  sodium  hydroxide 
solution  (free  from  Ag,  A12O3,  and  SiO2)  is  then  added,  meanwhile 
rotating  the  solution  until  the  dark  liquid  begins  to  lighten,  when 
it  is  heated  to  boiling  and  more  sodium  hydroxide  is  added  until 
the  liquid  is  perfectly  clear.  The  solution  now  contains  the  mer- 

cury   as    sulpho-salt,     Hg<    o      •      Ammonium    nitrate    is    then 


added  and  the  solution  boiled  until  the  ammonia  is  almost  entirely 
expelled,  and  the  precipitate  is  allowed  to  settle,  which  it  will 
do  much  more  quickly  than  if  it  were  produced  by  hydrogen 
sulphide  directly.  By  means  of  the  boiling  with  ammonium. 
nitrate,  the  sulpho-salt  is  decomposed  according  to  this  equation: 

Hg(SNa)2  +  2NH4N03  =  2NaNO3  +  (NH^S  +  HgS. 

The  clear  liquid  is  poured  through  a  Gooch  crucible,  and  the 
precipitate  washed  by  decantation  with  hot  water  until  the 
wash  water  no  longer  reacts  with  silver  nitrate  solution.  The 
precipitate  is  then  transferred  to  the  crucible,  dried  at  110°C.,and 
weighed.  In  case  the  precipitate  contains  free  sulphur,  it  should 
be  boiled  with  a  little  sodium  sulphite  before  filtering.* 

H.  Rauschenbach  tested  this  method,  analyzing  pure  mercuric 
chloride  with  the  addition  of  nitric  acid,  and  obtained  as  a  mean  of 
two  experiments  73.80  per  cent.  Hg  instead  of  the  theoretical  value, 
73.85  per  cent. 

A  still  better  way  of  removing  free  sulphur  from  the  precipitate 
consists  of  extracting  with  carbon  bisulphide.  In  this  case  the 
mercuric  sulphide,  together  with  the  sulphur,  is  filtered  through 
a  Gooch  crucible,  completely  washed  with  water  and  then  three 
times  with  alcohol.  The  crucible  is  now  placed  upon  a  glass 
tripod  in  a  beaker  containing  some  carbon  bisulphide  (Fig.  35)  ;t 

*  By  boiling  with  sodium  sulphite,  the  sulphur  is  changed  to  sodium 
thiosulphate,  Na2SO3  +  S  =  Na2S2O3. 

f  G.  Vortmann,  Uebungsbeispiele  aus  der  quantitative!!  chemischem 
Analyse,  p.  28,  Vienna,  1899. 


170 


GRAVIMETRIC  ANALYSIS. 


the  beaker  is  supported  over  a  vessel  filled  with  hot  water  and 

covered   with    a   round-bottomed   flask    con- 

taining cold  water  which  serves  as  a  condenser. 

After  about  an  hour  the  sulphur  will  be  com- 

pletely extracted.     The  carbon  bisulphide  is 

removed    from   the    precipitate   by  washing 

once  with  alcohol  and  once  with  ether.     The 

ether  is  driven  off  by  gently  warming,  and 

the    precipitate    then    dried  at    110°  C.  and 

weighed. 

H.  Rauschenbach  analyzed  pure  mercuric 
chloride  by  this  method  and  obtained  as  a 
mean  of  eight  experiments  73.79  per  cent. 
Hg  instead  of  73.85  per  cent.,  and  in  the  case 
of  eight  further  experiments  made  without 
removing  the  sulphur  he  obtained  74.17  per 
cent,  instead  of  the  theoretical  value,  73.84  per  cent. 


JTIG  35> 


Determination  of  Mercury  in  Non-Electrolytes. 

If  it  is  desired  to  determine  mercury  in  an  organic  non-electro- 
lyte, the  compound  is  decomposed  by  the  method  of  Carius  (see 
Elementary  Analysis)  by  heating  in  a  closed  tube  with  concen- 
trated nitric  acid,  and  the  mercury  precipitated  as  sulphide  by  the 
method  of  Volhard  ;  or  the  acid  solution  is  treated  with  pure  sodium 
hydroxide  solution  to  alkaline  reaction  and  then  with  pure  potas- 
sium cyanide  until  the  mercuric  oxide  has  dissolved,  after  which 
the  solution  is  saturated  with  hydrogen  sulphide,  ammonium 
acetate  added,  the  solution  boiled  until  the  ammonia  is  almost 
entirely  expelled,  the  precipitate  allowed  to  settle,  filtered,  and 
washed  first  with  hot  water,  then  with  hot  dilute  hydrochloric  acid, 
and  finally  with  water.  After  drying  at  110°  C.  the  precipitate  of 
mercuric  sulphide  is  weighed. 

Determination  as  Mercurous  Chloride. 

For  the  analysis  of  a  solution  containing  a  mercurous  salt,  the 
solution  is  treated  with  sodium  chloride,  diluted  considerably  with 
water,  filtered,  after  standing  twelve  hours,  through  a  Gooch  cruci- 
ble, dried  at  105°  C.,  and  weighed.  If  the  solution  contains  a  mer- 


DETERMINATION  OP  MERCURY  AS  METAL  171 

curie  salt,  it  is  first  reduced,  by  the  method  of  H.  Rose,  by  means 
of  phosphorous  acid  in  the  presence  of  hydrochloric  acid. 

Procedure. — The  mercury  solution  (which  almost  always  con- 
tains nitric  acid)  is  treated  with  hydrochloric  acid,  diluted  con- 
siderably with  water,  an  excess  of  phosphorous  acid  is  added,  and 
after  standing  for  twelve  hours  the  precipitate  is  filtered  through 
a  Gooch  crucible,  dried  at  105°  C.,  and  weighed. 

Remark  —The  results  obtained  by  this  method  are  always 
about  0.4  per  cent,  too  low,  but  in  spite  of  this  fact  the  method  is 
to  be  recommended. 

The  phosphorous  acid  necessary  for  this  method  is  obtained  by 
the  oxidation  of  phosphorus  in  moist  air  or  by  the  decomposition 
of  phosphorus  trichloride  with  water,  evaporating  the  solution  to 
remove  the  hydrochloric  acid  and  dissolving  the  residue  in  water. 

Determination  as  Metal. 

Almost  all  mercury  compounds  are  quantitatively  decompose .'. 
on  heating  with  lime  according  to  the  equation 

HgX + CaO  =  CaX  +  Hg  +  O. 

The  iodide  alone  is  not  readily  acted  upon. 

To  carry  out  this  determination,  a  glass  tube  50  cm.  long  and 
1.5  cm.  wide,  open  at  both  ends,  is  taken  and  in  one  end  an  asbes- 
tos plug  is  placed,  followed  by  8  cm.  of  pure  lime,  then  an  intimate 
mixture  of  a  weighed  amount  of  substance  with  lime,  finally  a  layer 
of  lime  30  cm.  long  and  at  the  other  end  of  the  tube  another  asbes- 
tos plug.  After  the  tube  has  been  filled,  the  end  nearest  this  sec- 
ond asbestos  plug  is  drawn  out  until  it  is  only  4  cm.  wide,  and  is 
connected  by  means  of  rubber  tubing  with  the  empty  narrower 
arm  of  a  Peligot  tube.  The  other  wider  end  of  the  Peligot  tube  is 
loosely  filled  with  pure  gold-leaf.  The  glass  tube  is  placed  in  a 
combustion-furnace  and  illuminating-gas  (carbon  dioxide  is  less 
suited)  is  passed  through  it  for  half  an  hour.  The  tube  is  heated, 
at  first  where  the  30  cm.  layer  of  lime  is,  then  the  other  burners  are 
lighted  one  after  another  until  finally  the  entire  contents  of  the 
tube  is  subjected  to  gentle  ignition.  During  the  whole  of  the  opera- 
tion illuminating-gas  is  being  passed  through  the  apparatus  at  the 
rate  of  about  three  bubbles  a  second.  The  greater  part  of  the 
mercury  collects  in  the  lower  empty  end  of  the  Peligot  tube  and 


172  GRAVIMETRIC  ANALYSIS. 

the  mercury  vapors  that  are  carried  further  amalgamate  with  the 
gold.  A  small  amount  of  the  mercury  condenses  in  the  drawn-out 
tube.  After  cooling  the  apparatus  (in  a  current  of  illuminating-gas) 
the  narrow  part  of  the  tube  is  cut  off  both  sides  of  the  condensed 
mercury  and  weighed.  It  is  then  heated  gently  while  air  is  passed 
through  it  to  volatilize  the  mercury  and  again  weighed.  The  dif- 
ference in  weight  gives  the  amount  of  mercury  condensed  in  the 
tube.  The  Peligot  tube  is  usually  moist;  dry  air  is,  therefore,  con- 
ducted through  it  for  some  time,  after  which  it  is  weighed. 

The  results  obtained  by  this  method  *  are  perfectly  satisfactory. 
Winteler  found  in  the  analysis  of  pure  mercuric  chloride  73.81, 
73.88,  73.74  per  cent,  instead  of  the  theoretical  value,  73.85  per 
cent. 

Experiments  made  attempting  to  condense  the  mercury  under 
water  invariably  gave  too  low  values  (about  1-2  per  cent.). 

Although  it  is  easy  to  obtain  good  results  by  this  method,  it 
can  be  dispensed  with,  for  the  sulphide  method  affords  just  as 
exact  results  in  much  less  time. 

In  case  it  is  desired  to  determine  the  amount  of  mercury  vapor 
present  in  a  given  space,  it  is  only  necessary  to  aspirate  the  gas 
through  a  calcium-chloride  tube  filled  with  gold-leaf.  The  gain 
in  weight  of  the  latter  shows  the  amount  of  mercury  present  in 
the  gas. 

Electrolytic  Determination  of  Mercury.f 

Mercury  can  be  determined  satisfactorily  by  the  electrolysis 
of  acid,  neutral,  or  alkaline  solutions.  The  metal  is  deposited  in 
the  form  of  little  drops,  which,  when  the  quantity  is  small,  adhere 
to  the  electrode,  or,  when  larger  amounts  are  present,  the  mercury 
may  collect  at  the  bottom  of  the  platinum  dish  used  as  cathode. 
The  use  of  silver-plated  electrodes  is  also  advised. 

The  electrolysis  takes  place  to  advantage  in  solutions  slightly 
acid  with  nitric  acid. 

*  First  proposed  by  Erdmann  and  Marchand,  J.  prakt.  Chem.,  31,  385. 

f  Luckow,  Z.  anal.  Chem.,  19,  15  (1880);  Smith  and  Knerr,  Am.  Chem. 
J.,  8,  206;  F.  W.  Clarke,  Ber.,  11,  1409  (1878);  Riiderff,  Z.  angew.  Chem., 
1894,  388;  Classen  and  Ludwig,  Ber.,  19,  324  (1886);  G.  Vortmann,  Ber., 
24,  2750  (1891). 


ELECTROLYTIC  DETERMINATION  OF  MERCURY.  173 

Procedure. — The  neutral  or  slightly  acid  solution  of  the  mer- 
curous  or  mercuric  salt  is  placed  in  a  beaker,  diluted  with  water 
to  150  c.c.,  treated  with  2  or  3  c.c.  of  concentrated  nitric  acid, 
and  elect roly zed  with  a  platinum  gauze  cathode  at  the  ordinary 
temperature  with  a  current  of  0.055-0.10  ampere.  The  voltage 
under  these  conditions  corresponds  to  3.5-5  volts.  If  the 
electrolysis  is  started  at  night,  it  will  be  finished  next  morning, 
provided  the  amount  of  mercury  does  not  exceed  1  gm.  By  using 
a  current  of  0.6-1  ampere  the  electrolysis  is  finished  at  the 
end  of  two  or  three  hours.  At  the  end  of  the  electrolysis,  the 
metal  is  washed  with  water  without  interrupting  the  current, 
then  with  alcohol*  and  dried.  The  metal  is  further  dried  by 
touching  it  with  filter  paper,  and  then  placing  it  in  a  desiccator  f 
over  fused  potassium  hydroxide  and  a  small  dish  of  mercury. 
In  this  way  correct  results  are  obtained.  Drying  at  100°  and  then 
over  sulphuric  acid  in  a  desiccator  gives  rise  to  low  results  because 
the  acid  absorbs  considerable  mercury  vapor. 

During  the  electrolysis  of  mercuric  chloride  J  the  solution 
often  becomes  turbid  in  consequence  of  the  formation  of  insoluble 
mercurous  chloride;  this  does  no  harm,  howrever,  as  the  metal 
is  subsequently  deposited  on  the  cathode. 

Mercury  can  also  be  electrolyzed  from  a  solution  in  potassium 
cyanide  in  the  presence  of  some  caustic  alkali,  and  similarly  from 

*  It  is  usually  stated  that  alcohol  is  not  to  be  used,  but  with  gauze 
electrodes  it  does  no  harm. 

f  Private  communication  from  A.  Miolati,  cf.  Borelli,  Revisto  tecnica, 
V,  Part  7  (1905).  Even  at  20°  the  tension  of  mercury  vapor  is  considerable. 
It  amounts  to  0.00133  mm. 

%  In  the  electrolysis  of  the  chloride,  it  is  better  to  use  a  platinum  dish 
with  dull,  unpolished  inner  surface  (Classen)  because  then  any  mercurous 
chloride  will  certainly  be  reduced  to  metal,  which  is  not  always  the  case 
with  gauze  electrodes.  When  a  dish  is  use4  as  cathode,  the  electrode  is 
washed  with  water,  without  breaking  the  current,  by  pouring  water  into  it 
from  a  wash-bottle  while  the  solution  is  being  siphoned  off.  As  soon  as. 
the  ammeter  (or  voltmeter  used  as  an  ammeter)  reaches  the  zero  mark,  the 
washing  is  finished.  The  current  is  then  turned  off,  the  water  carefully 
poured  off,  the  rest  of  it  removed  by  touching  it  with  filter-paper,  and 
the  electrode  dried  as  above  and  weighed.  The  drying  requires  several 
hours. 


174  GRAVIMETRIC  ANALYSIS. 

a  solution  formed  by  dissolving  mercuric  sulphide  in  50—60  c.c. 
of  concentrated  sodium  sulphide  solution. 

The  great  advantage  of  the  electrolytic  determination  of 
mercury  lies  in  the  fact  that  good  deposits  are  obtained  irrespective 
of  the  nature  of  the  acid  radical,  or  element,  which  is  combined 
with  mercury. 

LEAD,  Pb.    At.  Wt.  207.1. 

Forms:  PbO,  PbS04,  Pb02,  and  in  rare  cases  PbCl2.* 
i.  Determination  as  Lead  Oxide,  PbO. 

If  the  lead  is  present  as  carbonate,  nitrate,  or  peroxide,  it  is 
only  necessary  to  ignite  a  weighed  portion  in  a  porcelain  crucible 
over  a  small  flame  and  weigh  the  residue.  The  treatment  of  the 
nitrate  requires  care,  because  on  rapid  ignition  the  mass  decrepi- 
tates. 

2.  Determination  as  Lead  Sulphate,  PbS04. 

If  the  lead  is  present  in  solution  in  the  form  of  its  chloride  or 
nitrate,  it  is  placed  in  a  porcelain  dish,  an  excess  of  dilute  sulphuric 
acid  is  added  t  and  the  mixture  evaporated  on  the  water-bath  as 
far  as  possible,  then  over  a  free  flame  until  dense  white  fumes  of 
sulphuric  acid  are  evolved,  and  afterwards  allowed  to  cool.  A  little 
water  is  added,  the  mixture  stirred,  allowed  to  stand  some  hours, 
filtered  through  a  Gooch  crucible,  washed  at  first  with  4  per  cent, 
sulphuric  acid,  then  with  alcohol,  and  dried  at  100°  C.  The  dried 
precipitate  is  placed  in  a  larger  porcelain  crucible,  provided  with 
an  asbestos  ring,  and  ignited  over  the  full  flame  of  a  Teclu  burner. 

If  it  is  desired  to  use  an  ordinary  filter,  the  precipitate  is  finally 
washed  with  alcohol  until  the  wash  liquid  no  longer  gives  the  sul- 
phuric acid  reaction,  dried,  as  much  of  it  as  possible  is  transferred 
to  a  weighed  porcelain  crucible,  the  filter  ignited  in  a  platinum 
spiral  (p.  22),  and  the  ash  added  to  the  contents  of  the  crucible. 
By  means  of  the  reducing  action  of  the  burning  filter,  some  of  the 
lead  sulphate  adhering  to  it  is  always  reduced  to  lead,  which  must 

*  See  Analysis  of  Vanadinite. 

t  The  solution  at  the  time  of  filtering  should  contain  about  5  per  cent, 
of  free  sulphuric  acid. 


DETERMINATION  Of"  LEAD  AS   LEAD  SULPHATE.  I?S 

be  changed  back  to  sulphate  before  weighing.  For  this  purpose 
the  precipitate  in  the  crucible  is  moistened  with  dilute  nitric  acid, 
evaporated  on  the  water-bath  to  dryness,  a  few  drops  of  concen- 
trated sulphuric  acid  added  and  the  crucible  heated  over  a  free 
flame  until  no  more  fumes  are  given  off,  when  it  is  gently  ignited  and 
weighed. 

In  case  the  lead  is  originally  present  as  acetate,  the  solution 
is  treated  with  an  excess  of  dilute  sulphuric  acid  and  twice  its 
volume  of  alcohol,  filtered  after  standing  some  hours,  and  the 
precipitate  of  lead  sulphate  treated  exactly  as  described  above. 

In  order  to  determine  the  amount  of  lead  present  in  organic 
compounds,  the  substance  can  be  placed  in  a  large  porcelain  crucible, 
treated  with  an  excess  of  concentrated  sulphuric  acid,  and  very 
cautiously  heated  in  the  covered  crucible  over  a  free  flame  until 
the  sulphuric  acid  is  completely  expelled.  The  crucible  is  then 
gently  ignited,  and  if  the  residue  is  white  it  is  ready  to  be  weighed; 
otherwise  more  sulphuric  acid  is  added  and  the  process  repeated 
until  finally  a  white  residue  is  obtained. 

In  case  the  organic  lead  compound  is  soluble  in  water,  it  is 
preferable  to  first  precipitate  the  lead  by  means  of  hydrogen 
sulphide  and  then  transform  the  precipitate  into  sulphate.  For 
this  purpose,  as  much  as  possible  of  the  washed  and  dried  pre- 
cipitate is  placed  upon  a  watch  glass,  the  filter  and  remainder 
of  the  precipitate  are  heated  in  a  large  porcelain  crucible,  which  is 
supported  in  an  inclined  position,  and  heated  carefully  over  a 
small  flame  until  the  filter-paper  is  completely  consumed.  The 
main  part  of  the  precipitate  is  added  to  the  crucible,  which  is 
then  covered  with  a  watch-glass  and  treated  with  concentrated 
nitric  acid  at  the  temperature  of  the  water-bath.  When  the 
main  reaction  is  over,  the  treatment  with  fuming  nitric  acid  is 
repeated  until  the  contents  of  the  crucible  are  pure  white  in 
color.  The  watch-glass  is  then  removed,  five  or  ten  drops  of 
dilute  sulphuric  acid  are  added,  the  liquid  is  evaporated  as  far 
as  possible  on  the  water-bath,  the  excess  of  sulphuric  acid  is  re- 
moved by  heating  on  the  air-bath  (cf.  Fig.  11,  p.  27)  and  the  lead 
sulphate  is  weighed.  Should  the  precipitate  be  dark  colored 
after  the  ignition,  it  is  moistened  with  concentrated  sulphuric 
acid  and  the  excess  of  acid  again  expelled. 


176  GRAVIMETRIC  ANALYSIS. 

If  the  lend  is  present  in  an  organic  compound  \\hich  is  not 
capable  of  dissociation,  the  compound  should  be  decomposed  in  a 
closed  tube  with  strong  nitric  acid  according  to  the  method  of 
Carius  (see  page  287),  finally  washing  out  the  contents  of  the  tube, 
adding  sulphuric  acid,  and  treating  the  precipitate  as  above  de- 
scribed. 

Separation  of  Lead  Sulphate  from  Barium  Sulphate  and  Silicic 

Acid. 

In  the  analysis  of  sulphide  ores  containing  lead,  it  is  customary 
to  dissolve  the  finely  powdered  ore  in  nitric  acid,  or  aqua  regia, 
and  to  remove  the  volatile  acids  by  evaporation  with  sulphuric 
acid,  eventually  heating  over  the  free  flame  until  fumes  of  sul- 
phuric acid  come  off  thickly.  The  sulphuric  acid  should  be  diluted 
with  an  equal  volume  of  water  before  adding  it  to  the  original 
solution;  usually  5  c.c.  of  the  diluted  acid  is  sufficient.  After  the 
evaporation  the  moist  residue  is  allowed  to  cool,  then  water  is 
added  and  the  precipitate  filtered  and  washed  with  1  per  cent, 
sulphuric  acid.  The  precipitate  contains  all  the  lead  as  sulphate 
but  often  contains  silica  and  barium  sulphate  (also  strontium 
sulphate  and  sometimes  calcium  sulphate).  It  is  purified  by 
redissolving  the  lead  in  hot  ammonium  acetate  solution  (made  by 
neutralizing  acetic  acid,  sp.  gr.  1.04,  with  ammonia,  sp.  gr.  0.96, 
and  leaving  the  mixture  barely  ammoniacal) .  When  the  precip- 
itate is  large  in  amount  it  is  best  to  wash  it  into  a  beaker  or  flask 
and  heat  it  with  about  20  c.c.  of  the  ammonium  acetate  solution 
(or  enough  to  dissolve  all  the  lead  sulphate),  then  filter  through 
the  original  filter  and  wash  with  hot  ammonium  acetate  solution, 
and  finally  with  hot  water  until  the  filtrate  gives  no  blackening 
with  ammonium  sulphide.  Small  amounts  of  lead  sulphate  are 
dissolved  on  the  filter.  The  silica  and  barium  sulphate  will  all 
remain  behind. 

In  order  to  obtain  lead  from  the  acetate  solution,  it  is  precip- 
itated as  sulphide  by  hydrogen  sulphide,  and  transformed,  after 
drying,  into  sulphate  as  described  on  page  175. 

Or,  the  ammonium  acetate  solution  may  be  treated  with  10  c.c. 
of  50  per  cent,  sulphuric  acid,  the  acetic  acid  removed  by  evap- 
oration, the  residue  allowed  to  cool,  diluted  with  water,  and  the 


ELECTROLYTIC  DETERMINATION  OF  LEAD.  177 

lead  sulphate  filtered  into  a  Gooch  crucible,  washed  with  dilute 
sulphuric  acid,  heated  in  an  air  bath  and  weighed. 

If  the  amount  of  ammonium  acetate  solution  used  is  not  too 
large,  the  lead  may  be  precipitated  by  adding  enough  sulphuric 
acid  to  the  acetate  solution  to  make  the  solution  contain  from 
5-10  per  cent,  sulphuric  acid.  Sometimes  the  precipitate  is  not 
pure  lead  sulphate,  in  which  case  it  should  be  redissolved  in 
ammonium  acetate  and  the  precipitation  as  sulphate  repeated. 

3.  Electrolytic  Determination  of  Lead  as  Peroxide  (Pb02). 

Many  neutral  solutions  of  complex  lead  salts,  a  neutral  solu- 
tion of  lead  acetate,  also  alkaline  lead  solutions  yield  deposits  of 
metallic  lead  on  the  cathode  when  subjected  to  electrolysis;  but 
lead  is  never  determined  this  way;  partly  because  of  the  round- 
about process  necessary,  and  partly  on  account  of  the  fact  that 
the  deposited  lead  is  oxidized  so  readily.  If  a  neutral  or  slightly 
acid  (nitric  acid)  solution  of  lead  nitrate  is  electrolyzed,  the 
lead  is  deposited  partly  as  metal  upon  the  cathode  and  partly  as 
brown  peroxide  on  the  anode.  If,  however,  the  solution  contains 
sufficient  free  nitric  acid,  it  is  easily  possible  to  deposit  the  lead 
quantitatively  upon  the  anode  as  firmly -adhering  lead  peroxide. 

Procedure. — The  solution  of  lead  nitrate,  containing  not  more- 
than  0.5  gm.  lead,  is  placed  in  a  platinum  dish  whose  inner  sur- 
face is  unpolished  (as  recommended  by  Classen),  20-30  c.c.  of 
pure  nitric  acid,  sp.  gr.  1.4,  are  added,  the  solution  is  diluted  to 
150-200  c.c.  and  electrolyzed  in  the  cold  with  a  weak  current  of 
about  0.5-1  ampere  and  2-2.5  volts.  When  the  electrolysis  is 
carried  out  in  the  cold,  all  the  lead  will  be  deposited  as  the  peroxide 
at  the  end  of  two  hours  and  a  half  or  three  hours.  Only  an  hour 
or  an  hour  and  a  half  is  required  if  the  temperature  of  the  cell 
is  kept  at  50°-60°.  If  it  is  desired  to  let  the  electrolysis  run  over 
night,  a  current  of  0.05  ampere  is  sufficient. 

A  suitable  arrangement  of  the  electrolytic  apparatus  is  shown 
in  Fig.  36,  but  the  dish  should  serve  as  anode  and  the  platinum 
spiral  as  cathode.  The  resistance  W  is  made  by  taking  about 
10  m.  of  nickel  wire  of  about  0.5  mm.  diameter,  fastening  it  to  a 
board  as  shown  in  the  drawing  and  connecting  the  wires  in  pairs 
by  means  of  a  brass  hook,  of  which  only  one  is  shown  in  the 
sketch.  By  suitably  moving  these  hooks  it  is  possible  to  vary 


i78 


GRAVIMETRIC  ANALYSIS. 


the  resistance  at  will.  Instead  of  this  arrangement,  that  shown 
in  Fig.  31,  p.  132,  may  be  used;  such  an  apparatus  is  more 
convenient  but  also  more  expensive.  At  the  end  of  the  elec- 
trolysis, which  is  shown  by  the  fact  that  dilution  with  a  little 
water  so  as  to  expose  a  fresh  surface  of  platinum  causes  no  yel- 
lowish-brown coating  to  appear  at  the  end  of  half  an  hour,  the 
dish  is  washed  without  breaking  the  current.  This  is  accom- 
plished by  introducing  distilled  water  while  the  solution  is  being 
siphoned  off.  It  is  important  in  this  operation  to  keep  the  deposit 


FIG,  36. 

of  lead  peroxide  completely  covered  with  liquid.  When  the 
solution  that  is  being  siphoned  off  no  longer  reacts  acid,  or  at 
least  only  barely  acid,  the  washing  is  complete  and  the  circuit  can 
be  broken.  The  dish  is  finally  washed  once  more  with  distilled 
water,  dried  at  180°  C.,  and  weighed.  The  results  obtained  are 
always  slightly  high  on  account  of  the  lead  peroxide  not  being 
completely  anhydrous  when  dried  at  this  temperature,  so  that  it 
seems  to  the  author  to  be  advisable  to  gently  ignite  the  dish 
before  weighing,  thereby  readily  converting  the  peroxide  into 
lead  oxide.*  The  results  obtained  in  the  author's  laboratory 
leave  nothing  to  be  desired. 

*  Cf.  W.  C.  May,  Z.  analyt.  Chem.,  14,  347  (1875). 


DETERMINATION  OF  BISMU7H  AS  BISMUTH  OXIDE,  ETC.       179 

Results. — (a)  10  c.c.  lead  nitrate  solution  containing  0.0631 
gm.  lead  yielded  deposits  of  PbO2  weighing  0.0734,  0.073L  0.0735, 
0.0733  gm. ;  mean  0.07332  corresponding  to  0.0635  gm.  lead.  After 
ignition  the  lead  monoxide  formed  weighed  respectively  0.0679, 
0.0678,  0.0679,  0.0681 ;  mean  0.0679  gm.  corresponding  to  0.0630 
instead  of  0.0631  gm.  lead. 

(6)  10  c.c.  of  a  lead  nitrate  solution  containing  0.1898  gm. 
lead  yielded  deposits  of  PbO2  weighing  0.2202,  0.2200,  0.2203, 
0.2202;  mean  0.2202  corresponding  to  0.1907  gm.  lead.  After 
ignition  the  weights  of  lead  oxide  obtained  were  0.2042,  0.2046, 
0.2043.  0.2044;  mean  0.2044,  corresponding  to  0.1897  gm.  Pb 
instead  of  0.1898  gm.  These  experiments  were  performed  by  M. 
Stoffel. 

Remark. — By  employing  a  stronger  current  and  keeping  the 
solution  warm  during  the  electrolysis,  the  deposition  is  complete 
in  much  less  time,  but  according  to  the  author's  experience  the 
results  obtained  are  not  so  satisfactory.  By  rotating  one  of  the 
electrodes  and  using  a  stronger  current,  the  deposition  can  be  made 
to  take  place  in  a  short  time.  If  a  little  lead  deposits  on  the 
cathode,  this  is  remedied  by  stopping  the  current  for  a  short  time, 
toward  the  end  of  the  electrolysis. 

Besides  the  above-mentioned  forms,  lead  is  also  determined 
as  the  chromate  and  as  the  chloride.  The  latter  method  is 
sometimes  used  in  the  analysis  of  bearing  metal,  cf.  p.  252. 


BISMUTH,  Bi.    At.  Wt.  208.0. 

Forms:  Bi203,  Bi2S3,  Bi. 
I.  Determination  as  Bismuth  Oxide,  Bi20s. 

Solid  bismuth  nitrate  or  carbonate  is  readily  changed  to  the 
oxide  by  gentle  ignition.  When  bismuth,  however,  is  present  in 
solution  as  the  nitrate,  it  should  be  first  precipitated  as  the  basic 
carbonate  and  this  changed  by  ignition  to  the  oxide. 

Procedure. — The  solution  is  diluted  with  water  (if  a  turbidity 
ensues  it  makes  no  difference)  a  slight  excess  of  ammonium 
carbonate  is  added,  and  after  heating  to  boiling  the  precipitate 


i8o  GRAVIMETRIC  ANALYSIS. 

is  filtered  off.  washed  with  hot  water,  dried,  ignited,*  and  weighed 
as  Bi2O3.  If  the  solution  from  which  the  bismuth  is  to  be  pre- 
cipitated contains  besides  nitric  acid  other  acids  (HC1,  H2SO4,  etc.), 
the  precipitate  produced  by  ammonium  carbonate  always  con- 
tains basic  salts  of  these  acids  which  cannot  be  converted  to  the 
oxide  by  ignition.  In  this  case,  which  is  most  frequent  in  analysis, 
the  bismuth  should  be  determined  according  to  one  of  the  follow- 
ing  methods. 

2.  Determination  as  Sulphide,  Bi2S3. 

The  slightly  acid  solution  is  saturated  with  hydrogen  sulphide, 
filtered  through  a  Gooch  crucible  (or  a  filter  that  has  been  dried 
at  100°  C.  and  weighed) ,  washed  with  hydrogen  sulphide  water, 
then  with  alcohol  to  remove  the  water,  and  afterwards  with  freshly- 
distilled  carbon  bisulphide  f  to  remove  any  sulphur  that  may  be 
mixed  with  the  precipitate. 

The  washing  with  carbon  bisulphide  is  continued  until  a  few 
drops  of  the  filtrate  leave  no  residue  on  being  evaporated  to  dry- 
ness  on  a  watch-glass.  The  precipitate  is  then  washed  with 
alcohol  to  remove  the  carbon  bisulphide  and  finally  with  ether, 
dried  at  100°  C.,  and  weighed  as  Bi2S3. 

The  distillation  of  the  carbon  bisulphide  should  be  performed 
as  follows:  Ordinary  commercial  carbon  bisulphide  is  placed  in 
a  long-necked,  round-bottomed  flask,  provided  with  a  closely  fit- 
ting cork  (not  rubber)  stopper  which  is  bored  once.  Through  the 
hole  in  the  cork  is  placed  a  glass  tube  bent  twice  at  right  angles, 
whose  further  end  leads  into  a  dry  flask  (without  using  a  stopper 
for  this  receiver).  Two  large  beakers  are  placed  upon  the  table, 
one  filled  with  water  at  about  60-70°  C.  and  the  other  with  cold 
water.  If  the  flask  containing  the  carbon  bisulphide  is  placed 
in  the  beaker  containing  the  warm  water,  and  the  other  flask  in  the 
beaker  of  cold  water,  the  carbon  bisulphide  will  distil  rapidly  from 
one  flask  to  the  other.  Care  must  be  taken  during  this  operation 

*  If  the  precipitate  is  large  in  amount,  the  greater  part  is  placed  on  a 
watch-glass,  the  remainder  adhering  to  the  filter  is  dissolved  in  hot,  dilute 
nitric  acid,  the  solution  evaporated  to  dryness  in  a  weighed  platinum  dish, 
and  the  main  portion  of  the  precipitate  added.  The  dish  and  its  contents 
are  heated  at  first  gently  but  finally  over  the  full  flame  of  a  Bunsen  burner. 

t  As  described  on  p.  169  or  on  p.  223. 


DETERMINATION  OF  BISMUTH  AS  METAL.  181 

that  there  is  no  lighted  gas-burner  in  the  immediate  vicinity,  for 
otherwise  there  is  danger  of  the  vapors  of  carbon  bisulphide  taking 
fire. 

3.  Determination  as  Metal.     Method  of  H.  Rose.* 

The  bismuth  is  first  precipitated  as  basic  carbonate  as  described 
under  1,  and  the  dried  precipitate,  together  with  the  ash  of  the  filter, 
is  placed  in  a  porcelain  crucible  and  ignited  gently.  Five  times 
as  much  of  98  per  cent,  potassium  cyanide  is  added  to  the  con- 
tents of  the  crucible  and  the  mixture  is  fused,  whereby  the  oxide 
and  basic  salt  are  changed  to  metallic  bismuth : 

Bi203 + 3KCN  =  3KCNO + Bi, 
2BiOa+4KCN=2KCNO  f  2KC1+  (CN),-f  Bij. 

Since  bismuth  melts  at  268°  C.,  but  boils  at  1600°  C.,  it  is  possi- 
ble to  perform  this  operation  with  a  Bunsen  flame  of  about  half  the 
usual  height  without  running,  any  risk  of  losing  some  of  the  bismuth 
by  volatilization.  The  reduction  is  usually  complete  at  the  end  of 
twenty  minutes.  After  cooling,  the  melt  is  treated  with  water, 
which  dissolves  the  salts  and  leaves  the  metallic  bismuth  behind 
in  the  form  of  a  fused  metallic  globule.  Frequently,  however,  the 
fusion  will  have  loosened  some  of  the  glaze  of  the  porcelain  crucible, 
which  will  remain  behind  with  the  bismuth  after  the  treatment 
witii  water.  Consequently  the  aqueous  solution  is  filtered  through 
a  filter  that  has  been  dried  at  100°  C.  and  weighed  with  the  empty 
crucible.  After  washing  first  with  water,  then  with  absolute 
alcohol  and  ether  and  drying  at  100°  C.,  the  filter  is  again  placed 
in  the  crucible  and  weighed.  The  gain  in  weight  represents  the 
amount  of  metallic  bismuth. 

Bismuth  sulphide  can  also  be  reduced  by  potassium  cyanide, 
but  in  this  case  a  longer  and  stronger  heating  is  necessary. 

4.  Determination  as  Metal.     Method  of  Vanino  and  Treubert.t 

In  this  method  the  bismuth  is  precipitated  as  metal  by  means  of 
formaldehyde  in  alkaline  solution.  The  slightly  acid  bismuth  solu- 

*  Pogg.  Ann.,  110,  p.  425. 
f  Berichte,  31  (1898),  1303. 


182  GRAVIMETRIC  ANALYSIS. 

tion  is  treated  with  formaldehyde  and  a  considerable  excess  of  pure 
10  per  cent,  caustic  soda  solution  and  warmed  on  the  water-bath  until 
the  liquid  above  the  precipitate  has  become  perfectly  clear ;  more  for- 
maldehyde and  caustic  soda  solution  are  then  added  and  the  mixture 
heated  over  a  free  flame,*  decanted  repeatedly  with  water  to  which 
a  little  aldehyde  has  been  added,  again  boiled,  and  by  pressing  with 
a  glass  rod  the  partly  spongy,  partly  pulverulent  precipitate  is  made 
to  collect  together.  The  precipitate  is  then  filtered  through  a  fil- 
ter that  has  been  previously  dried  at  105°  C.  and  weighed,  washed 
with  absolute  alcohol,  dried  at  105°  C.  and  weighed. 

Remark. — Results  obtained  in  the  author's  laboratory  by  this 
method  were  as  a  rule  too  high.  Thus  W.  Urech  obtained  from 
pure  bismuth  nitrate  solution,  as  a  mean  of  four  experiments,  100.78 
per  cent,  instead  of  100  per  cent. 

The  high  results  are  caused  by  the  difficulty  in  removing 
the  last  traces  of  alkali.  Absolutely  accurate  results  may  be 
obtained  by  dissolving  the  precipitated  bismuth  in  nitric  acid, 
precipitating  by  ammonia  and  ammonium  carbonate  and  weighing 
as  the  oxide  according  to  (1).  Naturally  this  roundabout  process 
would  only  be  chosen  when  the  bismuth  solution  contained  other 
acids  (HC1,  H2SO4,  or  H3PO4) ;  the  necessity  of  fusing  with  potas- 
sium cyanide  would  then  be  avoided. 

COPPER,  Cu.    At.  Wt.  63.57. 
Forms:  CuO,  Cu^S,  Cu,  Cu2(CNS)2. 

i.  Determination  as  Copper  Oxide,  CuO.. 

The  solution,  which  must  be  free  from  organic  substances  and 
ammonium  salts,  is  heated  to  boiling  in  a  porcelain  dish  and  pure 
caustic  potash  solution  is  added,  drop  by  drop,  until  the  precipi- 
tate becomes  dark  brown  and  is  permanent,  while  the  solution 
itself  shows  an  alkaline  reaction  towards  litmus-paper.  After  the 
precipitate  has  settled,  the  upper  liquid  is  carefully  poured  through 
a  filter  and  the  precipitate  washed  by  decantation  with  hot  water 
until  the  wash  water  no  longer  shows  an  alkaline  reaction,  when  the 

*  Frequently,  particularly  on  long  boiling,  the  liquid  becomes  colored 
yellow  or  brown.  This  has  no  influence  upon  the  results. 


DETERMINATION  OF  COPPER  AS  COPPER   OXIDE.  183 

precipitate  is  transferred  to  the  filter  and  completely  washed. 
Usually  a  small  amount  of  copper  oxide  adheres  to  the  porcelain 
dish  so  firmly  that  it  can  be  removed  only  by  vigorous  rubbing 
with  a  glass  rod  covered  at  the  end  with  a  piece  of  rubber 
tubing,  and  finally  when  the  precipitate  is  removed  from  the  dish 
some  will  then  remain  on  the  rubber.  Consequently  it  is  better  to 
proceed  as  follows  :  As  much  of  the  precipitate  as  possible  is  removed 
by  a  stream  of  water  from  the  wash-bottle,  then  two  drops  of  dilute 
nitric  acid  are  added,  and  by  inclining  the  dish  and  rubbing  with 
the  glass  rod,  the  whole  of  the  precipitate  remaining  on  the  dish  is 
moistened  with  the  acid.  Two  drops  of  the  acid  are  sufficient,  with 
correct  manipulation,  to  dissolve  all  of  the  copper  oxide.  A  small 
fresh  filter  is  prepared  and  the  dish  is  held  in  an  inclined  position, 
so  that  the  liquid  remains  near  its  lip,  the  sides  are  washed  once 
with  hot  water  and  the  contents  of  the  dish  (which  is  continually 
maintained  in  this  inclined  position)  are  heated  to  boiling  over  a 
small  flame  and  precipitated  by  the  addition  of  caustic  potash, 
drop  by  drop.  (A  large  excess  of  alkali  is  to  be  avoided  on  account 
of  its  solvent  action  upon  the  precipitate.)  *  The  whole  contents 
of  the  dish  are  then  quickly  poured  through  the  small  filter  and 
the  dish  is  immediately  washed  once  with  water.  The  copper  oxide 
is  now  all  on  the  filter.  The  precipitate  is  washed  with  hot  water, 
both  filters  are  dried,  and  the  most  of  the  precipitate  transferred 
to  a  porcelain  crucible,  the  filters  ignited  in  a  platinum  spiral,  and 
the  ash  added  to  the  contents  of  the  crucible.  The  crucible  is  cov- 
ered and  ignited,  at  first  gently,  and  finally  with  the  full  heat  of  the 
Bunsen  burner  then  weighed.  If  the  process  is  carried  out  care- 
fully, the  results  obtained  are  almost  the  theoretical  values  but 
as  a  rule  they  are  a  trifle  high. 


2.  Determination  as  Cuprous  Sulphide, 

The  solution,  which  contains  for  every  100  c.c.  about  5  c.c.  of 
concentrated  acid  (best  sulphuric  acid),  is  heated  to  boiling  and 
hydrogen  sulphide  is  introduced  until  the  solution  becomes  cold. 
If  the  right  amount  of  acid  was  present,  the  precipitate  settles 
quickly  in  large  flocks  and  the  upper  liquid  appears  completely 

*Cf.  VoM. 


1 84  GRAVIMETRIC  ANALYSIS. 

colorless.  Before  filtering,  the  wash  liquid  is  prepared  by  passing 
hydrogen  sulphide  through  the  long  tube  of  a  wash-bottle  for  one 
minute,  then  closing  the  short  tube  with  a  piece  of  rubber  tubing 
and  shaking  vigorously.  As  soon  as  no  more  bubbles  pass  through 
the  liquid,  the  water  is  saturated ;  this  takes  about  a  minute  at  the 
most. 

A  filter  is  now  placed  in  a  funnel  containing  a  platinum  cone, 
the  funnel  is  fitted  to  a  suction-bottle  and  the  filtration  is  begun 
at  first  without  using  suction,  taking  care  that  the  filter  is  con- 
stantly kept  full.  When  all  the  precipitate  is  on  the  filter,  it  is 
washed  with  the  hydrogen  sulphide  water  containing  acetic  acid, 
and,  at  this  point  also,  the  filter  must  be  kept  full  of  liquid.  The 
washing  is  continued  until  1  c.c.  of  the  filtrate  shows  no  test  for 
mineral  acid.*  The  filter  is  now  for  the  first  time  allowed  to 
drain  completely,  and  it  is  dried  as  much  as  possible  by  means 
of  gentle  suction,  then  completely  by  heating  in  the  drying 
closet  at  90°-100°  C. 

As  much  of  the  precipitate  as  possible  is  now  transferred  to  a 
weighed  Rose  crucible  (of  unglazed  porcelain),  f  the  filter  is 
burned  in  a  platinum  spiral  and  the  ash  allowed  to  fall  at  first 
upon  an  unglazed  crucible  cover,  where  it  is  heated  gently  till  it 
glows,  in  order  to  make  sure  that  it  contains  no  unburned  car- 
bonaceous matter;  the  ash  is  then  added  to  the  main  portion 
of  the  precipitate  in  the  crucible.  A  little  sulphur  that  has  been 
recrystallized  from  carbon  bisulphide  is  added  to  the  contents 
of  the  crucible,  the  perforated  cover  is  now  placed  on  the  crucible 
(Pfg.  37),  a  stream  of  hydrogen  is  passed  through  it  (the  wash- 
bottle  shown  contains  concentrated  sulphuric  acidt),  and  the  cru- 
cible is  heated  at  first  over  a  small  flame  and  finally  so  that  the 


*  The  test  for  sulphuric  acid  is  made  with  barium  chloride.  To  test  for 
hydrochloric  acid,  the  solution  is  boiled  until  the  hydrogen  sulphide  is  ex- 
pelled and  is  then  treated  with  silver  nitrate. 

t  A  quartz  crucible  is  more  desirable,  as  the  transformation  of  CuS  into 
Cu2S  can  then  be  watched. 

%  If  the  hydrogen  is  prepared  from  zinc  and  hydrochloric  acid,  the  gas 
should  be  passed  first  through  water  and  then  through  a  wash-bottle  con- 
taining concentrated  sulphuric  acid. 


DETERMINATION  OF  COPPER  AS  CUPROUS  SULPHIDE.        185 

bottom  of  the  crucible  glows  faintly,  at  which  temperature  the 
cupric  sulphide  is  changed  to  cuprous  sulphide, 


Too  strong  heating  is  inadvisable  according  to  Hampe.  * 
When  the  excess  of  sulphur  has  been  driven  off  (which  can  be 
readily  ascertained  by  removing  the  cover  of  the  crucible  and 


FIG.  37. 

finding  no  blue  flame  to  be  perceptible  and  no  odor  of  burning 
sulphur),  the  current  of  hydrogen  is  increased  so  that  eight 
bubbles  per  second  pass  through  the  wash-bottle  (at  first,  not 
more  than  four  bubbles  per  second  should  have  been  the 
rate),  and  the  flame  is  removed.  The  crucible  is  allowed  to 
cool  in  the  current  of  hydrogen  and  weighed  after  remaining 
in  the  desiccator  for  fifteen  minutes.  The  cuprous  sulphide 
should  be  brownish  black  or  black,  and  should  show  no  reddish- 
brown  stains  (due  to  Cu  or  Cu2O);  this  is  the  case  if  the  cur- 
rent of  hydrogen  was  too  slow  during  the  cooling.  In  this  case 

f  Z.  anal.  Chem .,  38,  465  (1894). 


l86  GRAVIMETRIC  ANALYSIS. 

a  little  sulphur  must  be  added  to  the  precipitate  and  the  process 
repeated. 

Remark. — It  is  evident  that  the  sulphur  used  for  this  experi- 
ment should  leave  on  ignition  no  weighable  residue.  This  is  why 
the  sulphur  used  should  be  recrystallized  from  carbon  bisulphide. 

The  reason  why  it  is  necessary  to  keep  the  funnel  filled  with 
liquid  during  the  filtration  and  washing  of  the  cupric  sulphide  is  this : 
If  moist  copper  sulphide  is  exposed  to  the  air  it  is  quickly  oxidized 
and  the  hydrogen  sulphide  wash  water  acts  upon  the  salt  formed  by 
the  oxidation,  (CuS2O3  •  CuSOJ ,  and  transforms  it  into  colloidal 
cupric  sulphide,  which  forms  a  pseudo-solution,  passes  through  the 
filter,  .and  on  coming  in  contact  with  the  acid  filtrate  is  coagu- 
lated. If,  however,  the  precipitate  is  not  exposed  to  the  air  during 
the  filtration  there  is  no  oxidation  and  the  filtrate  remains  clear. 

Instead  of  changing  the  cupric  sulphide  into  cuprous  sulphide, 
it  has  been  proposed  to  convert  it  to  oxide  by  ignition  in  the 
air  and  weighing  the  copper  in  this  form.  If,  however,  the  highest 
degree  of  accuracy  is  desired,  this  should  not  be  done,  for  the 
ignited  product  always  contains  some  sulphate.  When  this 
method  is  chosen,  the  cupric  sulphide  should  be  heated  in  a  glazed 
porcelain  crucible,  at  first  over  a  small  flame,  so  that  the  mass 
does  not  melt,  and  the  heat  gradually  increased  until  finally  a 
blast-lamp  is  used  and  the  copper  weighed  as  CuO.  The  results 
are  about  0.1  per  cent,  too  high  when  not  more  than  0.2  gm.  of 
precipitate  is  present.  Holthof*  states  that  copper  oxide  abso- 
lutely free  from  sulphate  can  be  obtained  if  the  precipitate  is 
ignited  wet  in  an  inclined  porcelain  crucible. 

3.  Determination  as  Cuprous  Sulphocyanate,  Cu2(CNS)2. 
Method  of  Rivot.f 

The  solution,  slightly  acid  with  sulphuric  or  hydrochloric  acid 
(oxidizing  agents  must  not  be  present),  is  treated  with  an  excess 
of  sulphurous  acid,J  after  which  ammonium  sulphocyanate  is 

*  Z.  anal.  Chem.,  28,  680  (1889). 

t  Compt.  Rend.,  38,  868;  see  also  R.  G.  van  Name,  Zeit.  f.  anorg.  Chem., 
26,  230,  and  Busse,  Zeit.  f.  anal.  Chem.,  17,  53,  and  30,  122. 

t  Instead  of  sulphurous  acid,  ammonium  bisulphite  may  be  used.  The 
latter  is  prepared  by  saturating  aqueous  ammonia  with  SO2. 


ELECTROLYTIC  DETERMINATION  OF  COPPER.  187 

added  drop  by  drop  with  constant  stirring,  whereby  at  first  a 
greenish  precipitate  of  cupric  and  cuprous  sulphocyanate  is  pre« 
cipitated,  which  after  stirring  becomes  pure  white.  The  precipitate 
is  allowed  to  settle  completely  (this  requires  several  hours) ;  it  is 
then  filtered  and  washed  with  cold  water  until  the  filtrate  shows 
cnly  a  slight  reddish  coloration  when  ferric  chloride  is  added, 
cfter  which  it  is  washed  several  times  with  20  per  cent,  alcohol, 
dried  at  110-120°  C.,  and  weighed.  R.  Philipp  found  by  this 
method  99.95  per  cent,  instead  of  100  per  cent,  copper,  as  a  mean 
of  twelve  experiments.  The  cuprous  sulphocyanate  can  be  dried 
at  a  temperature  as  high  as  160°  C.,  but  at  180°  C.  it  begins  to 
decompose.  The  Munroe  crucible  can  be  used  to  advantage  in 
this  determination.  The  precipitate  permits  rapid  filtration,  and 
a  turbid  filtrate  is  never  obtained.  After  the  determination  is 
finished,  the  greater  part  of  the  precipitate  is  shaken  out  of  the 
crucible,  and  the  remainder  dissolved  in  hot  nitric  acid. 


4.  Electrolytic  Determination  of  Copper. 

This  most  accurate  and  most  convenient  of  all  methods  for 
the  determination  of  copper  was  first  proposed  by  W.  Gibbs  in 
1864.* 

Copper  may  be  deposited  by  means  of  the  electric  current 
from  acid,  alkaline,  and  neutral  solutions,  but  for  analytical  pur- 
poses only  the  use  of  acid  solutions  is  of  importance. 

Procedure. — The  safest  way,  according  to  F.  Forster,*  is  to 
deposit  the  copper  from  a  sulphuric  acid  solution.  To  the  neutral 
solution  containing  the  copper  in  the  form  of  sulphate,  10  c.c. 
of  twice  normal  sulphuric  acid  are  added,  the  solution  is  diluted  to 
a  volume  of  100  c.c.  and  electrolyzed  with  exactly  two  volts 
potential  at  the  electrodes  and  this  potential  is  kept  constant 
during  the  electrolysis.  These  conditions  are  fulfilled  by  simply 
connecting  the  electrodes  with  the  poles  of  a  single  storage  cell. 
The  electrolysis  requires  at  least  eight  hours  if  done  at  the  ordi- 
nary temperature,  but  by  keeping  the  solution  at  70°-80°,  0.2 
gm.  of  copper  is  deposited  in  60-80  minutes.  If,  therefore,  it 

*  Z.  anal.  Chem.,  3,  334. 


1 88  GRAVIMETRIC  ANALYSIS. 

is  desired  to  carry  on  the  electrolysis  over  night,  it  is  done  in  the 
cold.  It  is  very  easy  to  decide  when  the  electrolysis  is  finished 
by  adding  a  little  water  and  noticing  whether  there  is  any  more 
copper  deposited  upon  the  freshly  exposed  electrode  surface. 
The  cathode  is  then  washed  with  water,  without  breaking  the 
circuit,  exactly  as  was  described  under  the  electrolytic  determina- 
tion of  nickel  (p.  136).  Finally/  the  cathode  is  rinsed  with  alcohol, 
dried  by  holding  it  high  above  a  flame,  cooled  in  a  desiccator, 
and  weighed. 

If  these  directions  are  followed  closely,  the  copper  is  never 
deposited  in  a  spongy  condition.  The  presence  of  Ni,  Co,  Fe, 
Zn  and  Cd  does  not  influence  the  analysis  and  the  copper  may  be 
separated  from  these  elements  by  means  of  such  an  electrolysis. 

If  the  solution  to  be  analyzed  contains  copper  and  some  of 
the  above-mentioned  base  metals,  it  is  evaporated  to  dryness, 
heated  with  a  little  sulphuric  acid  until  dense  fumes  are  evolved, 
cooled,  treated  with  10  c.c.  of  2  N.  sulphuric  acid,  diluted  to  100 
c.c.  and  electrolyzed  as  described  above. 

If,  however,  only  copper  is  present  in  the  solution,  it  may 
be  deposited  very  nicely  in  the  following  manner.  The  solution 
should  contain  4-5  c.c.  of  concentrated  nitric  acid  in  100  c.c.  If, 
originally,  i:  contained  more  nitric  acid  than  this,  it  is  either 
evaporated  to  dryness  or  neutralized  with  ammonia,  and  then  the 
required  quantity  of  nitric  acid  added.  The  solution  is  heated 
to  50°-60°  and  electrolyzed  with  a  current  of  1  ampere  and  elec- 
trode potential  of  2-2.5  volts.  The  electrolysis  is  over  at  the 
end  of  two  hours,  when  not  more  than  0.3  gm.  of  copper  is  present. 
The  analysis  is  finished  as  above  but  there  is  more  danger  of 
traces  of  copper  being  dissolved  while  the  electrodes  are  being 
removed. 

Remark. — The  copper  may  be  deposited  electrolytic  ally  much 
more  rapidly  by  the  use  of  a  rotating  electrode  or  any  stirring 
arrangement.  The  use  of  a  gauze  cathode  has  also  been  rec- 
ommended. The  solution  should  not  be  diluted  too  much,  as 
spongy  deposits  are  obtained  from  very  dilute  solutions  unless 
a  very  weak  current  is  used.  As  a  general  rule,  the  more  con- 
centrated the  copper  solution,  the  stronger  the  current  that  can. 
be  used. 


ELECTROLYTIC  DETERMINATION  OF  CADMIUM.  189 

CADMIUM,  Cd.    At.  Wt.  112.4. 
Forms:   Cd,  CdS04,  CdO. 

I.  Electrolytic  Determination  of  Cadmium. 

Of  all  the  methods  for  the  determination  of  cadmium  the  electro- 
lytic method  is  not  only  the  most  convenient,  but  by  far  the  most 
accurate,  and  of  the  many  methods  proposed  that  of  Beilstein  and 
Jawein*  can  be  recommended.  From  the  experience  obtained  in  the 
author's  laboratory  the  best  procedure  is  as  follows:  To  the  solution 
of  the  sulphate  a  drop  of  phenolphthalein  is  added  and  then  pure 
caustic  soda  solution  until  a  permanent  red  color  is  obtained.  A  so- 
lution of  98  per  cent,  potassium  cyanide  is  now  added  with  constant 
stirring  until  the  precipitate  of  cadmium  hydroxide  produced  by  the 
caustic  soda  has  completely  dissolved  (an  excess  of  potassium  cya- 
nide should  be  scrupulously  avoided),  the  solution  is  diluted  with 
water  to  100-150  c.c.  and  electrolyzed  in  the  cold,  using  a  gauze 
cathode,  for  from  five  to  six  hours  with  a  current  of  0.5-0.7  ampere 
and  an  electromotive  force  of  4.8-5  volts;  at  the  end  of  this  time 
the  current  is  increased  to  from  1-1.2  amperes  and  the  solution 
is  electrolyzed  for  one  hour  more.  If  these  directions  are  followed, 
all  of  the  cadmium  (if  not  more  than  0.5  gm.  is  present)  will  be 
deposited  as  a  firmly  adhering  dull  deposit  of  almost  silver- 
white  metal.  The  current  is  then  stopped,  the  liquid  is  quickly 
poured  off  and  the  deposited  metal  washed  first  with  water, 
then  with  alcohol  and  finally  with  ether;  it  is  dried  and 
weighed.  Experiments  performed  by  von  Girsewald  gave  fault- 
less results. 

After  the  electrolysis  is  finished,  the  solution  should  always 
be  tested  for  cadmium.  For  this  purpose,  it  is  saturated  with 
hydrogen  sulphide.  If  much  cadmium  is  present,  a  yellow  pre- 
cipitate is  obtained,  but  if  very  little,  a  yellow  coloration  results. 
The  latter  is  due  to  the  formation  of  colloidal  cadmium  sulphide, 
and  the  color  is  so  intense  that  R.  Philip  estimates  the  quantity 
of  cadimum  not  precipitated,  by  comparing  the  shade  with  that 
produced  in  a  solution  containing  a  known  quantity  of  cadmium 
and  the  same  amounts  of  potassium  cyanide  and  caustic  potash 
as  in  the  solution  tested. 

*  Berichte,  12,  446. 


190  GRAVIMETRIC  ANALYSIS. 

Remark. — If  for  the  electrolysis  a  current  of  0.5  ampere  were 
used,  the  cadmium  will  not  be  all  deposited  at  the  end  of  twelve 
hours;  if,  however,  the  current  is  increased  at  the  end,  as  above 
stated,  to  1  ampere,  the  electrolysis  will  be  surely  finished  at  six 
to  seven  hours.  To  work  with  the  stronger  current  from  the 
beginning  is  not  to  be  recommended  unless  a  gauze  cathode  is 
used,  or  one  of  the  electrodes  is  rotated,  for  otherwise  the  metal 
is  deposited  in  a  spongy  form  and  on  washing  some  of  it  is  likely 
to  be  lost. 

A  solution  containing  0.4568  gm.  Cd.,  3  gm.  KCN,  1  gm.  NaOH, 
and  diluted  to  125  c.c.  with  water,  can  be  electrolyzed  in  fifteen 
minutes  with  a  current  of  5  amperes  and  5.5  volts  if  one  of  the 
electrodes  be  rotated.* 

From  neutral  and  weakly  acid  solutions,  cadmium  can  be 
deposited  electrolytically,  but  not  from  strongly  acid  solutions. 

2.  Determination  as  Cadmium  Sulphate,  CdS04. 

Next  to  the  electrolytic  method,  the  determination  of  cadmium 
as  the  sulphate  is  the  best.  If  the  cadmium  is  combined  with  a 
volatile  acid,  the  compound  is  treated  in  a  weighed  porcelain  cru- 
cible with  a  slight  excess  of  dilute  sulphuric  acid,  the  solution  evapo- 
rated on  the  water-bath  as  far  as  possible,  and  finally  the  excess 
of  sulphuric  acid  is  removed  by  heating  in  an  air-bath  (the  crucible 
is  placed  in  a  larger  crucible  that  is  provided  with  an  asbestos  ring).f 
The  heat  is  applied  at  first  slowly,  and  the  temperature  is  raised 
gradually  until  finally  no  more  fumes  of  sulphuric  acid  are  evolved. 
The  outer  crucible  can  even  be  heated  with  the  full  flame  of  a  Teclu 
burner  without  running  any  risk  of  decomposing  the  cadmium  sul- 
phate ;  it  is,  however,  not  necessary  to  heat  it  so  strongly.  As  soon 
as  the  fumes  of  sulphuric  acid  cease  to  come  off,  the  operation  is 
ended  and  the  crucible  and  its  contents  are  weighed  after  cooling 
in  a  desiccator.  The  cadmium  sulphate  should  be  pure  white 
and  should  dissolve  in  water  to  form  an  absolutely  clear  solution. 

If  the  cadmium  has  been  precipitated  from  a  solution  as  the 
sulphide,  the  greater  part  of  the  precipitate  is  placed  in  a  large 
porcelain  crucible,  covered  with  a  watch-glass,  and  treated  with 
hydrochloric  acid  (1:3)  on  the  water-bath.  After  the  precipitate 

*  See  Edgar  F.  Smith's  Electro- Analysis, 
t  Cf.  Fig.  11,  p.  27. 


THE  PRECIPITATION  OF  CADMIUM  AS  SULPHIDE.  191 

has  dissolved  and  the  evolution  of  hydrogen  sulphide  has  ceased, 
the  lower  side  of  the  watch-glass  is  washed,  the  crucible  is  placed 
under  the  funnel,  and  the  precipitate  which  adhered  to  the  filter- 
paper  is  dissolved  by  dropping  hot  hydrochloric  acid  (1: 3)  upon  it, 
finally  washing  the  filter  with  hot  water,  evaporating  the  solution 
upon  the  water-bath,  and  proceeding  as  above  described. 
The  results  obtained  by  this  method  are  excellent. 

The  Precipitation  of  Cadmium  as  Sulphide. 

The  frequently  recommended  determination  of  cadmium  as  the 
sulphide  must  be  rejected;  it  is  useless.  It  is  not  possible  to  precipi- 
tate pure  cadmium  sulphide  from  acid  solutions  by  means  of  hydro- 
gen sulphide;  the  precipitate  is  always  contaminated  with  a  basic 
salt  (Cd2Cl2S-Cd2SO4S,  etc.)  whether  the  precipitation  takes  place 
in  cold  or  hot  solutions,  whether  under  atmospheric  pressure  or 
under  increased  pressure  (in  a  pressure-flask),  and  in  fact  the  amount 
of  basic  salt  formed  increases  with  the  amount  of  free  acid  present. 
Results  are  obtained  as  much  as  5  per  cent,  too  high.  Follenius  * 
attempted  to  make  the  method  possible  by  igniting  an  aliquot  part 
of  the  dried  and  weighed  precipitate  in  a  stream  of  hydrogen  sul- 
phide. If  the  sulphide  was  contaminated  with  sulphate,  he  suc- 
ceeded in  changing  it  all  to  sulphide  and  obtained  results  that  were 
acceptable.  If,  however,  chloride  was  present,  a  considerable 
part  was  lost  by  sublimation,  so  that  the  results  obtained  were  too 
low.  It  is,  furthermore,  not  possible  to  ignite  the  cadmium  sul- 
phide with  sulphur  in  a  current  of  hydrogen,  as  was  described 
under  Zinc  and  Copper,  for  cadmium  sulphide  is  so  volatile  that 
some  of  it  is  lost. 

On  the  other  hand,  the  method  of  precipitating  the  cadmium  as 
sulphide  from  solutions  containing  2-7  c.c.  of  concentrated  sul- 
phuric acid  in  100  c.c.  is  to  be  recommended,  for  by  this  means  a 
precipitate  is  obtained  which  can  be  readily  filtered  and  which  by 
solution  in  hot  hydrochloric  acid  (1:1)  and  evaporation  with  sul- 
phuric acid  can  be  changed  without  loss  to  the  sulphate  and  weighed 
as  such. 

*  Zeit.  f.  anal.  Chem.,  XIII,  422. 


192  GRAVIMETRIC  ANALYSIS. 

3.  Determination  as  Cadmium  Oxide,  CdO. 

Cadmium  carbonate  and  cadmium  nitrate  can  be  changed  to 
the  oxide  by  strong  ignition. 

The  cadmium  is  precipitated  from  its  solutions  at  the  boiling 
temperature  by  the  addition  of  a  slight  excess  of  potassium  car- 
bonate, and  after  standing  for  some  time  on  the  water-bath,  and 
when  the  precipitate  has  completely  settled,  it  is  filtered  off,  washed 
with  hot  water,  and  dried.  As  much  of  the  dried  precipitate  as 
possible  is  transferred  to  a  watch-glass  and  set  aside  for  the  time 
being.  The  filter  is  washed  with  dilute  nitric  acid  to  dissolve 
the  small  amount  of  the  precipitate  which  still  adheres  to  it  and 
the  solution  is  received  in  a  weighed  porcelain  crucible  and  evap- 
orated to-  dryness.  The  main  portion  of  the  precipitate  is  now 
added,  and  the  crucible  is  at  first  very  gently  heated  by  placing 
the  open  crucible  high  above  a  small  flame  from  a  Teclu  burner/ 
until  the  whole  mass  has  become  a  uniform  brown  throughout- 
The  temperature  is  now  gradually  raised  until  finally  the  full  heat  of 
the  burner  is  reached.  It  is  important  during  this  operation  to 
take  care  that  the  inner  flame-mantle  does  not  touch  the  cruci- 
ble, for  otherwise  reducing  gases  may  enter  the  crucible  and  reduce 
a  part  of  the  oxide  to  metallic  cadmium,  which  is  volatile  at  this 
temperature.*  The  cadmium  oxide  is  obtained  as  a  brown 
powder  which  is  infusible,  insoluble  in  water,  but  readily  soluble 
in  dilute  acids,  f 

Remark. — It  is  not  advisable  to  precipitate  the  cadmium  by 
means  of  sodium  carbonate  solution,  for  in  that  case  it  is  difficult 
to  wash  the  precipitate  free  from  alkali. 

SEPARATION  OF  THE  SULPHO-BASES  FROM  THE  METALS  OF 
THE  PRECEDING  GROUPS. 

Hydrogen  sulphide  precipitates  only  the  metals  of  the  "  hydro- 
gen sulphide  group"  from  acid  solutions.  It  is  to  be  noted  that 
zinc  precipitates  with  this  group  if  the  solution  is  not  acid  enough; 

*  If  the  cadmium  carbonate  is  filtered  into  a  Munroe  crucible,  and  ignited 
in  an  electric  oven,  the  transformation  takes  place  readily  without  danger 
of  any  volatilization. 

f  The  oxide  after  ignition  is  a  black,  crystalline  powder.         -  -  — • 


ANALYSIS  OF  BRASS.  193 

while  if  the  solution  is  too  acid  lead  and  cadmium  are  often 
incompletely  precipitated.  A  suitable  concentration  is  5-7  c.c. 
of  concentrated  hydrochloric  acid  to  100  c.c.  of  liquid. 


EXAMPLE. 

Analysis  of  Brass  (Alloy  of  Copper  and  Zinc  with  Small 
Amounts  of  Lead,  Iron,  and  Nickel). 

About  0.4-0.5  gm.  of  the  alloy,  in  the  form  of  borings,* 
is  dissolved  in  about  20  c.c.  of  nitric  acid,  sp.  gr.  1.2,  in  a  200-c.c. 
casserole  which  is  covered  with  a  watch-^lass.  After  the  reaction 
begins  to  slacken,  complete  solution  is  effected  by  warming  on  the 
water-bath.  The  solution  is  then  evaporated  to  complete  dryness, 
moistened  with  a  little  nitric  acid,  dissolved  in  about  50  c.c.  of 
hot  water,  and  any  metastannic  acid  present  is  allowed  to 
settle,  is  filtered  off,  washed  with  hot  water,  dried,  and  the 
tin  determined  according  to  p.  228.  To  the  cold  filtrate  3  c.c. 
of  pure,  concentrated  sulphuric  acid  are  added,  the  solution  is 
evaporated  on  the  water-bath  as  far  as  possible,  and  then  heated 
cautiously  over  a  free  flame  until  dense  wrhite  fumes  of  sulphuric 
acid  are  evolved.  After  cooling  the  residue  is  treated  with  50  c.c. 
of  water  and  15  c.c.  of  alcohol,  stirred  well,  filtered,  washed,  and 
the  lead  sulphate  determined  according  to  p.  174.  The  filtrate 
is  evaporated  until  the  alcohol  is  completely  removed,  100  c.c.  cf 
water  are  added,  the  solution  is  heated  to  boiling,  and  hydrogen 
sulphide  is  conducted  into  it  until  it  becomes  cold,  when  the  copper 
sulphide  is  filtered  off,  wrashed  first  with  hydrogen  sulphide  water 
containing  in  every  100  c.c.  20  c.c.  of  double-normal  sulphuric  acid 
and  at  the  end  with  5  per  cent,  acetic  acid,  which  is  saturated  with 
hydrogen  sulphide,  until  the  filtrate  gives  no  precipitate  on  being 
treated  with  barium  chloride.  The  copper  is  determined,  accord- 
ing to  p.  183,  as  Cu2S. 

The  filtrate  from  the  copper  sulphide  is  evaporated  to  a  small 
volume  in  order  to  remove  completely  the  excess  of  hydrogen 

*  The  borings  are  usually  somewhat  greasy.  They  should  be  washed  with 
ether  before  weighing.  Cf.  p.  236,  foot-note. 


194  GRAVIMETRIC  ANALYSIS. 

sulphide,  the  iron  is  then  oxidized  by  the  addition  of  bromine 
water,  precipitated  by  ammonia,  and  filtered.  In  order  to  make 
sure  that  the  precipitate  of  ferric  hydroxide  contains  no  zinc,  it 
is  dissolved  in  a  little  hydrochloric  acid  and  the  precipitation  with 
ammonia  is  repeated.  The  filtered  and  washed  precipitate  is 
ignited  in  a  porcelain  crucible  and  weighed  as  ferric  oxide  (cf. 
p.  87). 

The  combined  filtrates  from  the  ferric  hydroxide  are  acidified 
with  a  little  sulphuric  acid,  heated  to  about  50°  C.,  and  the  zinc 
determined  as  zinc  sulphide  according  to  the  "  salting-out " 
method  described  on  p.  160.  For  the  determination  of  nickel, 
the  filtrate  from  the  zinc  sulphide  precipitation  is  boiled  to  expel 
the  hydrogen  sulphide  and  the  nickel  determined  as  the  salt  of 
dimethyl  glyoxime  according  to  p.  129. 


SEPARATION  OF  THE  SULPHO-BASES  FROM  ONE  ANOTHER. 

I.  Separation  of  Mercury  from  Lead,  Bismuth,  Copper, 

and  Cadmium. 

• 

Method  of  Gerhard  v.  Rath. 

Principle. — This  separation  is  based  upon  the  insolubility  of 
mercuric  sulphide  in  boiling,  dilute  nitric  acid  (sp.  gr.  1.2-1.3) 
and  the  solubility  of  the  remaining  sulphides. 

Procedure. — The  solution  (containing  the  mercury  entirely  in 
the  mercuric  form)  is  precipitated  by  means  of  hydrogen  sulphide, 
the  precipitate  filtered  off,  washed  with  hydrogen  sulphide  water, 
transferred  to  a  porcelain  dish  and  boiled  for  a  considerable  length 
of  time  with  nitric  acid  of  the  above  concentration,  then  diluted 
with  a  little  water  and  washed  with  water  containing  nitric  acid. 
The  residue  of  mercuric  sulphide  thus  obtained  always  contains 
sulphur,  and  in  case  considerable  lead  were  present  it  will  also 
contain  lead  sulphate.  It  is,  therefore,  dissolved  in  a  little  aqua 
regia,  diluted  with  water,  filtered  from  the  separated  sulphur  and 
lead  sulphate  and  the  mercury  precipitated  according  to  the  method 
of  Volhard,  with  ammonium  sulphide  (cf.  p.  169).  If  some  of  the 
lead  sulphate  should  go  into  solution  with  the  mercury  on  treating 


SEPARATION  OF  BISMUTH  FROM  LEAD.  195 

with  aqua  regia,  it  will  be  converted  by  the  ammonium  sulphide 
and  potassium  hydroxide  into  insoluble  lead  sulphide,  while  the 
mercury  will  be  in  the  form  of  its  soluble  sulpho-salt.  In  this 
case  the  lead  sulphide  is  filtered  off,  washed  with  dilute  potassium 
hydroxide  solution,  and  the  mercury  then  precipitated  as  sulphide, 
as  described  on  p.  169. 

2.  Separation  of  Bismuth  from  Lead. 
(a)  Method  of  Lowe. 

Principle. — Bismuth  nitrate  is  changed  by  the  action  of  water 
into  an  insoluble  basic  salt,  while  lead  nitrate  undergoes  no  such 
transformation. 

Procedure. — The  solution  of  the  two  metals  in  nitric  acid  is 
evaporated  on  the  water-bath  until  it  reaches  a  syrupy  consist- 
ency, water  is  added,  and  after  thorough  stirring  with  a  glass 
rod  the  evaporation  is  repeated  and  the  process  continued  until 
the  addition  of  the  water  fails  to  produce  any  further  turbidity; 
a  sign  that  the  bismuth  has  been  completely  converted  into  the 
basic  salt  Bi2O2NO3OH.  A  cold  solution  of  ammonium  nitrate 
(1  NH4NO3:500  H2O)  is  now  added,  and  after  standing  some  time, 
with  frequent  stirring,  in  order  to  make  sure  that  the  lead 
nitrate  is  completely  dissolved,  the  solution  is  filtered.  The 
precipitate  is  washed  with  the  dilute  ammonium  nitrate  solution 
and  dried.  As  much  of  it  as  possible  is  transferred  to  a  weighed 
porcelain  crucible  and  together  with  the  ash  of  the  filter  is  ignited,* 
at  first  gently,  and  finally  with  the  full  flame  of  a  Bunsen  burner. 
It  is  weighed  as  Bi203. 

From  the  filtrate  the  lead  is  precipitated  according  to  p.  174, 
as  sulphate,  and  weighed  as  such.  It  is  less  satisfactory  to  pre- 
cipitate the  lead  as  sulphide  and  weigh  it  in  this  form  after 
gentle  heating  with  sulphur  in  a  Rose  crucible. 

(b)  Method  of  Jannasch.^ 

Principle. — The  separation  depends  upon  the  different  vola- 
tility of  the  two  bromides.  Bismuth  bromide  is  fairly  readily 
volatile;  lead  bromide  is  only  difficultly  so. 

*  It  is  still  better  to  proceed  as  in  the  determination  of  cadmium  oxide,  p.  192. 
t  Praktischer  Leitfaden  der  Gewichtsanalyse. 


196  GRAVIMETRIC  ANALYSIS. 

Procedure. — The  solution  of  the  nitrates  is  evaporated  to  dry- 
ness,  100  c.c.  of  water,  sufficient  hydrochloric  acid  to  afford  a 
clear  solution,  and  a  few  drops  of  fuming  nitric  acid  are  added,* 
after  which  hydrogen  sulphide  is  introduced.  The  precipitated 
sulphides  are  immediately  filtered,  the  precipitate  is  dried  at  100°  C. 
in  a  stream  of  carbon  dioxide,  after  which  as  much  of  the  precipitate 


FIG.  38. 

as  possible  is  placed  in  an  agate  mortar  and  the  ash  of  the  filter  added 
to  it.  The  whole  of  the  precipitate  is  ground  fine  and  transferred 
without  loss  to  a  weighed  porcelain  boat,  which  is  then  introduced 
into  the  decomposition  tube  R^  (Fig.  38),  made  of  difficultly-fusible 
glass.  At  first  a  stream  of  dry  carbon  dioxide  is  passed  through  the 
apparatus  and  the  substance  is  gently  heated  by  means  of  a  small 
flame,  in  order  to  completely  dry  it.  The  water  condensing  in  the 
front  part  of  the  tube  is  driven  over  into  E  by  careful  heating. 

The  bottle  A  containing  bromine  J  is  now  connected  with  the 
apparatus  and  the  stream  of  carbon  dioxide  is  passed  through 
it;  the  gas,  carrying  bromine  vapors  with  it,  is  passed  through 
the  vertical  calcium  chloride  tube  filled  with  pieces  of  calcite, 

*  By  the  addition  of  the  fuming  nitric  acid  the  precipitated  sulphide  is 
contaminated  with  considerable  sulphur;  such  a  precipitate  is  more  readily 
decomposed  by  the  action  of  bromine. 

t  In  this  determination,  the  bulb  of  the  tube  is  unnecessary ;  it  should  be 
replaced  by  one  such  as  is  shown  in  Fig.  38,  //.  For  other  analyses  it  is 
better  to  have  the  bulb. 

J  For  this  experiment  the  bromine  used  must  be  absolutely  free  from 
chlorine  and  is  prepared  as  follows:  50-60  c.c.  of  commercial  bromine  are 
treated,  in  a  tightly  stoppered  separatory  funnel,  with  a  10  percent,  potassium 
bromide  solution.  The  funnel  is  shaken  vigorously,  and  the  bromine  sepa- 
rated from  the  aqueous  alkali  solution.  After  washing  two  or  three  times  with 
water  it  is  ready  for  use. 


SEPARATION  OF  BISMUTH  FROM  LEAD.  197 

then  through  the  concentrated  sulphuric  acid  contained  in  B,  after 
this  through  the  tube  C  containing  glass  beads  moistened  with 
sulphuric  acid,  and  finally  through  the  tube  D  filled  with  glass 
wool,  and  from  this  the  dry  bromine  vapors  reach  the  sub- 
stance. The  latter  is  heated  over  a  small  flame  (kept  in  con- 
stant motion)  and  the  yellow  bismuth  bromide  distils  off  and 
condenses  partly  in  the  narrow  part  of  the  tube  and  partly  in  the 
/eceiver  E,  which  contains  dilute  nitric  acid  (1  HN(>3:2  H^O).  The 
substance  is  heated  hotter,  whereby  more  bismuth  bromide  is  vola- 
tilized, and  this  is  again  distilled  as  completely  as  possible  into  the 
receiver.  FinaJly  the  substance  is  heated  more  strongly  still,  until 
the  lead  bromide  begins  to  melt.  When  no  more  of  the  yellow 
sublimate  is  formed,  the  decomposition  is  shown  to  be  complete 
and  the  substance  is  allowed  to  cool  in  a  stream  of  carbon  dioxide. 
The  bromine  that  escapes  from  the  tube  K  is  passed  into  alcohol 
contained  in  the  beaker  F.  When  the  apparatus  has  become  cold, 
the  bromine  bottle  is  removed,  and  the  bromine  is  removed  from 
the  apparatus  by  passing  carbon  dioxide  through  it  for  some 
time.  The  boat  filled  with  lead  bromide  is  then  weighed,  and 
from  the  weight  of  the  PbBr2  that  of  the  lead  is  computed.  To 
check  this,  the  lead  bromide  is  dissolved  in  freshly-prepared 
chlorine  water,  an  excess  of  dilute  sulphuric  acid  is  added,  and 
the  solution  is  evaporated  to  remove  the  hydrochloric  acid,  at 
first  on  the  water-bath  and  finally  over  a  free  flame  until  dense 
fumes  of  sulphuric  acid  are  evolved. 

After  cooling,  water  and  alcohol  are  added,  the  precipitate 
filtered  off  and  the  weight  of  the  lead  sulphate  determined  as 
described  on  p.  174.  For  the  bismuth  determination,  the  nitric 
acid  solution  contained  in  E  and  K  is  poured  into  a  beaker,  filtered 
if  necessary  from  any  sulphur,  evaporated  to  a  small  volume,  and 
the  bismuth  precipitated  by  the  addition  of  ammonium  carbonate 
and  determined  as  metal  as  described  on  p.  181. 

There  have  been  many  other  methods  proposed  for  the  separa- 
tion of  lead  and  bismuth,*  all  of  which  are  less  satisfactory  than 
the  two  methods  just  described,  so  that  they  will  not  be  discussed 
in  this  book. 

*  Cf.  O.  Steen,  Z.  angew.  Chem.,  1895,  p.  530. 


198  GRAVIMETRIC  ANALYSIS. 

3.  Separation  of  Bismuth  from  Copper. 

The  solution  is  treated  with  an  excess  of  ammonium  carbonate, 
warmed  gently,  and  filtered.  The  precipitate  of  basic  bismuth 
carbonate  almost  always  contains  small  quantities  of  copper,  so 
that  it  is  dissolved  in  nitric  acid  and  the  separation  by  means  of 
ammonium  carbonate  is  repeated.  The  basic  bismuth  salt  is 
fused  with  potassium  cyanide  and  weighed  as  metal,  according 
to  p.  181. 

For  the  copper  determination,  the  two  nitrates  are  combined, 
evaporated  to  remove  the  excess  of  ammonium  carbonate,  acidified 
with  sulphuric  acid,  and  the  copper  precipitated  by  means  of 
hydrogen  sulphide,  being  determined  as  cuprous  sulphide  accord- 
ing to  p.  183,  or  the  sulphuric  acid  solution  is  subjected  to  elec- 
trolysis as  described  on  p.  187. 

According  to  Fresenius  and  Haidlin,  bismuth  can  be  separated 
from  copper  very  nicely  by  means  of  potassium  cyanide.  For 
this  purpose  the  acid  solution  is  precipitated  by  the  addition  of  a 
slight  excess  of  sodium  carbonate,  potassium  cyanide  is  added, 
and  the  solution  warmed  and  filtered.  All  of  the  copper  is  found 
in  the  filtrate,  while  the  precipitate  contains  bismuth  oxide  con- 
taminated with  alkali.  The  residue  is,  therefore,  dissolved  in 
nitric  acid,  the  bismuth  precipitated  by  means  of  ammonium  car- 
bonate and  determined  as  metal  according  to  p.  181.  The 
filtrate  containing  the  copper  is  evaporated  with  nitric  acid,  in 
order  to  destroy  the  cyanide,  and  the  copper  determined  electro- 
lytically  according  to  p.  187. 


4.  Separation  of  Lead  from  Copper  by  Means  of  Electrolysis. 

This  separation  depends  upon  the  fact  that  the  electric  current 
deposits  lead  quantitatively  as  Pb(>2  upon  the  anode  from 
solutions  containing  a  definite  amount  of  nitric  acid,  while 
the  copper  is  either  not  deposited  at  all  under  these  conditions 
or  is  found  upon  the  cathode  to  some  extent.  After  the  lead  is 
completely  deposited,  the  copper  solution  is  poured  into  a  second 
weighed  platinum  dish,  the  excess  of  the  acid  is  neutralized  with 


SEPARATION  OF  LEAD  FROM  COPPER.  199 

ammonia,  and  the  solution  again  electrolyzed.  The  copper  will 
now  deposit  quantitatively  upon  the  cathode. 

Procedure. — The  solution  of  the  two  nitrates  is  placed  in  a 
platinum  dish  (of  the  form  recommended  by  Classen  with  the 
inner  surface  unpolished)  and  15  c.c.  of  nitric  acid  (sp.  gr.  1.35- 
1.38)  are  added,  after  which  the  solution  is  diluted  to  150  c.c.  and 
electrolyzed  at  50°-60°  C.  with  a  current  of  1-1.5  amperes  and 
an  electrode  potential  of  1.4  volts.  After  1-1.5  hours  practically 
all  the  lead  will  be  deposited  upon  the  anode  (dish)  in  the  form  of 
a  firmly  adhering,  brown  coating  of  lead  peroxide,  PbO2.  At 
the  cathode  (a  plate  electrode)  a  considerable  part  of  the  copper 
will  be  deposited,  but  the  remainder  will  still  be  in  solution.  The 
circuit  is  broken  and  the  solution  poured  as  quickly  as  possible 
into  a  second  weighed  platinum  dish,  and  the  washings  added 
to  this  dish.  After  washing  the  electrodes  with  water,  the  first 
dish  with  the  PbO2  deposit  is  dried  at  180°  and  weighed.  The 
solution  in  the  second  dish  contains  a  little  lead  and  some  copper, 
It  is  made  slightly  ammoniacal,  4  c.c.  of  concentrated  nitric  acid 
are  added,  and  the  solution  electrolyzed  at  60°.  The  platinum  dish 
now  serves  as  the  cathode,  while  the  plate  electrode  *  serves  as 
the  anode;  in  case  traces  of  lead  remain  in  solution  after  the  first 
electrolysis,  it  will  now  be  deposited.  After  an  hour  or  two 
with  a  current  of  one  ampere  all  the  remaining  copper  and  lead 
will  be  deposited.  When  the  electrolysis  is  complete  the  elec- 
trodes are  washed  without  breaking  the  circuit  and  the  weight 
of  the  copper  and  PbO2  is  determined. 

If  only  small  amounts  of  lead  and  copper  are  present,  the 
electrolysis  should  take  place  under  the  conditions  described  on 
p.  187,  except  in  this  case  a  weighed  plate  electrode  should  be 
employed  as  the  anode.  Under  these  conditions  the  lead  will  be 
deposited  as  the  peroxide  upon  the  anode,  while  the  copper  will 
separate  out  upon  the  dish. 

*The  plate  electrode  with  copper  upon  it  was  weighed,  cleaned,  and 
then  weighed  again. 


200  GRAVIMETRIC  ANALYSIS. 

5.  Separation  of  Lead  from  Copper  and  Cadmium. 

(From  Bismuth  less  satisfactorily.) 

The  solution  of  the  nitrates  or  chlorides  is  treated  with  an  excess 
of  sulphuric  acid,  evaporated  to  remove  the  nitric  or  hydrochloric 
acid,  and  the  lead  determined  as  sulphate  as  described  on  p. 
174. 

6.  Separation  of  Copper  from  Cadmium. 

(a)  Method  of  A.  W.  Hofmann* 

A.  W.  Hofmann  states  that  copper  and  cadmium  can  be  sepa- 
rated from  one  another  by  boiling  their  sulphides  with  sulphuric 
acid  (1:5)  whereby  cadmium  sulphide  is  dissolved  while  copper 
sulphide  is  unacted  upon.  Hofmann  seems  to  have  tested  this 
separation  only  qualitatively  and  not  quantitatively,  but  neverthe- 
less this  method  is  given  in  all  early  text-books  without  submitting 
any  analyses  to  prove  its  accuracy.  Experiments  performed  in  the 
author's  laboratory  showed  that  in  the  form  proposed  by  Hofmann 
this  method  cannot  be  used  for  the  quantitative  separation  of 
the  two  metals ;  on  the  other  hand,  if  it  is  carried  out  according  to 
the  following  modifications,  excellent  results  are  obtained. 

Procedure. — Sufficient  sulphuric  acid  is  added  to  the  solution 
of  the  sulphates  so  that  one  part  of  the  acid  is  contained  in  four 
parts  of  the  solution.  The  latter  is  now  heated  to  boiling,  and 
during  the  boiling  hydrogen  sulphide  is  passed  through  it  for 
twenty  minutes,  after  which  the  solution  is  boiled  for  fifteen 
minutes  longer.  The  solution  is  filtered  while  hot  through  a 
funnel  kept  filled  with  carbon  dioxide  and  the  precipitate  is  washed 
with  boiled,  hot  water  to  the  disappearance  of  the  acid  reaction. 
The  copper  sulphide  thus  obtained  is  easy  to  filter  and  wash;  it 
however,  always  contains  small  amounts  of  cadmium,  so  that  the 
separation  must  be  repeated.  The  copper  sulphide  is,  therefore, 
transferred  to  a  porcelain  dish  by  means  of  a  stream  of  water  from 

*  Ann.  d.  Chem.  und  Pharm.,  115,  286. 


SEPARATION  OF  COPPER  FROM  CADMIUM.  201 

the  wash-bottle,  where  it  is  dissolved  in  nitric  acid,  the  solution 
evaporated  to  dryness,  the  dry  mass  treated  with  sulphuric  acid 
(1:4)  and  again  evaporated  on  the  water-bath  as  far  as  possible 
to  remove  the  greater  part  of  the  nitric  acid.  After  this,  without 
regard  to  the  separated  sulphur,  the  mass  is  washed  with  as  little 
water  as  possible  into  an  Erlenmeyer  flask,  for  every  0.3—0.5  gm. 
of  copper  about  150-200  c.c.  of  sulphuric  acid  (1:4)  are  added, 
and  the  separation  by  means  of  hydrogen  sulphide  is  repeated 
exactly  as  above  described.  The  pure  copper  sulphide  that  is 
finally  obtained  is  dried  and  the  copper  determined  as  cuprous 
sulphide  as  described  on  p.  183,  or  it  is  dissolved  in  nitric  acid 
and  the  solution  electrolyzed  as  described  on  p.  187. 

For  the  cadmium  determination,  hydrogen  sulphide  is  passed 
into  the  cold  filtrate,  the  precipitated  cadmium  sulphide  after 
being  washed  is  transferred  by  means  of  a  spatula  to  a  porcelain 
dish,  hydrochloric  acid  (1 : 3)  is  poured  over  it,  the  dish  covered 
with  a  watch-glass  and  heated  on  the  water-bath  until  the  precipi- 
tate is  dissolved  and  until  the  hydrogen  sulphide  is  all  expelled. 
The  dish  is  now  placed  under  the  funnel  and  the  cadmium  sulphide 
which  remained  upon  the  filter  is  dissolved  by  dropping  hot  hydro- 
chloric acid  (1:3)  upon  it,  finally  washing  the  filter  with  water. 
The  contents  of  the  dish  are  evaporated  to  dryness,  the  dry  mass 
dissolved  in  a  little  sulphuric  acid,  washed  into  a  weighed  porcelain 
crucible,  and  treated  with  1  c.c.  of  concentrated  nitric  acid*  and 
a  little  more  sulphuric  acid.  After  this  the  contents  of  the  crucible 
are  evaporated  as  far  as  possible  upon  the  water-bath,  the  excess  of 
sulphuric  acid  removed  by  heating  in  an  air-bath,  and  the  cadmium 
determined  as  sulphate  according  to  p.  190. 

The  above  method  was  tested  by  Oberer  in  the  author's  labora- 
tory and  the  following  results  obtained: 

*  The  nitric  acid  is  added  to  oxidize  the  fibres  of  filter-paper;  if  these 
are  not  destroyed  they  will  cause  a  partial  reduction  of  the  cadmium  sul- 
phate. 


202 


GRAVIMETRIC  ANALYSIS. 


Amount  Taken. 

Found. 

Difference. 

Amount  Found  in 
Per  Cent,  of  the 
Theoretical  Value. 

1     Cu—  0  3126  gm.         .    . 

0  3130  gm. 

4-0  0004 

100   12 

Cd—  0  2504    "       

0.2506    " 

+  0.0002 

100  08 

2.    Cu—  0  3126    "     

0.3125    " 

-0.0001 

99.97 

Cd—  0  2504    " 

0  2501    " 

—0  0003 

99  88 

3     Cu—  0  3126    "       .    .    . 

0  3134    " 

+  0  0008 

100  25 

Cd—  0  2504    " 

0.2496    " 

—  0.0008 

99  68 

4    Cu—  0  3126    "     

0.3120    " 

—  0.0006 

99  81 

Cd=0  6259    "     

0.6252    " 

-0.0007 

99.88 

5.    Cu—  0  3142    "     

0.3147    " 

+  0.0005 

100.16 

Cd—  0  6259    " 

0  6248    " 

-0  0011 

99  82 

6.    Cu—  0  3142    "     

0  3150    " 

+  0.0008 

100  25 

Cd=0  6259    "     

0.6240    " 

-0.0019 

99.69 

(6)  Method  of  Rivot-Rose. 

The  copper  is  precipitated  as  sulphocyanide  according  to  p. 
186,  and  from  the  filtrate  the  cadmium  is  precipitated  as  sulphide 
by  means  of  hydrogen  sulphide  and  determined  as  sulphate 
according  to  p.  190.  The  results  are  good. 

(c)  Method  of  Fresenius  and  Haidlen. 
(The  Potassium  Cyanide  Method.) 

The  neutral  solution  containing  salts  of  both  metals  is  treated 
with  potassium  cyanide  until  the  precipitate  that  is  first  formed 
redissolves,  after  which  more  potassium  cyanide  is  added  (about 
three  times  as  much  as  was  necessary  for  the  precipitation  and  so- 
lution of  the  precipitate)  and  either  ammonium  or  hydrogen  sul- 
phide is  added  to  the  cold  solution.  The  cadmium  is  precipitated 
as  the  yellow  sulphide,  while  the  copper  remains  in  solution.  * 

*  The  copper,  however,  remains  entirely  in  solution  only  when  more  than 
enough  potassium  cyanide  is  present  than  is  required  to  form  the  complex  salt 
K3Cu(CN)4.  If  the  pure  potassium  cuprocyanide  is  dissolved  in  consider- 
able water  and  hydrogen  sulphide  passed  into  the  solution,  there  is  a  partial 
precipitation  of  Cu.jS;  the  more  dilute  the  solution,  the  more  the  precipi- 
tation. By  the  addition  of  an  excess  of  potassium  cyanide,  the  precipitation 
is  prevented.  A  cold,  concentrated  solution  of  the  above  salt  is  not  precipi- 
tated by  hydrogen  sulphide  (v.  Girsewald,  Zurich,  1902) 


SEPARATION  OF  COPPER  PROM  CADMIUM.  203 

The  cadmium  sulphide  thus  precipitated  shows  a  great  tendency 
of  passing  through  the  filter-paper  even  when  a  "hardened"  filter 
is  used,  so  that  it  is  "salted  out."  A  considerable  amount  of  pure, 
solid  potassium  chloride  is  stirred  into  the  solution,  the  precipitate 
is  allowed  to  stand  overnight,  and  in  the  morning  it  is  filtered 
through  a  Schleicher  &  Schull  "hardened  filter."  The  precipi- 
tate is  washed  first  by  decantation  with  concentrated  potassium 
chloride  solution,  it  is  then  transferred  to  the  filter  and  washed 
with  the  same  solution.  For  the  cadmium  determination  this 
precipitate  cannot  be  used  on  account  of  the  potassium  chloride 
which  adheres  to  it,  and  it  is  not  advisable  to  wash  the  salt  out 
with  water,  for  in  this  case  a  turbid  filtrate  will  be  obtained.  It  is, 
therefore,  dissolved  in  hot  hydrochloric  acid  (1:3)  from  a  wash- 
bottle,  the  solution  is  evaporated  to  dryness,  the  residue  dissolved 
in  water,  filtered  if  necessary  from  separated  sulphur,  and  for 
every  100  c.c.  of  the  solution  5-7  c.c.  of  concentrated  sulphuric 
acid  are  added,  and  the  cadmium  is  precipitated  by  passing  hydro- 
gen sulphide  into  the  cold  solution.  This  time  the  cadmium 
sulphide  is  easily  filtered.  The  cadmium  is  determined  as  sulphate 
according  to  p.  190. 

The  filtrate  is  evaporated  with  nitric  acid  until  the  odor  of 
hydrocyanic  acid  can  no  longer  be  detected,  and  the  copper  is  most 
conveniently  determined  according  to  p.  183  as  cuprous  sulphide. 

Remark. — The  results  obtained  by  this  method  are  good,  but 
considerable  time  and  patience  are  required. 


(d)  By  Electrolysis. 

The  experiments  of  R.  Philipp  in  the  author's  laboratory  show 
that  a  very  accurate  separation  can  be  made,  as  recommended  by 
Neumann,  by  electrolyzing  the  nitric  acid  solution. 

The  solution,  containing  not  more  than  0.2  gm.  cadmium,  is 
treated  with  4-5  c.c.  of  concentrated  nitric  acid  or  10  c.c.  nitric 
acid  sp.gr.  1.2,  and  diluted  to  150  c.c.  in  a  platinum  dish.  The 
anode,  a  disk  electrode,  is  placed  so  that  it  only  dips  into  the 
liquid  a  short  way.  Under  these  conditions,  0.2  gm.  of  copper 
is  deposited  perfectly  free  from  cadmium,  within  12  or  14  hours 


204  GRAVIMETRIC  ANALYSIS. 

by  a  current  of  0.2-0.3  ampere  and  a  voltage  of  1.9-2.3  volts, 
with  a  current  of  1  to  1.5  amperes  and  2.5-2.6  volts  electrode 
potential,  the  cadmium  is  deposited  in  about  five  hours.  The 
solution  is  siphoned  off,  while  pure  water  is  poured  into  the  dish 
without  breaking  the  current ;  the  dish  is  finally  rinsed  with  alcohol, 
dried  and  weighed  with  the  deposited  copper.  The  solution  is 
treated  with  sufficient  sulphuric  acid,  evaporated  to  expel  the 
nitric  acid,  cooled,  diluted  and  the  cadmium  electrolyzed  from 
cyanide  solution  as  described  on  p.  189. 

Remark. — If  considerably  more  than  0.2  gm.  Cd  is  present  in 
150  c.c.  of  the  solution,  there  is  danger  of  small  amounts  of 
cadmium  separating  out  upon  the  copper  during  the  washing  of 
the  deposit,  especially  when  the  anode  extends  well  into  the 
solution.  This  is  because  the  concentration  of  the  acid  becomes 
less  during  the  washing.  In  analyzing  a  solution  containing  a 
large  amount  of  cadmium  and  small  amount  of  copper,  therefore, 
it  is  best  to  wash  at  first  with  2  per  cent,  nitric  acid  rather  than 
with  distilled  water. 

The  separation  requires  but  a  few  minutes  with  a  rotating 
anode  or  cathode,  and  a  stronger  current. 


DETERMINATION   OF  ARSENIC  AS   TR1SULPHIDE,  ETC.       205 

B.  DIVISION  OF  THE  SULPHO-ACIDS. 
Arsenic,  Antimony,  Tin. 

SELENIUM,  TELLURIUM.   GOLD.   PLATINUM,  TUNGSTEN, 
MOLYBDENUM,  VANADIUM.) 

ARSENIC,  As.  At.  Wt.  74.96. 
Forms: 


I.  Determination  as  Arsenic  Trisulphide, 

For  the  determination  of  arsenic  in  this  form,  it  must  be  present 
in  its  trivalent  state,  i.e.,  as  arsenious  acid  or  as  arsenite. 

The  solution  is  made  strongly  acid  with  hydrochloric  acid  and 
the  arsenic  precipitated  in  the  cold  with  hydrogen  sulphide.  The 
excess  of  the  latter  is  removed  by  passing  a  stream  of  carbon 
dioxide  through  the  solution,  which  is  then  filtered  through  a 
Gooch  crucible  that  has  been  previously  dried  at  105°  C.  The 
precipitate  is  washed  with  hot  wrater,  dried  at  105°  C.  to  constant 
weight,  and  weighed  as  As2S3. 


2.  Determination  as  Arsenic  Pentasulphide,  A^Sg,  according  to 

Bunsen.* 

Modified  by  Fr. 


The  solution,  which  must  contain  all  of  the  arsenic  as  arsenic 
acid,  is  treated  with  hydrochloric  acid  little  by  little  (it  is  best  to 
keep  the  solution  cooled  by  surrounding  the  flask  with  ice)  until 
the  solution  contains  at  least  two  parts  of  concentrated  hydro- 
chloric acid  for  each  part  of  water.  A  very  rapid  stream  of  hydro- 
gen sulphide  is  conducted  into  this  solution  (contained  in  a  large 
Erlenmeyer  flask)  until  it  is  saturated  with  the  gas,  after  which 

*  Ann,  d.  Chem.  und  Pharm.,  192,  305. 

t  Z.  anal.  Chem.,  32,  45;  see  also  Brunner  and  Tomicek,  Monatshefte, 
8,  607;  McCay,  Z.  anal.  Chem.,  27,  682,  and  J.  Thiele,  Ann.  d.  Chem.  u. 
Pharm.,  265,  65. 


206  GRAVIMETRIC  ANALYSIS. 

the  flask  is  stoppered  and  allowed  to  stand  two  hours.  The  arsenic 
pentasulphide  is  then  filtered  through  a  Gooch  crucible  which  has 
been  dried  at  105°  C.,  and  the  precipitate  is  washed  completely 
with  water,  then  with  hot  alcohol  (to  hasten  the  subsequent  dry- 
ing). After  drying  at  105°  C.  the  precipitate  is  weighed  as  As2S4. 
It  is  not  necessary  to  wash  it  with  carbon  bisulphide. 

Remark. — If  the  above  directions  are  conscientiously  followed, 
this  method  gives  faultless  results.  If,  on  the  other  hand,  the 
directions  are  deviated  from  in  the  slightest  respect,  the  precipitate 
is  likely  to  contain  some  arsenic  trisulphide,  whereby  low  results 
will  be  obtained.  If  the  solution  is  not  kept  cool  and  the  hydro- 
chloric acid  is  added  too  rapidly,  the  heat  of  the  reaction  suffices 
to  change  a  part  of  the  arsenic  chloride  (this  compound  probably 
exists  in  solution)  to  arsenious  chloride  and  chlorine,  so  that 
on  passing  hydrogen  sulphide  into  the  solution  a  mixture  of  arsenic 
trisulphide  and  arsenic  pentasulphide  will  be  obtained. 

3.  Determination  of  Arsenic  as  Magnesium  Pyroarsenate, 
according  to  Levol. 

The  solution,  which  must  contain  all  of  the  arsenic  as  arsenate, 
and  have  a  volume  of  not  over  100  c.c.  per  0.1  gm.  arsenic,  is 
treated  drop  by  drop,  under  constant  stirring,  with  5  c.c.  of 
concentrated  hydrochloric  acid  and  then,  for  each  0.1  gm.  of 
arsenic,  there  is  added  7-10  c.c.  of  magnesia  mixture  *  and  a 
drop  of  phenolphthalein  solution,  ftow,  with  constant  stirring, 
10  per  cent,  ammonia  is  added  from  a  burette  until  the  phenol- 
phthalein imparts  a  permanent  red  color  to  the  solution,  and  then 
enough  more  of  the  10  per  cent,  ammonia  is  added  to  make  one- 
third  the  volume  of  the  neutralized  solution.  After  standing 
twelve  hours  the  liquid  is  filtered  through  a  Gooch  or  Monroe 
crucible.  The  precipitate  in  the  beaker  is  transferred  to  the 
crucible  by  squirting  upon  it  some  of  the  original  solution  from 
a  small  wash  bottle.  The  precipitate  is  then  washed  with  2.5 
per  cent,  ammonia  until  free  from  chloride.  It  is  drained  as 

*  Prepared  by  dissolving  55  gms.  crystallized  magnesium  chloride  and 
70  gms.  ammonium  chloride  in  650  c.c.  water  and  diluting  this  to  a  volume 
of  one  liter  with  ammonia,  sp.gr.  0.96. 


DETERMINATION  OF  ARSENIC  AS  MAGNESIUM  PYROARSENATE.  207 

completely  as  possible  by  suction,  dried  at  100°  and  heated  in 
an  electric  oven  quite  gradually  to  a  temperature  of  about  400° 
to  500°,  until  there  is  no  more  ammonia  evolved.  Then  the 
temperature  is  raised  to  800°  to  900°  and  kept  there  for  about 
10  minutes.  The  crucible  is  then  cooled  in  a  desiccator  and 
the  precipitate  weighed  with  the  precipitate  in  the  form  of 

M&PA- 

If  an  electric  oven  is  not  available  the  crucible  with  the 
precipitate  is  placed  in  an  air-bath  (cf.  Fig.  11,  p.  27),  having 
the  bottom  of  the  Gooch  crucible  [come  within  about  2-3  mm. 
of  the  bottom  of  the  outer  crucible.  A  thin  layer  of  ammonium 
nitrate  powder  *  is  added  to  the  precipitate,  which  is  then 
heated,  at  first  gently,  gradually  increasing  the  temperature 
until  a  light-red  glow  on  the  outer  crucible  is  obtained,  after 
which  the  precipitate  is  allowed  to  cool  in  a  desiccator  and 
is  weighed  as  Mg2As,O7.  The  results  obtained  are  excellent. 

Remark. — The  precipitate  produced  by  the  magnesia  mixture 
has  the  formula  MgNH4AsO4  +  6H2O  and  loses  5J  molecules  of  water 
at  102°  C.;  it  has,  therefore,  been  proposed  to  dry  the  precipitate 
at  this  temperature  and  to  compute  the  amount  of  arsenic  present 
as  follows: 

[MgNH4  AsO4 + iH2O] :  As = p :  x. 

It  is,  however,  impossible  to  obtain  a  constant  weight  at  this 
temperature,  so  that  the  procedure  is  not  to  be  recommended. 
If  the  precipitate  is  dried  at  105-110°  C.  the  salt  is  obtained  almost 
entirely  free  from  water  and  at  a  slightly  higher  temperature  it 
begins  to  decompose.  The  only  form  in  which  the  precipitate 
should  be  weighed  is  as  magnesium  pyroarsenate. 


*  Instead  of  using  ammonium  nitrate,  the  crucible  may  be  provided  with 
a  perforated  cover  and  heated  in  a  current  of  oxygen. 


208  GRAVIMETRIC  ANALYSIS. 

Solubility  of  Magnesium  Ammonium  Arsenate,  according  to 

Levol. 

600  parts  of  water  dissolve  1  part  of  the  salt. 

In  1\  per  cent,  ammonia  it  is  almost  entirely  insoluble.  Accord- 
ing to  J.  F.  Virgili,*  1  part  of  anhydrous  magnesium  ammonium 
arsenate  dissolves  in  24,558  parts  of  ammonia  water. 

Colorimetric  Determination  of  Arsenic. 

Small  quantities  of  arsenic,  such  as  are  present  in  wall  papers, 
may  be  estimated  very  accurately  by  means  of  the  Marsh  apparatus,, 
comparing  the  mirror  with  a  series  of  standards  formed  with 
known  quantities  of  arsenic. f  It  is  just  as  accurate,  however, 
to  use  the  much  simpler  apparatus  used  for  the  Gutzeit  test. 
Treadwell  and  Comment  %  allow  the  arseniuretted  hydrogen 
to  react  with  disks  containing  silver  nitrate  and  compare  the 
Suiting  color  with  a  standard  which,  unfortunately,  must  be 
produced  freshly  with  each  analysis,  as  it  does  not  keep  very 
well.  Almost  equally  accurate,  and  much  more  convenient 
is  the  method  of  F.  Hefti  §  and  that  of  C.  R.  Sanger  and  O.  F. 
Black  1 1  in  which  the  arseniuretted  hydrogen  is  allowed  to  act  upon 
mercuric  chloride  paper. 

(a)  Method  of  Hefti. 

In  the  first  place,  all  the  organic  matter  is  destroyed  by  heat- 
ing the  sample  in  a  tube  with  fuming  sulphuric  and  nitric  acids 
(see  Vol.  I),  both  of  which  must  be  free  from  arsenic.  The 
resulting  liquid  is  evaporated  with  sulphurous  acid  on  the  water- 
bath  in  order  to  reduce  the  arsenic  acid  to  arsenious  acid  and 
when  all  the  excess  of  S02  has  been  expelled,  the  solution  is 
poured  into  the  graduated  tube  T  of  the  apparatus  shown  in 
OT^Tn  ~the  100-150  c.c.  flask  K  are  placed  6-8  gm.  ofJ 


*  Z.  anal.  Chem.,  44,  504  (1905). 

,fC.  R.  Sanger,  Am.  Chem.  J.,  13,  431  (1891);  Z.  anal.  Chem.,  38,  137 
and  377;  G.  Lockemann,  Z.  angew.  Chem.,  1905,  429  and  491. 
J  This  method  was  given  in  the  former  editions  of  this  book. 
§  Inaug.  Dissert.  Zurich,  1907. 
II  Proc.  Amer.  Acad.  Arts  and  Sciences,  No.  8,  1907. 


COLOR/METRIC  DETERMINATION  OF  ARSENIC. 


209 


granulated  zinc  coated  with  copper*  and  about  20  c.c.  of  sul- 
phuric acid  free  from  arsenic  (1  vol.  cone,  acid +  7  vols.  water). 
At  the  end  of  ten  minutes  all  the  air  should  be  expelled  from  the 
apparatus.  The  outlet  Z)f  is  now  covered  with  a  piece  of  mercuric 
chloride  paper  and  kept  in  place  by  a  small  piece  of  ground  glass. 
According  to  whether  little  or  much  arsenic  is  present,  all  or  a 


FIG.  39. 

part  of  the  solution  in  T  is  allowed  to  flow  into  the  flask  K.  At 
the  end  of  twenty  minutes  the  experiment  is  finished.  By  com- 
paring the  color  of  the  spot  produced  on  the  mercuric  chloride 

*  Cf.  Vol.  I. 

t  For  quantities  of  arsenic  under  0.02  mg.,  the  upper  diameter  of  the 
tube  D  should  be  8  mm.  and  for  larger  quantities  it  should  be  16  mm.*  The 
upper  edge  of  the  tube  is  ground  perfectly  flat. 


210  GRA^  METRIC  ANALYSIS. 

paper,  with  the  standard  spots,  the  quantity  of  arsenic  present 
is  determined. 

The  disks  of  mercuric  chloride  paper  are  prepared  by  dipping 
pieces  of  pure  filter  paper  into  a  saturated  solution  of  mercuric 
chloride  and  drying  them  in  an  oven  at  a  temperature  of  60°-70°. 

The  standards  are  prepared  by  carrying  out  a  series  of  exper- 
iments with  known  quantities  of  arsenic.  The  spots  thus  obtained 
soon  lose  their  color  when  exposed  to  moist  air,  but  when  dry 
can  be  kept  in  the  dark  for  several  days.  An  older  standard 
is  not  reliable,  but  can  be  used  to  estimate  the  approximate 
quantity  of  arsenic  and  then,  by  making  two  or  three 
standards  with  known  quantities,  the  exact  amount  of  arsenic 
can  be  determined.  The  standard  solution  of  arsenious  acid 
used  in  preparing  the  scale  should  contain  20  mg.  of  As2O3  in  a 
liter.  t  Then 

0.05  cm.  =  0.001  mg.  As2O3> 
0.1  c.c.   =0.002        etc. 

For  smaller  amounts  of  arsenic  the  above  solution  is  diluted  with 
nine  times'  as  much  water  and  thus  made  one-tenth  as  strong. 

(6)  Method  of  C.  R.  Sanger. 

Three  grams  of  uniformly  granulated,  pure  zinc  is  placed  in  the 
30  c.c.  evolution  flask  (Fig.  40)  which  is  fitted  with  a  stopper 
holding  a  thistle  tube  and  a  gas  delivery  tube.  The  enlargement 
of  the  horizontal  tube  contains  a  wad  of  cotton  and  at  the  outer 
end  is  a  piece  of  thick  filter-paper  which  has  been  dipped  into 
mercuric  chloride  solution  and  dried.  Through  the  thistle  tube 
is  poured  15  c.c.  of  arsenic-free,  dilute  hydrochloric  acid*  (1:6) 
and  the  hydrogen  evolution  is  allowed  to  proceed  for  at  least  ten 
minutes  to  drive  out  the  air  from  the  apparatus.  At  the  end 
of  this  time,  a  measured,  or  weighed,  quantity  of  the  arsenic 
solution  to  be  tested  is  added  to  the  flask,  which  is  then  nearly 
filled  with  water.  After  a  few  minutes,  the  mercuric  chloride 
paper  begins  to  color  and  at  the  end  of  thirty  minutes  will  attain 

*  When  hydrochloric  acid  is  used  it  is  unnecessary  to  plate  the  zinc  with 
copper. 


COLOR/METRIC  DETERMINATION   OF  4RSENIC.  211 

the  maximum  coloration.     By  comparison  with  standards,  the 
quantity  of  arsenic  present  is  estimated. 

Inasmuch  as  the  color  of  the  standards  is  strongly  influenced 
by  moisture,  Sanger  recommends  preserving  the  strips  in  a  per- 
fectly dry  condition.  For  this  purpose  a  little  phosphorus  pent- 
oxide  is  placed  in  a  small  test-tube,  followed  by  a  little  cotton, 
and  then  the  strip  of  paper  is  shoved  in  with  the  colored  part  at 


FIG.  40. 

the  bottom.  The  upper  end  of  the  paper  is  moistened  with  a 
drop  of  Canada  balsam,  after  which  the  tube  is  closed  and  sealed. 
In  this  way  the  color  can  be  preserved  for  several  months,  although 
the  freshness  disappears  after  a  few  weeks.  See  the  colored 
chart  at  the  end  of  the  book,  upper  row. 

The  color  holds  a  little  better  if  the  strips  of  paper,  after  being 
colored,  are  moistened  with  strong  hydrochloric  acid  and  then 
dried.  Some  6N-HC1  is  placed  in  a  small  test  tube,  heated  to 
at  least  60°,  the  strips  of  paper  dipped  in  the  acid  and  allowed 
to  remain  there  two  minutes,  washed  thoroughly  in  running  water, 
dried  and  sealed  in  tubes  as  described  above.  After  the 
drying,  the  color  is  a  little  duller.  See  the  colored  chart,  middle 
row. 


212  GRAVIMETRIC  ANALYSIS. 

If,  however,  the  strips  of  colored  paper  are  treated  with  normal 
ammonja  solution,  the  spot  which  was  originally  red  turns  black. 
After  drying,  such  strips  are  kept  in  small  test-tubes  over  calcium 
chloride.  These  standards  are  much  more  permanent  than  when 
prepared  as  above.  See  the  colored  chart,  bottom  row. 

Remark. — To  make  the  strips  uniform  as  regards  the  length 
of  the  spot  and  the  color,  the  following  conditions  must  be 
fulfilled: 

1.  The  evolution  flask  must  be  kept  the  same  size  and  the 
delivery  tubing  must  be  of  uniform  bore. 

2.  The  same  quantity  of  zinc  of  the  same  size  must  be  used  in 
all  the  tests. 

3.  The  volume  and  concentration  of  the  acid  must  remain 
the  same. 

4.  The  wad  of  cotton  must  not  get  too  moist.     After  10  or 
12  experiments  it  should  be  renewed. 

5.  H2S,  SbH3  and   PH3  must  not  be  present,  as  they  give  a 
colored  spot  with  the  HgCl2  paper. 

(c)  Electrolytic  Determination  of  Arsenic.  * 

Instead  of  producing  the  arseniuretted  hydrogen  by  means  of 
zinc  and  acid,  it  may  be  formed  with  the  aid  of  cathodic  hydrogen. 
Thorpe  passes  the  arsine  through  a  heated  tube  and  produces  an 
arsenic  mirror,  but  Hefti  f  allows  the  gas  to  react  with  mercuric 
chloride  paper.  In  both  cases  the  apparatus  devised  by  Thorpe 
is  used  and  is  shown  in  Fig.  41. 

As  cathode  a  perforated  cone  of  thin  lead  foil  is  used.  This 
is  suspended  from  the  platinum  wire  that  has  been  fused  into  the 
ground-glass  stopper  of  the  cathode  compartment.  The  anode 
consists  of  platinum  foil,  two  or  three  centimeters  wide,  which 
is  wrapped  around  the  porous  cell. 


*  Cf.  Bloxam.,  Z.  anal.  Chem.,  1,  483  (1862);  T.  E.  Thorpe,  Proc.  Chem. 
Soc.,  19,  183  (1903);  W.  Thomson,  Manch.  Memoirs,  48,  No.  17  (1904); 
S.  R.  Tootmann,  Chem.  Zentr.,  1904, 1,  1295;  H.  J.  S.  Sand  and  E.  Hackford, 
Chem.  Zentr.,  1904,  II,  259. 

t  Inaug.  Dissert.,  Zurich,  1907. 


ELECTROLYTIC  DETERMINATION  OF  ARSENIC. 


213 


Procedure. — Pure,  dilute  sulphuric  acid  (1:7)  is  poured  into 
the  earthenware  cell  and  into  the  glass  outer  vessel,  E;  the  level 
of  the  acid  should  be  about  2  or  3  cm.  from  the  bottom  in  the 
former,  and  about  0.5  cm.  higher  in  the  latter.  For  the  colori- 
metric  determination,  the  arsenic  solution  is  poured  directly 
into  the  acid  of  the  inner  cell.  It  must  be  present  as  arsenious 
acid,  and,  if  this  is  not  the  case,  it  must  be  reduced  with  sulphurous 
acid  and  the  excess  of  the  latter  expelled  by  heating.  For  the 


FIG.  41. 

production  of  mirrors,  the  air  must  all  be  expelled  by  hydrogen 
before  the  arsenic  solution  is  added.  The  tube  C  is  filled  with 
crystallized  calcium  chloride.  The  outlet  at  D  is  covered  with  a 
disk  of  mercuric  chloride  paper  (see  Method  a)  and  then  the 
circuit  is  closed.  The  potential  should  be  about  7  volts  and  the 
current  about  2  to  3  amperes.  The  analysis  is  finished  at  the 
end  of  twenty  minutes  and  the  quantity  of  arsenic  estimated  by 
comparing  the  spot  with  a  standard  scale.  (See  Method  a.) 
If  the  apparatus  is  connected  with  a  horizontal  delivery  tube, 
Sanger's  method  can  be  used.  (See  Method  6.) 

Remark. — As  regards  the  influence  of  the  cathode  material, 
Thorpe  recommends  bright  platinum  foil  and  Hefti  uses  lead. 


214  GRAVIMETRIC  ANALYSIS. 

Polished  platinum  does  not  hold  arsenic'back,  but  platinum  with  a 
rough  surface  does,  and  since  bright  platinum  becomes  dull  with 
use,  it  is  easily  possible  for  low  results  to  be  obtained.  Exper- 
iments performed  by  Hefti  in  the  author's  laboratory  showed 
that  zinc  alloyed  with  a  trace  of  copper  or  platinum,  bright 
platinum  foil  and  lead  did  not  hold  back  arsenic  when  used  as  the 
cathode;  on  the  other  hand,  zinc  in  the  presence  of  chloroplatinic 
acid  and  platinum  foil  with  spongy  platinum  held  back  a  con- 
siderable quantity  of  arsenic. 

To  determine  arsenic  in  a  mineral  water,  100  c.c.,  or  more  if 
necessary,  are  evaporated  to  a  small  volume  in  a  procelain  dish. 
The  resulting  solution  is  acidified  with  sulphuric  acid,  reduced 
with  sulphurous  acid,  the  excess  of  the  latter  expelled,  and  the 
analysis  continued  by  one  of  the  above  three  methods. 

Determination  of  Larger  Quantities  of  Arsenic  as  Arsine. 
Method  of  F.  Hefti. 

In  the  electrolysis  of  larger  quantities  of  arsenic  it  was  not 
possible,  in  the  past,  to  recover  all  the  arsenic  in  the  form  of 
arseniuretted  hydrogen;  some  arsenic  was  deposited  upon  the 
cathode  in  the  form  of  the  element  arsenic  and  was  not  trans- 
formed into  arsine  by  the  further  action  of  the  electric  current. 
The  quantity  of  arsenic  deposited  as  metal  depends  upon  the 
potential  of  the  electric  current  at  the  electrodes,the  temperature,, 
and  the  concentration  of  the  arsenic  solution.  At  high  potentials, 
low  temperature,  and  low  concentration  of  the  solution,  the 
quantity  of  arsenic  deposited  becomes  zero  and  the  yield  of 
arseniuretted  hydrogen  is  then  quantitative.  The  estimation  of 
the  latter  is  best  accomplished  iodimetrically.  If  arseniuretted 
hydrogen  is  passed  through  a  solution  of  iodine  in  potassium 
iodide,  it  is  immediately  oxidized  to  arsenic  acid  in  the  cold. 

AsH3 + 4H2O  +  4I2  =  SHI  +  H3AsO4. 

If  the  excess  of  iodine  is  titrated  with  sodium  thiosulphate 
(see  lodimetric  Methods)  it  is  possible  to  determine  the  quantity 
of  iodine  that  has  reacted  with  the  arsine.  If  T  c.c.  of  O.lN" 
iodine  were  used  at  the  start,  and  t  c.c.  of  0.1N"  thiosulphate 


DETERMINATION  OF  ARSENIC  AS  ARSINE. 


215 


solution  were  used  for  the  titration,  then  the  quantity  of  arsenic 
or  arsenic  trioxide,  present  is 

(T  -0X0.000937  gm.  arsenic, 
(T -0X0.001237  gm.  As2O3. 

The  apparatus  necessary  is  shown  in  Fig.  42.  The  decom- 
position cell  is  also  shown  in  Fig.  43  and  consists  of  a  wide 
U-tube  capable  of  holding  120  c.c.  of  solution.  The  tube  is 


FIG.  42. 

made  in  two  halves,  the  edges  of  the  bottom  being  ground  so 
that  they  fit  tightly  together.  Between  these  edges  is  placed 
a  piece  of  thin  parchment  paper,  the  extending  edges  of  which 
are  folded  over  one  side  of  the  tube.  A  piece  of  rubber 
tubing  holds  the  two  halves  of  the  U-tube  together  and  also 
the  parchment  paper  in  place.  This  tubing  is  wired  tightly  in 
place,  taking  care  that  the  edges  of  the  parchment  paper  are 
also  covered  by  the  wire.  In  one  arm  of  the  tube  (the  anode 
compartment)  which  remains  open  during  the  whole  experi- 
ment, is  suspended  a  platinum  plate  electrode  as  anode,  and 
the  other  arm  (the  cathode  compartment)  is  tightly  stoppered 
with  a  three-holed  rubber  stopper.  Through  one  hole  passes 
a  glass  tube  containing  mercury;  at  the  bottom  of  this  tube 


2l6 


GRAVIMETRIC  ANALYSIS. 


a  platinum  wire  is  sealed  in  and  from  this  a  plate  electrode  of 
lead  foil  is  suspended  to  serve  as  cathode.  The  wire  from  the 
negative  pole  of  the  battery  dips  into  the  mercury.  Through 
the  second  hole  in  the  stopper  is  passed  a  gas  delivery  tube 
leading  to  the  absorption  vessel  A.  The  third  hole  in  the  stopper 
carries  a  tube  that  leads  to  the  Erlenmeyer  flask  E  which,  in 
turn,  is  connected  with  the  empty  flask  F,  and  the  latter  with 
the  rubber  tubing  shown  in  Fig.  42.  This  tubing  leads  to  the 
hood.  Such  an  arrangement  provides  for  the  regulation  of  the 


FIG.  43. 

pressure  in  the  cathode  space.  If  the  pressure  there  exceeds 
that  of  the  anode  space,  a  part  of  the  arsenic  solution  will  pass 
into  the  anode  compartment  and  will  be  lost  in  the  analysis. 
If  suction  is  applied  at  the  extreme  end  of  the  absorption 
apparatus,  so  that  bubble  after  bubble  of  air  passes '  through 
the  Erlenmeyer,  then  it  is  very  easy  to  overcome  the  pressure  in  the 
absorption  vessel  without  having  diminished  the  pressure  in  the 
cathode  compartment  enough  to  tear  the  parchment  membrane. 

Procedure. — The  arsenic  solution  to  be  tested  must  contain 
all  the  arsenic  in  the  trivalent  condition. 

In  the  first  place,  the  anode  compartment  is  filled  to  within 
3  cm.  of  the  top  with  10  per  cent,  sulphuric  acid,  the  arsenic 
solution  is  placed  in  the  cathode  compartment  and  this  is  filled 
to  within  3.5  cm.  of  the  top  (in  other  words,  the  level  in  the 


DETERMINATION  OF  ARSENIC  AS  ARSINE.  217 

cathode  compartment  is  about  0.5  cm.  lower  than  on  the  other 
side  of  the  U-tube) ;  the  concentration  of  the  arsenic  solution 
in  the  U-tube,  after  this  dilution  with  acid,  should  not  ex- 
ceed 80  mgm.  As2O3  in  50  c.c.  of  solution.  The  U-tube  is 
placed  in  ice-water  and  the  gas  delivery  tube  is  connected  with 
two  ten-bulb  absorption  tubes,  of  which  only  one  is  shown  in 
the  drawing.  Into  the  first  absorption  tube  is  now  placed  an 
accurately  measured  volume  of  tenth-normal  iodine  solution, 
and  into  the  second  tube,  which  is  not  shown  in  the  drawing, 
10  c.c.  of  sodium  thiosulphate  solution,  and  about  40  c.c.  of 
water.  The  purpose  of  the  sodium  thiosulphate  solution  is  to 
catch  any  iodine  that  may  escape  from  the  first  absorption 
tube.  While  the  apparatus  absorption  vessels  are  being  filled, 
the  arsenic  solution  should  be  in  the  ice-water,  and  its  tem- 
perature should  be  about  0°  when  the  analysis  is  ready  to 
begin.  Gentle  suction  is  started  at  the  end  of  the  second 
absorption  tube,  the  electric  circuit  is  closed*  and  the  suction 
is  regulated  so  that  bubble  after  bubble  of  air  slowly  streams 
through  the  pressure  regulator  and  into  the  cathode  compart- 
ment throughout  the  whole  duration  of  the  electrolysis.  More- 
over, care  is  taken  that  enough  ice  remains  in  the  cooling  bath. 
When  all  the  conditions  are  maintained  satisfactorily,  the  liquid 
in  the  cell  should  remain  perfectly  clear,  or  at  the  worst  be 
colored  only  by  a  slight  brownish  turbidity,  which  eventually 
disappears.  If  a  black  turbidity  is  formed  that  settles  to  the 
bottom  of  the  U-tube,  something  has  gone  wrong  and  it  is 
useless  to  continue  the  experiment.  In  a  normal  experiment, 
the  evolution  of  the  arsine  is  finished  in  an  hour,  when  not 
more  than  50  mgm.  of  As2O3  are  present.  The  current  is  then 
stopped,  the  contents  of  the  two  absorption  tubes  (first  the 
iodine  and  then  the  thiosulphate  solution)  are  poured  into  a 
beaker  containing  5  c.c.  of  a  saturated  solution  of  pure  XaHCO3 
and  the  excess  of  iodine  is  titrated  with  O.lX  sodium  thiosul- 
phate solution  using  starch  solution  as  indicator.  If  on  mixing 
the  contents  of  the  two  absorption  bulbs  the  solution  is  decolorized, 
the  tit  ration  is  finished  with  O.lX  iodine. 

*  A  current  of  2  to  3  amperes  and  7  volts  is  used. 


2i8  GRAVIMETRIC  ANALYSIS. 

This  method  can  be  carried  out  very  easily  and  gives  accurate 
results  in  the  presence  of  iron,  so  that  it  is  suitable  for  a  rapid 
determination  of  the  arsenic  present  in  iron  minerals. 

Determination  of  Arsenic  in  Mispickel. 

One  gram  of  the  finely  powdered  mineral  is  fused  in  a  nickel 
crucible  with  6  gm.  of  sodium  carbonate  and  1  gm.  of  potas- 
sium nitrate.  The  resulting  melt  is  extracted  with  hot  water 
and  the  residue  (Fe2O3,  NiO)  washed  with  hot  sodium  carbonate 
solution.  To  the  filtered  solution  200  c.c.  of  water  saturated 
with  S02  are  added  to  reduce  the  arsenic,  the  solution  is  boiled 
to  expel  the  excess  of  SO2,  allowed  to  cool,  diluted  to  500  c.c. 
with  sulphuric  acid  so  that  the  entire  solution  contains  10  to 
12  per  cent,  of  H2S04,  and  the  arsenic  is  then  determined  as 
outlined  above,  using  one-tenth  of  the  solution. 

Instead  of  extracting  the  melt  with  water,  it  may  be  treated 
with  dilute  sulphuric  acid,  whereby  all  the  iron  goes  into  solution. 
After  this  solution  has  been  reduced  with  sulphurous  acid,  the 
analysis  of  an  aliquot  part  gives  the  same  result  as  when  the 
first  procedure  is  followed. 

Hefti  found  42.67  per  cent.  As  by  the  former  process  and 
42.73  per  cent,  by  the  latter.  The  mineral  analyzed  was  sup- 
posed to  contain  42.72  per  cent,  arsenic. 

ANTIMONY,  Sb.    At.  Wt.  120.2. 
Forms:  Sb2S3,  Sb204,  and  Sb. 
i.  Determination  as  Trisulphide,  Sb2S3. 
Method  of  F.  Henz.  * 

The  best  method  for  the  determination  of  antimony  is,  in  the 
author's  opinion,  the  following: 

Hydrogen  sulphide  is  passed  for  twenty  minutes  into  the  cold 
solution  of  an  antimonite  or  antimonate,  then,  without  stopping 
the  current  of  hydrogen  sulphide,  the  solution  is  slowly  heated  to 

*  F.  Henz,  Z.  anorg.  Chem.,  37,  18  (1903). 


DETERMINATION   OF  ANTIMONY  AS    TRISULPH1DE.  219 

boiling  and  the  gas  passed  through  it  for  fifteen  minutes  more,  after 
which  the  now  dense  precipitate  is  allowed  to  settle  and  filtered 
through  a  Gooch  crucible  which  has  been  heated  at  280-300°  and 
weighed.  The  precipitate  is  washed  four  or  five  times  by  decanta- 
tion  with  50—75  c.c.  of  hot,  very  dilute  acetic  acid  into  which 
hydrogen  sulphide  has  been  passed,  and  washed  on  the  filter  with 
the  same  wash  liquid  until  all  chloride  is  removed.  At  first  the 
filtrate  runs  through  perfectly  clear,  but  after  all  the  mineral  acid 
has  been  removed,  the  filtrate  shows  a  slightly  orange  tint,  owing 
to  an  unweighable  amount  of  the  antimony  sulphide  passing 
through  in  colloidal  solution.  As  soon  as  this  point  is  reached 
the  washing  is  stopped. 

F.  Henz  then  proceeds  as  follows: 

The  crucible,  after  the  precipitate  has  been  dried  as  much  as 
possible  by  suction,  is  placed  in  the  tube  R,  Fig.  44,  which  is 
fitted  to  a  drying  oven  (about  18  cm.  long  and  10  cm.  high; 
covered  with  asbestos  paper).  The  tube  R  is  then  closed  with  a 
rubber  stopper  that  holds  a  glass  delivery  tube,  and  R  is  pushed 
into  the  drying  closet  until  the  end  of  the  stopper  is  reached.  To 
protect  the  rubber  stopper  during  the  subsequent  heating,  its 
inner  surface  is  provided  with  a  Rose  crucible  cover,  which  is 
held  in  place  by  wrapping  the  tube  a  with  a  strip  of  asbestos 
paper. 

The  air  is  now  expelled  from  the  tube  by  a  stream  of  dry, 
air-free  carbon  dioxide*  and  heated  for  two  hours  at  100°-130°. 

t  In  order  to  obtain  accurate  results  it  is  neceesary  to  have  the  carbon 
dioxide  perfectly  free  from  air.  This  may  be  prepared  by  the  use  of  the 
Kipp  generator  as  modified  by  Henz  (Chem.  Ztg.,  1902,  386)  see  Fig.  45. 

This  differs  from  the  ordinary  form  of  the  Kipp  apparatus  only  as  regards 
the  siphon  tube  a;  but  herein  lies  a  distinct  advantage.  The  apparatus  is 
charged  as  follows:  First  of  all,  pieces  of  pure  marble  are  placed  in 
the  middle  compartment,  the  stop-cock  is  opened,  and  water  is  poured 
through  the  upper  compartment,  until  it  begins  to  run  out  through  the 
stop-cock,  which  is  then  closed.  By  this  means  all  the  air  has  been  expelled 
from  the  lower  parts  of  the  apparatus  and  it  only  remains  to  introduce  the 
hydrochloric  acid.  To  accomplish  this,  the  water  is  allowed  to  run  out 
through  the  siphon  while  hydrochloric  acid  (1:4)  is  poured  in  at  the  top 
of  the  generator.  As  soon  as  carbon  dioxide  begins  to  be  evolved,  the  tube  a 
is  closed  and  the  apparatus  is  ready  for  use.  When  the  acid  has  become 


220 


GRAVIMETRIC  A 'N 'A LYSIS. 


Inasmuch  as  the  tube  R  extends  so  far  into  the  drying  oven,  there 
is  no  danger  of  water  condensing  in  the  tube,  but  it  is  all  expelled 
as  vapor  at  b. 

The  precipitate  is  now  dry  and  the  air  completely  expelled 
from  the  heating  tube. 

The  tube  R  is  now  withdrawn  a  little  from  the  oven,  about 


FIG.  44.  FIG.  45. 

5  cm.,  as  shown  in  the  drawing,  and  the  temperature  is  raised  to 
280-300°  and  kept  there  for  two  hours. 

Hereby  some  sulphur  is  volatilized  and  collects  in  the  tube  R 
outside  the  oven.  The  antimony  pentasulphide  is  also  com- 
pletely changed  into  the  black  modification  of  the  trisulphide  by 
this  heating.*  The  crucible  is  allowed  to  cool  in  the  stream  of 

too  weak,  it  is  removed  through  the  siphon  while  a  fresh  supply  is  poured 
in  at  the  top;  there  is  no  need  of  taking  the  apparatus  apart  during  this 
operation.  It  is  obvious  that  the  same  apparatus  can  be  employed  to 
advantage  for  generating  hydrogen  or  hydrogen  sulphide. 

*  According  to  Paul  (Z.  anal.  Chem.,  31,  540  (1892)),  the  transformation 
of  antimony  pentasulphide  can  be  accomplished  in  his  drying  oven  (shown 


DETERMINATION  OF  ANTIMONY  AS   TRISULPHIDE.         221 

carbon  dioxide,  transferred  to  the  balance  case,*  and  after  standing 
half  an  hour  is  weighed.  The  black  antimony  trisulphide  is  not 
at  all  hygroscopic.  A  further  heating  in  the  current  of  carbon 
dioxide  will  rarely  show  any  change  in  weight. 

(6)  Method  of  Vortmann  and  Metzel. 

When  antimony  is  precipitated  by  hydrogen  sulphide  from  a 
hot  solution  which  is  strongly  acid  with  hydrochloric  acid,  the 
sulphide  eventually  becomes  grayish  black  in  color,  is  crystalline, 
and  can  be  filtered  easily  and  washed  with  water  without  the 
slightest  tendency  to  pass  into  the  hydrosol  condition. 

The  solution,  in  an  Erlenmeyer  flask,  is  treated  with  concen- 
trated hydrochloric  acid,  adding  24  c.c.  of  the  concentrated  acid  to 
each  100  c.c.  of  the  neutral  solution.  It  is  heated  to  boiling,  and 
the  hot  solution  subjected  to  the  action  of  hydrogen  sulphide  gas. 
The  Erlenmeyer  flask  containing  the  solution  is  placed  in  a  dish  of 
boiling  water  and  the  water  in  the  latter  is  kept  boiling  during  the 
precipitation.  It  is  advisable  to  introduce  the  hydrogen  sulphide 
gas  quite  rapidly  at  first,  but  towards  the  end  a  slow  stream  is 
sufficient.  The  antimony  sulphide  as  it  comes  down  is  yellow  at 
first,  but  as  the  precipitation  proceeds,  it  becomes  redder;  grad- 
ually it  becomes  heavier  and  denser,  assumes  a  crystalline  form 
and  becomes  darker,  and  finally  black  in  color.  The  transforma- 
tion into  the  crystalline  form  is  hastened  by  shaking  the  flask. 
At  first,  while  the  precipitate  is  of  a  yellowish  color,  there  is  no 
need  of  shaking  the  flask  but  later  on  it  is  very  desirable  to  do  so. 
The  shaking,  however,  should  not  be  too  violent,  as  otherwise 
some  of  the  precipitate  is  likely  to  adhere  to  the  upper  portions  of 
the  flask  and  escape  the  transformation.  The  duration  of  the 

in  Fig.  20  of  this  book)  by  heating  to  a  temperature  of  230°.  This  is  per- 
fectly true,  but  the  transformation  takes  place  more  readily  at  a  temperature 
of  280°.  It  is  more  difficult  to  replace  the  air  completely  with  carbon  dioxide 
in  Paul's  drying  oven  and  often  some  white  antimony  oxide  is  noticeable  in 
the  crucible. 

*  A  piece  of  writing-paper  should  be  rolled  up  and  placed  in  the  tube  R, 
so  that  the  crucible  does  not  come  in  contact  with  any  of  the  sulphur  sub- 
limate, on  withdrawing  it.  The  crucible  is  removed  with  the  paper. 

t  Z.  anal.  Chem.,  44,  526  (1905). 


222  GRAVIMETRIC  ANALYSIS. 

whole  process  amounts  to  from  30  to  35  minutes.  Finally  a 
heavy,  dense,  crystalline  precipitate  of  antimony  trisulphide 
is  obtained  which  settles  well  and  permits  a  rapid  filtration.  The 
solution  is  diluted  with  an  equal  volume  of  water,  which  is  allowed 
to  flow  around  the  walls  of  the  flask  in  order  to  wash  down  any 
adhering  sulphide.  The  dilution  almost  always  causes  the  forma- 
tion of  a  slight  yellow  turbidity.  The  reason  for  this  is  that  a 
little  of  the  antimony  is  held  in  solution  by  the  strong  acid  and 
as  the  solution  is  diluted  this  is  caused  to  precipitate  by  the 
dissolved  hydrogen  sulphide.  The  flask  is,  therefore,  once  more 
shaken,  placed  in  the  vessel  of  boiling  water  and  more  hydrogen 
sulphide  is  introduced.  In  two  or  three  minutes  the  solution 
above  the  precipitate  will  become  clear.  It  is  filtered  through 
a  Gooch  crucible,  washed  with  water  to  remove  >the  acid,  then 
with  alcohol,  and  placed  in  the  drying  oven. 

2.  Determination  as  Tetr oxide,  Sb204  (Bunsen). 

This  method  is  based  upon  the  fact  that  antimony  pentoxide, 
when  ignited  at  a  definite  temperature,  changes  into  Sb2C>4. 
Bunsen,*  who  first  proposed  the  method,  later  abandoned  it 
because  his  assistant  succeeded  in  volatilizing  more  than  0.1  gm. 
of  the  precipitate  by  heating  it  over  the  blast  lamp.f  Brunck,J 
Rossing§  and  Henz  II  have  shown,  however,  that  under  certain 
conditions  accurate  results  can  be  obtained,  although  they  did 
not  specify  the  exact  temperature  at  which  the  precipitate 
should  be  ignited.  If  the  pentoxide  is  ignited  in  a  large  porcelain 
crucible  over  the  blast  lamp,  it  is  possible  to  change  the  antimony 
pentoxide  quantitatively  into  the  tetroxide;  if,  however,  a  small, 
thin-walled,  procelain  crucible  is  used,  the  tetroxide  loses  oxygen 
and  is  transformed  into  the  volatile  trioxide,  whereby  low  results 
are  obtained.  It  is,  therefore,  purely  accidental  if  exact  results 
are  obtained  by  such  a  procedure.  In  1897,  Baubignyf  dis- 

*  Ann.  Chem.  u.  Pharm.,  106,  3  (1858). 

t  Ibid.,  192,  316  (1878). 

J  Z.  anal.  Chem.,  34,  171  (1895). 

§  Ibid.,  41,  9  (1902). 

||  Loc.  cit. 

If  Compt.  rend.,  124,  499  (1897) 


DETERMINATION  OF  ANTIMONY  AS    TETROXIDE.  223 

covered  that  antimony  pentoxide  is  converted  quantitatively 
into  the  tetroxide  at  a  temperature  of  750°-SOO°  and  begins  to 
form  the  volatile  trioxide  at  a  little  above  950°.  The  author's 
assistant,  Dr.  E.  G.  Beckett,*  has  confirmed  the  work  of  Bau- 
bigny.  At  7oO°-SOO°  the  transformation  is  complete  and  at  1000° 
it  is  possible  to  volatilize  0.35  gm.  of  the  precipitate  in  about 
thirty  minutes.  If  these  facts  are  borne  in  mind,  it  is  possible 
to  get  accurate  results,  although  even  then  the  method  is  less 
satisfactory  than  the  determination  as  trisulphide. 

Procedure. — In  the  majority  of  cases  it  is  desired  to  determine 
the  amount  of  antimony  present  in  a  mixture  of  its  tri-  and  penta- 
sulphides,  or  in  a  mixture  of  one  or  the  other  of  the  two  compounds 
with  sulphur.  It  is  best  to  proceed  as  follows:  The  sulphide 
of  antimony,  precipitated  from  hot  solution,  is  washed  first  with 
hot  water,  then  with  alcohol,  afterwards  with  a  mixture  of 
alcohol  and  carbon  disulphide  (in  order  to  remove  the  sulphur), f 
again  with  alcohol  and  finally  with  ether,  afterwards  drying  the 
precipitate.  The  bulk  of  the  precipitate  is  separated  from  the 
filter  and  placed  upon  a  watch-glass  and  the  filter  is  placed  in  a 
small  porcelain  dish  and  boiled  with  a  little  of  a  freshly  prepared 
solution  of  ammonium  sulphide,  stirring  constantly  with  a  glass 
rod.  The  resulting  solution  is  poured  through  a  small  filter  into 
a  30  c.c.  porcelain  crucible,  and  the  filter  is  treated  repeatedly 
with  ammonium  sulphide  until  it  is  no  longer  colored  brownish 
red  at  the  edge  of  the  paper,  where  it  begins  to  dry;  the  extrac- 
tion of  the  antimony  sulphide  is  then  complete.  The  solution 
in  the  crucible  is  evaporated  to  dryness  and  the  main  part  of 
the  precipitate  is  added.  To  oxidize  the  antimony  sulphide, 
Beckett  places  the  crucible,  with  a  dish  of  fuming  nitric  acid 
beside  it,  under  a  bell-jar  and  allows  it  to  stand  over  night.  The 
vapors  of  fuming  acid  slowly  oxidize  the  precipitate  in  the  crucible 
and  in  the  morning  it  is  possible  to  complete  the  oxidation  by 
means  of  nitric  acid  (sp.gr.  1.4)  without  the  reaction  being  too 
violent.  The  crucible  is  then  heated  on  the  water-bath  until 
the'precipitate  becomes  white  and  the  greater  part  of  the  acid  is 

*  Inaug.  Dissert.  Zurich,  1909. 

fThiele,  Ann.  d.  Chem.  und  Pharm.,  263,  372. 


224  GRAVIMETRIC  ANALYSIS. 

expelled.  A  little  water  is  added  and,  with  stirring,  enough 
concentrated  ammonia  to  give  an  alkaline  reaction.  The  con- 
tents of  the  crucible  are  now  evaporated  to  dryness  on  the  water- 
bath,  carefully  heated  in  an  air-bath  (Fig.  11,  p.  27)  until  no 
more  fumes  of  sulphuric  acid  are  evolved,  and  then  for  half  an 
hour  at  800°  in  an  electric  oven.  After  cooling  in  a  desiccator, 
the  crucible  is  transferred  quickly  to  a  glass-stoppered  weighing 
beaker,  allowed  to  stand  twenty  minutes  in  the  balance  case, 
and  then  weighed.*  The  ignition  and  weighing  are  repeated 
until  a  constant  weight  is  obtained. 

3.  Determination  of  Antimony  as  Metal. 

Antimony  may  be  deposited  from  acid  solutions  by  means  of 
the  electric  current;  the  metal,  however,  does  not  adhere  well  to 
the  electrode,  so  that  this  method  cannot  be  used  for  its  quantita- 
tive determination.  On  the  other  hand,  the  following  method 
is  suitable;  it  was  first  proposed  by  Parrodi  and  Mascazzini,f 
then  modified  by  Luckow,J  and  afterwards  improved  by  Classen 
and  Reiss.§  According  to  the  experience  in  the  author's  laboratory, 
it  is  not  so  accurate  as  the  trisulphide  method. 

If  a  solution  of  sodium  or  ammonium  sulphoantimonite,  or 
antimonate,  containing  not  more  than  0.3  gm.  Sb  in  a  volume  of 
about  140  c.c.  is  subjected  to  electrolysis  with  a  current  of  1-1.5 
amperes  at  70°  for  90  minutes,  the  antimony  will  be  deposited 
upon  a  platinum  dish,  which  has  been  gently  sand-blasted,  as  steel- 
gray,  metallic  antimony,  and  the  deposit  adheres  so  firmly  that 
it  can  be  dried  and  weighed  without  loss.  The  chief  condition  for 
the  success  of  this  operation  is  the  absence  of  polysulphides.  In 
case  these  substances  are  present  the  antimony  is  incompletely 
deposited  and  in  some  cases  not  at  all,  or  the  deposited  antimony 
may  pass  into  solution,  on  account  of  being  oxidized  to  sodium 

*  Sb2O4  is  hygroscopic  and  must  be  weighed  in  a  weighing  beaker,  as 
Finkener,  Dexter,  and  Beckett  have  all  found. 

t  Z.  anal.  Chem.,  18,  587  (1879). 

t  Ibid,,  19,  13  (1880). 

gBerichte,  14,  1629  (1881);  17,  2474  (1884);  18,  408  (1885);  27,  2074 
(1894). 


DETERMINATION  OF  ANTIMONY  AS  METAL.  22$ 

antimonite  by  means  of  the  sodium  polysulphide  which  is  formed 
at  the  anode  during  the  electrolysis: 

2Sb  +  3Na2S2 = 2Na3SbS3. 

It  is  necessary,  therefore,  to  prevent  the  formation  of  poly- 
sulphides  during  the  electrolysis.  For  this  reason  Lecrenier  * 
added  sodium  sulphide  to  the  bath,  whereby  the  polysulphide  is 
transformed  into  thiosulphate : 

Na2S2  +  Na2SO3  =  Na2S2O3 + Na2S. 

Ost  and  Klapproth  f  carry  out  the  electrolysis  with  the  aid  of 
a  diaphragm  to  keep  the  polysulphide  away  from  the  cathode. 

It  is  better,  however,  to  make  use  of  potassium  cyanide  for  this 
purpose.  { 

Na2S2  +  KCN = Na2S  +  KCNS. 

Procedure. — In  most  cases  the  antimony  is  first  isolated  as 
the  sulphide,  which  is  either  precipitated  by  hydrogen  sulphide 
from  acid  solution  or  obtained  by  acidifying  an  alkaline  solution 
of  the  thio-salt.  The  filtered  and  washed  precipitate,  cor- 
responding to  not  over  0.2  gin.  Sb,  is  dissolved  on  the  filter  by 
pouring  pure  sodium  sulphide  solution  (sp.gr.  1.14)  over  it.§ 

*  A.  Lecrenier,  Chem.  Ztg.,  13,  1219  (1889). 

t  Z.  angew.  Chem.,  1900,  828. 

JCf.  A.  Fischer,  Ber.,  36,  2048  (1903);  Z.  anorg.  Chem.,  42,  363  (1904); 
Hollard,  Bull.  Soc.  Chem.,  23  [3]  292  (1900);  F.  Henz,  Z.  anorg.  Chem.,  37, 
31  (1903). 

§  A.  Inhelder  prepares  the  solution  of  sodium  sulphide  as  follows:  666  gms. 
of  purest  sodium  hydroxide  (prepared  from  sodium)  are  dissolved  in  2  liters 
of  water  and  the  solution  divided  into  halves.  One-half  is  placed  in  a  lon-g- 
necked  flask  of  such  a  size  that  the  solution  just  reaches  the  neck  of  the  flask. 
The  flask  is  closed  with  a  two-holed  rubber  stopper  and  a  rapid  current  of 
well-washed  hydrogen  sulphide  is  introduced  through  a  glass  tube  1  cm. 
wide,  keeping  out  the  air  as  much  as  possible.  When  the  solution  ceases 
to  increase  in  volume  (1000  c.c.  of  NaOH  solution  should  give  1218  c.c.  of 
sodium  XaSH  solution).  When  this  is  accomplished,  the  other  half  of  the 
original  sodium  hydroxide  solution  is  added.  The  solution  of  Na,S  thus 
prepared  is  colored  a  pale  yellow,  and  after  standing  some  time,  or  sooner 


226  GRAVIMETRIC  ANALYSIS. 

The  solution  is  caught  in  a  weighed  platinum  dish  with  unpolished 
inner  surface,  or  in  a  beaker  if  a  platinum  gauze  electrode  is  to  be 
used.  After  washing  the  filter  with  the  sodium  sulphide  solu- 
tion, the  total  volume  of  the  liquid  in  the  platinum  dish  should 
not  be  over  80  c.c.;  if  less  than  this,  enough  more  sodium  sul- 
phide solution  is  added.  The  solution  is  diluted  with  60  c.c.  of 
water  and  2-3  gms.  of  the  purest  potassium  cyanide  are  added 
and  the  liquid  is  stirred  with  the  anode  until  all  the  cyanide  has 
dissolved  and  the  solution  is  well  mixed.  It  is  heated  to  60°-70° 
and  electrolyzed  with  a  current  of  1-1.5  amperes  and  electrode 
potential  of  2-3  volts.  After  1.5  to  2  hours  all  the  antimony 
will  be  upon  the  cathode  in  the  form  of  a  firmly-adhering,  light- 
gray  deposit.*  Now,  without  breaking  the  circuit,  the  electrolyte 
is  siphoned  off,  while  water  is  added  until  the  current  ceases  to 
pass  through  the  liquid  (the  voltmeter  connected  as  ammeter 
points  to  the  zero  reading).  The  cathode  is  removed,  washed 
thoroughly  with  water,  then  with  absolute  alcohol,  dried  at  about 
80°,  cooled  in  a  desiccator,  and  weighed. 

The  results  obtained  by  this  method  are  invariably  too  high, 
as  F.  Henz  f  showed  in  the  author's  laboratory,  the  error  amount- 
on  shaking,  tetragonal  crystals  of  Na2S  +  9H2O  are  deposited.  The  solution 
keeps  indefinitely  in  a  well-stoppered  flask;  a  slight  precipitate  of  black 
metal  sulphide  (FeS,  NiS,  Ag2S)  is  formed  after  some  time. 

If  the  sodium  sulphide  solution  is  prepared  from  caustic  soda  purified  by 
alcohol,  the  final  solution  is  colored  a  deep  yellow  or  brown  and  it  contains 
more  foreign  sulphides,  partly  in  suspension  and  partly  in  colloidal  solution. 
If  the  suspended  sulphides  are  removed  by  filtration,  a  further  precipitate 
will  appear  on  standing. 

For  the  antimony  determination,  the  saturated  sodium  sulphide  solution, 
which  has  a  specific  gravity  of  1.22,  is  diluted  until  the  specific  gravity  is  1.14. 
According  to  A.  Classen,  a  satisfactory  solution  can  be  prepared  by  dissolving 
the  purest  grade  of  commercial  Na2S.  Classen  formerly  recommended  that 
the  solution  be  prepared  by  boiling  a  solution  of  sodium  hydroxide  which 
had  been  saturated  with  hydrogen  sulphide.  Such  a  solution  after  an  hour's 
boiling,  while  introducing  a  stream  of  hydrogen,  contained  69  per  cent. 
NaSH  and  31  per  cent.  Na2S  (F.  Wegelin). 

*  To  make  sure  that  the  deposition  is  complete,  the  liquid  may  be  trans- 
ferred quickly  to  a  second  dish  and  electrolyzed  for  half  an  hour  longer. 
It  is  seldom  that  there  will  be  any  gain  in  weight  shown  by  this  dish. 

t  Z.  anorg.  Chem.,  37,  31  (1903). 


DETERMINATION  OF  ANTIMONY  AS  METAL.  227 

ing  to  about  1.5  to  2  per  cent,  of  the  total  antimony  present.  If, 
however,  the  antimony  deposit  is  dissolved  and  precipitated  as 
the  trisulphide,  the  weight  of  the  latter  corresponds  to  the 
theoretical  value,  showing  that  the  deposit  contained  all  the  anti- 
mony. If,  as  A.  Fischer  *  recommends,  the  deposit  is  dissolved 
in  alkali  polysulphide,  and  again  electrolyzed  with  addition  of 
potassium  cyanide,  the  same  weight  of  antimony  is  obtained  as 
at  first,  but  the  antimony  is  not  pure. 

The  error  in  the  analysis  is  so  constant  that  values  not  far 
from  the  truth  will  be  obtained  by  subtracting  1.6  per  cent,  of 
the  weight  of  antimony  deposited  electrolytically. 

This  error  of  the  electrolytic  antimony  determination  was 
first  detected  by  Henz,  but  has  been  confirmed  by  a  number  of 
other  investigators,  including  O.  M.  M.  Dormaar,f  F.  Forster 
and  O.  Wolff, %  and  recently  by  A.  Inhelder.§ 

According  to  Dormaar,  Forster  and  Wolff,  the  high  values  are 
due  to  the  presence  of  a  little  sulphur  and  more  oxygen.  Forster 
and  Wolff  assert  that  the  metal  contains  from  1  to  1.5  per  cent, 
of  oxygen.  The  error  is  greater  in  proportion  to  the  quantity 
of  free  sodium  hydroxide  present  in  the  electrolyte.  According 
to  the  work  of  Scheen,'!  which  was  suggested  by  Classen,  the 
error  is  due  to  enclosed  mother-liquor  and  is  greater  in  propor- 
tion as  the  electrode  surface  is  rough.  Scheen,  therefore,  rec- 
ommends a  bright  electrode  surface,  or  one  that  is  dulled  but 
slightly. 

A.  Inhelder  1f  has  carefully  repeated  the  experiments  of 
Scheen,  using  new,  polished  dishes,  but  has  not  been  able  to 
confirm  Scheen's  conclusions,  D.  Karl  Mayr  also  obtained 
high  values  no  matter  whether  the  electrode  surface  was  bright 
or  dull. 

Cleaning  the  Electrodes.     Ost  **  recommends  heating  with  a 

*  Berichte,  36,  2348  (1903). 

t  Z.  anorg.  Chem.,  53,  349  (1907).  I 

J  Z.  Elektrochem.,  13,  205  (1907). 

§  Inaug.  Dissert.  Zurich,  1910.  J 

||  Z.  Elektrochem.,  14,  257  (1908). 

Tf  Inaug.  Dissert.  Zurich,  1910. 

**  Z.  angew.  Chem.,  1901,  827. 


228  GRAVIMETRIC  ANALYSIS. 

mixture  of  equal  parts  concentrated  nitric  acid  and  a  saturated 
solution  of  tartaric  acid.  The  antimony  will  also  dissolve 
readily  by  heating  with  a  solution  of  alkali  polysulphide. 

TIN,  Sn.     At.  Wt.  119.0. 

Forms:  Sn02,  Sn. 
i.  Determination  of  Tin  Dioxide,  Sn02. 

Two  cases  are  to  be  distinguished: 

(a)   The  Tin  is  Present  as  Metal  (in  an  Alloy). 
(6)    The  Tin  is  Present  in  Solution. 

(a)   The  Tin  is  Present  in  an  Alloy. 
Method  I. 

In  case  the  tin  is  present  in  an  alloy  it  may  be  treated  accord- 
ing to  the  method  of  Busse*  as  follows: 

About  0.5  gm.  of  the  alloy,  in  the  form  of  borings,  is  treated  in 
a  beaker  with  6  c.c.  of  nitric  acid  (sp.  gr.  1.5),  3  c.c.  of  water  are 
slowly  added  and  the  beaker  is  then  quickly  covered  with  a  watch- 
glass.  As  the  water  is  mixed  with  the  acid  a  violent  reaction  takes 
place.  When  the  evolution  of  nitric  oxide  (brown  vapors  on  coming 
in  contact  with  the  air)  has  ceased,  the  solution  is  heated  to  boiling 
and  diluted  with  50  c.c.  of  boiling  water;  the  precipitate  is  allowed 
to  settle  completely,  then  filtered,  washed,  and  dried.  After  burning 
the  filter,  moistening  the  ash  with  nitric  acid  and  drying  on  the 
water-bath,  the  precipitate  is  ignited,  at  first  gently  and  finally 
strongly,  over  the  Teclu  burner  or  blast-lamp.  It  is  weighed  as 
SnO2. 

The  tin  dioxide  thus  ob tamed  is  never  pure ;  it  always  contains 
small  amounts  of  other  oxides  and  must  be  purified  as  follows: 
After  weighing,  the  precipitate  is  mixed  with  six  times  as  much  of  a 
mixture  consisting  of  equal  parts  calcined  sodium  carbonate  and 
pure  sulphur,  and  this  mixture  is  heated  in  a  covered  crucible  over 
a  small  flame  until  the  excess  of  sulphur  is  almost  entirely  removed. 
This  point  is  easily  recognized  by  there  being  no  longer  any  odor 

*  Zeit.  f.  anal.  Chem.,  17,  53. 


DETERMINATION  OF  TIN  AS  TIN  DIOXIDE.  229 

of  S02  and  no  blue  flame  of  burning  sulphur  evident  between  the 
cover  and  the  crucible.  After  cooling  the  melt  is  treated  with  a 
little  hot  water,  whereby  the  tin  goes  into  solution*  as  sodium 
sulphostannate  (cf.  Vol.  I,  p.  222),  together  with  some  copper  and 
iron.  The  deep-brown  liquid,  therefore,  is  treated  with  sodium 
sulphite  t  solution  until  it  becomes  only  slightly  yellow  in  color,  after 
which  any  iron  or  copper,  etc.,  will  be  quantitatively  precipitated 
as  sulphides.  The  latter  are  filtered  off  and  washed,  first  with  water 
to  which  a  little  sodium  sulphide  has  been  added  and  finally  with 
hydrogen  sulphide  water.  As  a  rule  the  amount  of  insoluble  sul- 
phide formed  by  this  treatment  is  so  small  that  after  drying  it  can 
be  ignited  in  the  air  and  changed  to  oxide  without  introducing  any 
appreciable  error.  If  this  weight  is  subtracted  from  the  original 
amount  of  impure  stannic  oxide,  the  weight  of  pure  stannic  oxide 
will  be  obtained.  If,  however,  the  amount  of  impurity  present 
with  the  residue  of  metastannic  acid  should  be  large,  the  different 
metals  must  be  separated  according  to  one  of  the  methods  for  the 
separation  of  the  sulpho-bases  and  the  weight  of  each  oxide  deter- 
mined separately  and  the  sum  of  their  weights  subtracted  from  the 
original  weight  of  the  tin  dioxide.  Instead  of  determining  the 
amount  of  impurity  present  with  the  tin  dioxide,  the  filtrate  from 
the  insoluble  sulphides  can  be  acidified  with  acetic  acid  and  the  tin 
precipitated  as  yellow  stannic  sulphide,  which,  after  it  has  com- 
pletely settled,  is  filtered  off  and  changed  by  careful  ignition  into 
tin  dioxide,  as  described  on  p.  233,  and  weighed  as  such. 


Method  II. 

The  alloy  is  dissolved  in  nitric  acid,  the  insoluble  metastannic 
acid  is  filtered  off  and  washed  as  in  the  first  method.     Instead  of 

*  Frequently  a  single  fusion  with  sodium  carbonate  and  sulphur  is  insuf- 
ficient; this  is  recognized  by  obtaining  a  sandy  residue  insoluble  in  water. 
In  this  case  the  residue  is  filtered,  washed,  dried,  and  the  fusion  repeated 
until  all  the  tin  is  brought  into  solution. 

t  The  sodium  sulphite  changes  the  sodium  polysulphide  to  monosulphide, 
in  which  copper  and  iron  sulphides  are  insoluble. 


230  GRAVIMETRIC  ANALYSIS. 

drying  and  igniting  the  precipitate,  however,  it  is  washed  into  a 
porcelain  evaporating  dish,  evaporated  on  the  water-bath  almost 
to  dryness,  and  then  treated  with  1  c.c.  of  pure  sodium  hydroxide 
solution  and  10-15  c.c.  of  concentrated  sodium  sulphide  solution 
(see  foot-note,  p.  225).  The  evaporating  dish  is  covered  with 
a  watch-glass,  and  the  dish  with  its  contents  is  heated  for  about 
45  minutes  on  the  water-bath,  whereby  all  the  tin»should  pass  into 
solution,  and  the  other  metals  remain  undissolved  as  sulphides; 
they  are  removed  by  filtration. 

The  filter,  upon  which  the  metastannic  acid  was  filtered,  still 
retains  some  of  the  precipitate.  It  is,  therefore,  laid  in  a  second 
small  evaporating  dish,  covered  with  about  1  c.c.  of  sodium 
sulphide  solution  and  heated  on  the  water-bath.  After  half  an 
hour,  the  tin  should  all  be  dissolved.  The  solution  is  poured 
through  a  small  filter  and  the  latter  is  washed  with  a  little  hot 
water. 

The  two  filters  are  dried,  ignited  in  a  platinum  spiral,  the  ash 
treated  with  concentrated  hydrochloric  acid  and  the  resulting 
solution  is  added  to  that  obtained  by  the  solution  of  the  alloy  in 
nitric  acid. 

For  the  determination  of  the  tin,  the  two  solutions  containing 
sodium  thiostannate  are  combined,  acidified  with  acetic  acid  and 
boiled  to  expel  the  hydrogen  sulphide.  The  precipitated  stannic 
sulphide  is  filtered  off,  washed  once  with  water  to  remove  the 
most  of  the  alkali  salts,  then  transferred  back  to  the  original 
beaker  and  dissolved  in  10  c.c.  of  50  per  cent,  caustic  potash, 
and  1  gm.  tartaric  acid,  these  quantities  sufficing  for  0.1  to  0.2 
gm.  of  tin.  (The  last  traces  of  precipitate  adhering  to  the  filter 
are  dissolved  in  a  very  little  sodium  sulphide  solution.)  To 
the  solution,  pure  30  per  cent,  hydrogen  peroxide  (Perhydrol, 
Merck)  is  added  until  the  yellow  liquid  becomes  perfectly  colorless, 
then  a  cubic  centimeter  in  excess.  The  solution  is  boiled  for 
about  ten  minutes  to  make  sure  that  the  oxidation  is  complete, 
and  that  the  excess  of  peroxide  is  decomposed.  As  soon  as  no 
more  bubbles  of  oxygen  are  evolved,  the  solution  is  allowed  to 
cool  somewhat  and  15  g.  of  oxalic  acid  dissolved  in  a  little  hot 
water  are  cautiously  added.  The  warm  solution  is  electrolyzed 
directly  as  described  on  page  234. 


DETERMINATION  OF  TIN  AS  TIN  DIOXIDE.  231 

The  precipitated  stannic  sulphide,  as  obtained  above  by 
acidifying  the  sodium  thiostannate  solution,  may  be  ignited  in  a 
porcelain  crucible  and  weighed  as  SnC>2.  The  results  are  usually 
a  little  high  and  the  method  is  not  as  accurate  as  the  electro- 
lytic determination.  Cf .  page  233 ;  /?. 


Method  III. 

The  translator  prefers  to  use  a  more  dilute  nitric  acid  for  dis- 
solving the  alloy  than  was  recommended  by  Busse.  It  is  possible 
to  obtain,  in  this  way,  residues  of  metastannic  acid  which  are 
fully  as  pure  and  the  work  is  not  as  unpleasant  as  when  the  more 
concentrated  acid  is  employed. 

Procedure. — 0.5  gm.  of  borings  are  dissolved  in  a  small  beaker 
with  15  c.c.  off  nitric  acid,  sp.  gr.  1.2.  The  solution  is  evaporated 
just  to  dryness  on  the  water-bath,  and  the  beaker  removed  as 
soon  as  this  stage  is  reached.  A  mixture  is  prepared  of  20  c.c. 
nitric  acid  sp.  gr.  1.2,  and  40  c.c.  water  and  this  is  divided  into 
three  portions.  The  residue  is  treated  successively  with  each 
portion  of  the  above  mixture,  each  time  heating  to  boiling  and 
decanting  off  the  solution  through  a  hardened  filter  paper.  The 
washing  is  completed  by  boiling  and  decanting  with  a  1  per  cent, 
solution  of  ammonium  nitrate.  As  much  of  the  precipitate  as 
possible  is  allowed  to  remain  in  the  original  beaker,  and  the 
total  volume  of  the  filtrate  should  not  exceed  150  c.c.  The  first 
portions  of  the  filtrate  are  carefully  examined  for  metastannic 
acid,  refiltered  if  necessary,  and  each  successive  portion  removed 
from  below  the  funnel  before  new  wash  water  is  added. 

The  residue  of  slightly  impure  metastannic  acid  is  treated 
as  described  under  either  of  the  above  methods.  In  case  Method 
II  is  chosen,  however,  it  is  advisable  to  treat  the  filters  containing 
the  residue  from  the  sodium  sulphide  treatment  with  15  c.c.  of 
hot  dilute  nitric  acid  (7  c.c.  HNOs,  sp.  gr.  1.2  and  8  c.c.  water) 
instead  of  burning  and  treating  with  hydrochloric  acid  as  directed 
above.  The  resulting  solution  of  the  impurities  that  were 
originally  present  in  the  metastannic  acid,  is  filtered  and  added 
to  the  original  nitric  acid  solution.  The  filters  are  dried,  ignited 


232  GRAVIMETRIC  ANALYSIS. 

and  the  ash  weighed  as  Sn02.  The  amount  thus  found  is 
added  to  the  result  obtained  from  the  sodium  thiostannate 
solution. 

Remark. — Sometimes  a  little  metastannic  acid  is  left  undis- 
solved  by  the  treatment  with  alkaline  sulphide.  It  is  not  safe, 
therefore,  to  discard  the  filters. 

(6)    Tin  is  Present  in  Solution, 
(a)   The  Solution  Contains  Tin  only. 

If  the  solution  contains  only  tin  in  the  form  of  stannic  salt 
(chloride  or  bromide),  a  few  drops  of  methyl  orange  are  added 
and  then  ammonia  until  the  pink  color  of  the  indicator  is 
changed  to  yellow.  Ammonium  nitrate  (obtained  by  the  neutral- 
ization of  20  c.c.  of  concentrated  ammonia  with  nitric  acid)  is  added 
and  the  solution  is  diluted  to  a  volume  of  300  c.c.,  heated  to  boiling, 
filtered  after  the  precipitate  has  settled,  washed  with  hot  water 
containing  ammonium  nitrate,*  dried,  ignited  in  a  porcelain 
crucible,  and  weighed  as  Sn02. 

Remark. — If  the  solution  contains  non-volatile  organic  acids, 
this  method  cannot  be  used  for  the  determination  of  tin.  In  this 
case  the  tin  must  be  first  precipitated  as  sulphide  by  means  of 
hydrogen  sulphide  (cf.  p.  233).  If  the  tin  is  not  in  solution  as 
stannic  salt,  but  as  stannous  salt,  the  solution  must  be  first  oxidized 
by  the  addition  of  bromine  water  until  a  permanent  yellow  color  is 
obtained,  after  which  the  solution  is  neutralized  with  ammonia  and 
treated  as  above  described. 

According  to  J.  Lowenthal,  tin  may  be  precipitated  from  slightly 
acid  stannic  chloride  or  bromide  solutions  in  the  presence  of  ammo- 
nium nitrate.  Methyl  orange  is  added  to  the  solution  and  then 
ammonia  until  a  yellow  solution  is  obtained ;  f  dilute  nitric  acid  is 
now  added,  drop  by  drop,  until  the  solution  just  becomes  pink  again, 
more  ammonium  nitrate  solution  is  added  (20  c.c.  of  concentrated 
ammonia  exactly  neutralized  with  nitric  acid),  the  solution  is 

*  The  ammonium  nitrate  prevents  the  formation  of  soluble,  amorphous 
stannic  acid;  it  "salts  out"  the  precipitate  (cf.  Vol.  I,  p.  71). 

t  The  excess  of  acid  cannot  be  removed  by  evaporation  on  account  of  the 
volatility  of  stannic  chloride. 


DETERMINATION  OF  TIN  AS   TIN  DIOXIDE.  233 

diluted  to  300  c.c.,  boiled  for  some  time,  filtered,  washed  with  water 
containing  ammonium  nitrate,  dried,  ignited,  and  weighed  as  SnO2. 
This  method  is  employed  when  the  solution  contains  small  amounts 
of  alkaline  earths ;  they  remain  in  solution.  Sodium  sulphate  can 
be  used  instead  of  ammonium  nitrate  to  "salt  out"  the  tin  precipi- 
tate, but  although  the  tin  will  be  quantitatively  precipitated,  some 
sodium  sulphate  will  be  also  thrown  down,  so  that  high  results  will 
be  obtained. 


(/?)  The  Solution  Contains,  besides  Tin,  Metals  of  the  Preceding 
Groups  or  Organic  Substances. 

In  this  case,  independent  of  whether  the  tin  is  present  in  the 
form  of  stannic  or  stannous  salts,  hydrogen  sulphide  is  conducted 
into  the  dilute  solution  until  it  is  saturated  with  the  gas;  the  solu- 
tion is  allowed  to  stand  until  the  odor  of  hydrogen  sulphide  has 
almost  disappeared  and  then  filtered.  The  precipitate  is  washed 
with  a  solution  of  ammonium  nitrate  (or  ammonium  acetate), 
dried,  transferred  as  completely  as  possible  to  a  porcelain  crucible, 
and  the  ash  of  the  filter  added.  The  tin  sulphide  is  at  first  gently 
heated  in  a  covered  crucible  to  avoid  loss  by  decrepitation,  and  after- 
wards in  an  open  crucible  until  the  odor  of  sulphur  dioxide  is  no 
longer  perceptible.  The  temperature  now  is  raised  gradually  until 
finally  the  full  heat  of  a  good  Teclu  burner  is  obtained  or  the  cruci- 
ble is  heated  over  the  blast-lamp.  As  tin  dioxide  holds  fast  to 
some  sulphuric  acid  with  great  tenacity,  after  cooling  the  crucible 
somewhat  a  piece  of  ammonium  carbonate  the  size  of  a  pea  is  added, 
the  crucible  covered  and  again  heated,  after  which  it  is  weighed  as 
Sn02.  The  heating  with  the  addition  of  ammonium  carbonate  is 
repeated  until  a  constant  weight  is  obtained. 

Remark. — F.  Henz  *  in  testing  this  method  always  obtained 
results  which  were  a  little  too  high.  This  is  due  to  the  fact  that 
it  is  difficult  to  wash  the  stannic  sulphide  precipitate  free  from 
sodium  salts.  The  author  recommends,  therefore,  dissolving 
the  well-washed  stannic  sulphide  precipitate  in  a  little  sodium 
sulphide,  transforming  this  solution  into  potassium  stannioxalate, 
and  determining  the  tin  by  electrolysis,  according  to  page  234. 
*  Z.  anorg.  Chem.,  37,  39  (1903). 


234  GRAVIMETRIC  ANALYSIS. 


2.  Determination  of  Tin  as  Metal. 

The  electrolytic  deposition  of  tin  from  a  solution  of  the 
ammonium  double  oxalate  gives  excellent  results.*  It  is  necessary, 
however,  that  some  free  oxalic  acid  is  always  present  while  the 
solution  is  undergoing  electrolysis.  During  the  process,  ammo- 
nium oxalate  is  changed  by  anodic  oxidation  into  ammonium 
carbonate  and  carbon  dioxide, 

(NH4)2C204  +  0=  (NH4)2C03  +  C02, 

and  the  solution  will  smell  of  ammonia  as  a  result  of  the  hydrolysis- 
of  ammonium  carbonate.  When  this  point  is  reached  no  more 
tin  is  deposited.  The  ammonia  often  precipitates  some  stannic 
acid,  which  escapes  the  electrolysis.  It  is  necessary,  therefore, 
to  avoid  letting  the  bath  become  ammoniacal,  and  this  is  best  ac- 
complished by  adding  a  little  solid  oxalic  acid  from  time  to  time. 

Procedure. — In  the  course  of  an  analysis  it  is  usually  necessary 
to  precipitate  the  tin  from  a  solution  of  alkali  thiostannate.  This 
is  best  accomplished  as  follows:  The  thio-salt  is  decomposed 
by  acidifying  with  acetic  acid,  the  deposited  sulphide  dissolved 
in  caustic  potash  solution,  the  solution  oxidized  with  hydrogen 
peroxide,  and  finally  acidified  with  oxalic  acid,  all  exactly  as 
described  on  page  230.  The  final  solution,  about  150  c.c.  in 
volume,  is  heated  to  60°-70°  and  electrolyzed  with  a  current  of 
1  ampere  and  3-4  volts  potential  at  the  electrodes;  from  time 
to  time  a  little  solid  oxalic  acid  is  added.  At  the  end  of  about 
six  hours  all  the  tin  will  have  been  deposited  upon  a  gauze 
electrode.  The  deposit  is  washed  with  water,  exactly  as  pre- 
scribed for  nickel  on  page  136,  then  with  water,  dried  by  holding 
above  a  flame,  cooled  in  a  desiccator,  and  weighed.  The  results 
are  excellent. 

Remark. — If  ammonium  oxalate  is  used  in  place  of  the  potas- 
sium oxalate,  the  electrolysis  requires  more  tim£  (eight  to  ten 

*  Cf.  Classen's  "Quant.  Anal,  by  Electrolysis"  and  his  "  Ausgewahlte 
Methoden  der  analytischen  Chemie." 


SEPARATION  OF  ARSENIC,  ANTIMONY,  AND    TIN.  235 

hours).     By  the  addition  of  hydroxylamine  the  duration  of  the 
electrolysis  is  shortened  (Engel). 

F.  Henz  proposed  to  prevent  the  bath  becoming  ammoniacal 
by  adding  sulphuric  acid  during  the  course  of  the  electrolysis, 
but  subsequent  experiments  have  shown  that  it  is  better  to  pro- 
ceed as  described  above.  If  too  much  sulphuric  acid  is  added, 
all  the  tin  is  not  precipitated.  The  chief  conditions  are  the 
presence  of  enough  oxalate  and  a  slightly  acid  solution. 

Separation  of  Arsenic,  Antimony,  and  Tin  from  the  Members  of 
the  Ammonium  Sulphide  Group. 

The  separation  is  effected  by  passing  hydrogen  sulphide  into 
the  acid  solution  of  the  above  metals  whereby  arsenic,  antimony, 
and  tin  are  precipitated  as  sulphides,  while  the  remaining  metals 
remain  in  solution. 

From  an  alloy,  or  the  solid  sulpho-salts  of  the  above  jnetals, 
arsenic,  antimony,  and  tin  may  be  readily  volatilized  by  heating 
in  a  stream  of  chlorine;  the  chlorides  of  these  three  metals  are 
readily  volatile,  while  those  of  the  remaining  metals  are  only 
difficultlv  so. 


Separation  of  Arsenic,  Antimony,  and  Tin  from  Mercury,  Lead, 
Copper,  Cadmium,  and  Bismuth. 

If  the  metals  are  all  in  solution,  they  are  precipitated  by  means 
of  hydrogen  sulphide  and  the  precipitated  sulphides  after  being 
filtered  and  washed  are  treated  with  alkali  sulphide  solution. 
If  mercury  is  present,  ammonium  polysulphide  should  be  used, 
but  in  the  absence  of  this  metal  sodium  polysulphide  works  better 
(cf.  Vol.  I.) 

If  the  metals  of  this  group  are  in  the  form  of  an  alloy  (arsenic 
and  mercury  are  seldom  met  with  to  any  extent),  the  antimony 
and  tin  are  separated  from  the  remaining  metals  on  treating 
the  alloy  with  nitric  acid.  The  tin  is  left  behind  as  meta- 
stannic  acid,  insoluble  in  dilute  nitric  acid,  with  the  antimony 
as  nearly  insoluble  antimonic  acid.  In  the  presence  of  tin,  all 
phosphorus  and  arsenic  are  thrown  down  in  the  insoluble  residue 
as  phosphate  and  arseniate  of  metastannic  acid.  The  small 


236 


GRAVIMETRIC  ANALYSIS 


amount  of  the  latter  (and  the  remaining  metals  of  this  group)  are 
precipitated  by  hydrogen  sulphide  and  separated 
from  the  copper  group  by  means  of  alkaline 
sulphide  solution. 

The   separation   of   tin  from  the   remaining 
metals   of  the   group   can  be   illustrated   by   a 
\      practical  example. 


Analysis  of  Bronzes. 

A  bronze  is  an  alloy  of  tin  and  copper  in 
varying  proportions.  It  almost  always  con- 
tains besides  these  metals  more  or  less  lead, 
aluminium,  iron,  manganese,  zinc,  and  phos- 
phorus. 

Procedure. — About  0.5-1  gm.  of  the  alloy  in 
the  form  of  borings  *  is  placed  in  a  beaker, 
treated  with  6  c.c.  of  nitric  acid,  sp.  gr.  1.5,t  and 
3  c.c.  of  water  are  added,  after  which  the 
beaker  is  immediately  covered  with  a  watch- 
glass.  When  the  reaction  begins  to  diminish, 
the  liquid  is  heated  to  boiling,  until  no  more 
brown  fumes  are  evolved,  when  50  c.c.  of 
boiling  water  are  added;  the  precipitate  (con- 
FIG.  46.  taining  all  the  tin,  the  phosphoric  acid,  and  always 

small  amounts  of  copper  oxide)  is  allowed  to  settle  completely, 


*  The  borings  are  usually  somewhat  oily,  in  which  case  they  should  be 
washed  with  ether  that  has  been  distilled  over  potash,  dried  at  about  80°  C., 
and  weighed  after  cooling  in  a  desiccator.  The  washing  with  ether  is  best 
accomplished  in  a  Soxhlet's  fat-extraction  apparatus,  as  shown  in  Fig.  46. 
The  borings  are  placed  in  the  extraction-tube,  which  is  filled  with  ether  nearly 
up  to  the  bend  b  of  the  siphon-arm.  The  tube  is  then  connected  with  the 
condenser  K.  After  this  from  20  to  30  c.c.  of  ether  are  added  to  the  flask 
and  this  is  heated  gently  on  the  water-bath.  The  ether  vapors  pass  through 
the  wide  side-arm  to  the  condenser  A',  where  they  are  condensed  and  drop 
upon  the  borings.  As  soon  as  the  ether  in  tfye  tube  has  reached  the  height 
b,  it  is  siphoned  back  into  the  flask,  where  it  is  again  distilled.  All  the  oil 
will  be  removed  from  the  borings  in  from  half  an  hour  to  an  hour. 

t  See  pages  228  and  231. 


ANALYSIS  OF  BRONZES.  237 

is  filtered,  washed  with  hot  water,  dried,  ignited  in  a  porcelain 
crucible,  and  weighed.  In  this  wray  the  weight  of  the  SnO2  +  P2O5+ 
foreign  oxide  is  obtained.  In  order  to  obtain  the  weight  of  foreign 
oxide  (chiefly  copper  oxide)  the  precipitate  is  fused  with  a  mix- 
ture of  sodium  carbonate  and  sulphur  as  described  on  p.  22S. 
The  sulphides,  remaining  after  the  solution  of  the  melt  in  hot 
water,  are  filtered  off,  converted  into  oxides  by  ignition  in  the 
air,  and  weighed.  By  subtracting  this  weight  from  that  pre- 
viously obtained,  the  weight  of  SnO2  +  P2O5  is  obtained.  In  order 
to  obtain  the  weight  of  the  SnO2  a  separate  portion  is  analyzed 
according  to  the  method  of  Oettel  as  described  below  for  phos- 
phoric acid,  and  the  amount  of  phosphoric  anhydride  subtracted 
from  the  weight  of  SnO2  +  P2O5. 

The  oxides  obtained  by  the  ignition  of  the  insoluble  sulphides 
are  dissolved  in  a  little  nitric  acid  (in  case  Fe2O3  is  present  a 
little  hydrochloric  acid  is  also  necessary)  and  the  solution  of  the 
nitrates  is  added  to  the  first  filtrate  from  the  impure  metastannic 
acid.  To  this  solution  an  excess  of  dilute  sulphuric  acid  is  added, 
and  it  is  evaporated  on  the  water-bath  as  far  as  possible  and  then 
heated  over  a  free  flame  until  dense,  white  fumes  of  sulphuric 
acid  are  evolved.  After  cooling,  50  c.c.  of  water  and  20  c.c.  of 
alcohol  are  added,  the  precipitate  of  lead  sulphate  is  filtered  off 
and  its  weight  determined  as  described  on  p.  174.  The  filtrate 
from  the  lead  sulphate  is  heated  to  remove  the  alcohol  and  the 
copper  precipitated  by  means  of  hydrogen  sulphide  and  weighed 
as  Cu2S  according  to  p.  183.  In  the  filtrate  from  the  copper  sul- 
phide the  iron,  aluminium,  and  zinc  (also  manganese)  will  be 
found.  It  is  evaporated  to  a  small  volume  in  order  to  expel  the 
hydrogen  sulphide,  oxidized  by  the  addition  of  a  few  drops  of 
concentrated  nitric  acid,  and  the  iron  and  aluminium  separated 
from  the  zinc  by  means  of  a  double  precipitation  with  ammonia,* 
whereby  the  iron  and  aluminium  are  left  behind  as  hydroxides 

*  If  considerable  zinc  is  present,  the  above  separation  is  inexact.  In  this 
case  the  filtrate  from  the  copper  sulphide  is  treated  with  sodium  acetate, 
heated  to  60°,  saturated  with  hydrogen  sulphide,  and  the  iron  and  aluminium 
determined  in  the  filtrate,  the  zinc  in  the  precipitate.  If  manganese  is 
present  in  the  alloy,  it  should  be  separated  from  iron  and  aluminium  as 
described  on  pp.  149  to  155. 


238  GRAVIMETRIC  ANALYSIS. 

and  are  separated  and  determined  according  to  p.  107.  The 
zinc  is  precipitated  from  the  filtrate  after  acidifying  with  acetic 
acid,  by  passing  hydrogen  sulphide  into  the  boiling  solution.  The 
precipitated  zinc  sulphide  is  filtered  off,  dissolved  in  hydrochloric 
acid,  evaporated  to  dryness  in  a  weighed  platinum  dish,  and  trans- 
formed to  oxide  by  heating  with  mercuric  oxide  by  Volhard's 
method  (cf.  p.  142). 

For  the  phosphorus  determination  Oettel  *  recommends  the  fol- 
lowing procedure:  From  2-5  gms.  of  the  substance  are  dissolved, 
as  before,  in  nitric  acid,  and  the  impure  metastannic  acid  with  all 
the  phosphorus  is  filtered  off,  dried,  and  transferred  as  com- 
pletely as  possible  to  a  porcelain  crucible.  The  ash  of  the  filter 
is  added,  and  the  contents  of  the  crucible  ignited.  After  cooling, 
the  substance  is  mixed  with  three  times  as  much  solid  potassium 
cyanide,  the  crucible  covered,  and  the  contents  fused;  the  stannic 
oxide  is  reduced  to  metal, 


Sn02  +  2KCN  =  2KCNO  -f  Sn, 

while    the   phosphoric  acid   is   converted   into   potassium  phos- 
phate. 

By  skilfully  rotating  the  crucible  during  the  fusion,  it  is  possi- 
ble to  unite  the  small  particles  of  molten  tin  into  a  larger  button 
whereby  the  subsequent  filtration  is  greatly  facilitated.  After 
cooling,  the  melt  is  treated  with  water  and  filtered.  The  filtrate 
is  cautiously  treated  with  hydrochloric  acid  under  a  good  hood 
and  boiled  to  remove  the  hydrocyanic  acid.  It  is  then  saturated 
with  hydrogen  sulphide  in  order  to  remove  traces  of  copper  and 
tin  which  almost  always  remain  in  the  solution.  The  filtrate  is 
freed  from  hydrogen  sulphide  by  boiling,  made  ammoniacal,  and 
the  phosphoric  acid  precipitated  as  magnesium  ammonium  phos- 
phate by  the  addition  of  magnesia  mixture.  After  standing  for 
twelve  hours,  the  latter  is  filtered  off,  washed  with  2J  per  cent, 
ammonia  water,  dried,  and  changed  by  ignition  to  magnesium 
pyrophosphate,  in  which  form  it  is  weighed. 

*  Chemiker-Zeitung  (1896),  p.  19. 


DETERMINATION  OF  PHOSPHORUS.  239 

Ordinary  bronzes  may  be  analyzed  very  nicely  in  the  fol- 
lowing manner:  The  alloy  is  treated  with  nitric  acid  as  described 
above,  the  metastannic  acid  removed  by  filtration  and  the 
filtrate  electrolyzed,  using  a  dull  platinum  dish  as  cathode,  and 
a  plate  as  anode,  both  of  which  are  weighed.  The  electrolysis 
is  carried  out  with  a  current  of  1  to  1.2  amperes  at  about  60°  and 
at  the  end  of  two  and  one-half  to  three  hours  the  electrodes  are 
washed  without  breaking  the  circuit.  On  the  anode  will  be  found 
all  the  lead  as  PbO2  and  on  the  cathode  will  be  found  the  copper. 
The  siphoned  solution  contains  the  iron,  aluminium  and  zinc, 
which  are  determined  as  above.  The  phosphorus  is  determined 
in  a  special  sample. 

Remark. — The  method  just  outlined  will  give  exact  results 
only  when  the  metastannic  acid  is  purified  and  the  recovered 
solution  of  copper  and  lead  nitrates  added  to  the  main  solution. 
In  the  electrolysis,  the  chief  dangers  to  be  feared  are  having 
the  solution  so  acid  that  the  copper  is  not  all  precipitated,  or 
so  dilute  that  a  spongy  deposit  is  obtained. 

Determination  of  Phosphorus. 

Method  of  Dudley  and  Pease  Modified* 

One  gm.  of  the  borings  are  weighed  into  a  small  beaker  and  dis- 
solved in  20  c.c.  of  aqua-regia,  made  by  mixing  equal  volumes  of  the 
concentrated  acids  just  previous  to  use.  The  beaker  is  covered 
with  a  watch-glass,  and,  after  solution  is  complete,  the  contents 
heated  nearly  to  boiling  for  fifteen  minutes.  After  cooling,  25  c.c. 
of  water  are  added,  and  then  just  sufficient  ammonia  (sp.  gr.  0.90) 
to  redissolve  all  of  the  copper  hydroxide  and  to  produce  a  deep  blue 
colored  solution;  thereupon  50  c.c.  of  colorless  ammonium  sul- 
phide are  introduced.  This  should  be  enough  to  precipitate  the 
sulphides,  and  the  supernatant  liquid  should  show  no  blue  color. 
If  it  does,  more  ammonium  sulphide  must  be  added.  The  solu- 
tion is  digested  at  a  temperature  near  the  boiling  point  for  fifteen 
minutes,  the  precipitated  sulphides  of  copper  and  lead  allowed  to 
settle,  and  then  filtered  into  a  300  c.c.  Erlenmeyer  flask,  decanting 

*  Dudley  and  Pease,  Eng.  and  R.  R.  Journ.,  March,  1894. 


240  GRAVIMETRIC  ANA LYSIS. 

the  clear  liquid  carefully  from  the  precipitate,  and  finally  throwing 
the  precipitate  upon  the  filter.  When  the  filter  has  drained  the 
filter  and  precipitate  is  returned  to  the  beaker,  50  c.c.  of  ammonium 
sulphide  wash  water  (one  part  colorless  ammonium  sulphide  to 
three  parts  of  water)  are  added,  and  the  mixture  is  heated,  and 
stirred  occasionally,  for  ten  minutes;  it  is  then  poured  upon 
another  filter,  washed  with  50  c.c.  of  ammonium  sulphide  wash 
water  and  allowed  to  drain  completely.  The  total  volume  should 
not  be  over  250  c.c.,  but  it  is  not  necessary  to  evaporate  in  case 
this  volume  is  slightly  exceeded.  To  the  filtrate  10  c.c.  of  mag- 
nesia mixture  are  added  and  the  solution  shaken.  The  flask  is 
placed  in  ice  water  and  allowed  to  stand  with  occasional  shaking 
for  two  hours.  The  precipitate  of  magnesium  ammonium  phos- 
phate is  filtered  upon  a  small  filter  and  washed  with  ammonia 
water  (one  part  0.96  sp.  gr.  ammonia  to  three  parts  water)  until 
nearly  free  from  sulphide.  10  c.c.  of  hydrochloric  acid  (one  part 
HC1,  sp.  gr.  1.20,  to  four  parts  water)  are  placed  in  the  flask, 
taking  care  that  all  of  the  precipitate  adhering  to  the  walls  of  the 
flask  is  dissolved,  and  then  poured  through  the  filter,  allowing  the 
solution  to  run  into  a  No.  1  beaker.  The  flask  and  filter  are 
washed  with  10  c.c.  more  of  the  same  acid.  3  c.c.  of  magnesia 
mixture  are  added  to  the  filtrate,  which  is  heated  to  boiling, 
removed  from  the  flame,  and  then  treated  with  ammonia  (sp.  gr. 
0.90=  10  per  cent.  NH3)  until  the  latter  is  present  in  large  excess. 
The  solution  is  allowed  to  stand  in  ice  water  for  two  hours,  and 
stirred  occasionally.  The  precipitate  is  then  filtered  and  washed 
with  2J  per  cent,  ammonia  water  until  free  from  chlorides,  and 
ignited  with  the  usual  precautions,  weighing  as  Mg2P207. 

An  excellent  method  for  the  analysis  of  ordinary  bronzes 
consists  in  dissolving  the  alloy  as  described  under  Tin,  Method  III., 
determining  the  tin  as  there  described.  The  copper  and  lead  are 
determined  in  the  nitric  acid  solution  by  electrolysis  with  a 
current  of  0.2  ampere,  the  copper  being  deposited  on  the  cathode 
and  the  lead  as  peroxide  on  the  anode.  The  electrolysis  is  usually 
finished  in  twelve  hours,  but  it  is  well  to  clean  the  electrodes  after 
weighing  the  deposits  and  then  to  test  the  solution  with  the  current 
for  an  hour  or  so  longer  to  see  whether  any  lead  or  copper  remains 
in  the  solution.  Often  a  little  more  copper  will  be  found,  especially 


SEPARATION  OF  ARSENIC  FROM  ANTIMONY.  241 

if  the  solution  was  a  little  too  acid.  During  the  electrolysis  the 
concentration  of  the  acid  gradually  diminishes  so  that  eventually 
all  the  copper  will  be  thrown  down.  The  iron,  aluminium,  and 
zinc  remain  in  solution,  and  are  determined  as  above  outlined. 

SEPARATION  OF  THE  SULPHO-ACIDS  FROM  ONE  ANOTHER. 

i.  Arsenic  from  Antimony. 

(a)  Method  of  Bunsen* 

Principle. — If  a  slightly  acid  solution  of  an  alkali  arsenate 
and  antimonate  is  treated  with  hydrogen  sulphide  in  the  cold 
and  the  excess  of  the  latter  immediately  removed  by  conduct- 
ing air  through  the  solution,  the  antimony  is  quantitatively 
precipitated  as  pentasulphide,  while  the  arsenic  remains  in  solu- 
tion. 

Procedure. — Assume  the  arsenic  and  antimony  to  be  present 
in  the  solution  as  arsenious  and  antimonous  acids.  Both  ele- 
ments are  precipitated  by  hydrogen  sulphide,  filtered,  and  washed 
with  water.  The  greater  part  of  the  precipitate  is  transferred 
by  means  of  a  spatula  to  a  200-c.c.  porcelain  casserole,  and  the 
precipitate  remaining  on  the  filter  is  dissolved  into  the  casserole 
by  dropping  a  solution  of  hot  dilute  pure  caustic  potash  upon  it. 
From  3-5  gms.  of  pure  solid  caustic  alkali  are  added,  and  the 
precipitate  dissolves  to  a  clear  solution.! 

The  casserole  is  now  covered  with  a  perforated  watch-glass. 
It  is  placed  upon  the  water-bath,  and  chlorine  is  conducted  into 
the  solution  until  all  the  alkali  is  decomposed;  this  takes  from 
one-half  to  three-quarters  of  an  hour.  By  this  operation  the 
arsenite  and  antimonite  are  oxidized  to  arsenate  and  antimonate 
and  a  small  amount  of  potassium  chlorate  is  formed.  Concen- 
trated hydrochloric  acid  is  now  added  to  the  warm  solution  drop  by 
drop  from  a  pipette  until  all  the  chlorate  is  decomposed  and  no 
more  chlorine  is  evolved.  The  watch-glass  is  removed,  the  solu- 
tion is  evaporated  to  half  its  volume,  and  then  an  equal  amount  of 
concentrated  hydrochloric  acid  is  added  and  the  solution  again 

*  Ann.  d.  Chem.  und  Pharm.,  192,  305. 

t  If  alkaline  earths  were  the  only  metals  present  besides  the  arsenic  and 
antimony,  the  first  precipitation  with  hydrogen  sulphide  would  be  omitted. 


242  GRAVIMETRIC  ANALYSIS. 

evaporated  to  half  its  volume.  The  contents  of  the  casserole 
are  washed  by  means  of  dilute  hydrochloric  acid  into  a  large  beaker, 
diluted  with  water  to  a  volume  of  600  c.c.  and  for  every  decigram 
or  less  of  the  antimony  100  c.c.  of  freshly  prepared  hydrogen 
sulphide  water  are  added.  An  orange  precipitate  of  antimony 
pentasulphide  is  formed  at  the  end  of  a  short  time.  A  strong 
current  of  air  (filtered  through  a  wad  of  cotton)  is  then  passed 
through  the  solution  without  delay  until  the  excess  of  hydrogen 
sulphide  is  completely  removed;  this  usually  requires  about 
twenty  minutes.  In  order  to  avoid  loss  during  this  operation 
a  large  beaker  should  be  used  to  contain  the  solution  and  it  should 
be  covered  with  a  perforated  watch-glass.  The  precipitate  of 
antimony  pentasulphide  is  likely  to  contain  traces  of  arsenic 
pentasulphide  so  that  it  is  dissolved  once  more  in  caustic 
potash  and  the  above  operation  repeated.  The  precipitate  now 
obtained  will  be  pure  antimony  pentasulphide.  It  is  filtered 
through  a  Gooch  crucible,  dried  at  280°  C.  in  a  stream  of  car- 
bon dioxide  as  described  under  antimony,  and  weighed  as 
Sb2S3.* 

For  the  arsenic  determination,  the  combined  filtrates  are  con- 
centrated somewhat  by  evaporation,  a  few  drops  of  chlorine 
water  are  added  and  hydrogen  sulphide  is  passed  into  the  warm 
solution  (being  kept  on  the  water-bath)  for  from  six  to  eight 
hours,  after  which  it  is  allowed  to  cool  in  a  rapid  stream  of 
hydrogen  sulphide.  After  allowing  the  precipitate  to  settle  for 
twenty-four  hours,  it  is  filtered  through  a  Gooch  crucible,  washed 
with  water,  then  three  times  writh  alcohol,  four  times  with  a  mix- 
ture of  pure  carbon  bisulphide  and  alcohol  (cf.  p.  180),  and 
finally  three  times  with  pure  alcohol.  After  drying  at  110°  C., 
the  precipitate  is  weighed  as  As2S5. 

Remark. — If  the  solution  contains  no  very  large  excess  of 
hydrogen  sulphide,  the  precipitate  will  always  contain  trisulphide, 
so  that  it  is  safer  to  dissolve  it  in  ammoniacal  hydrogen  per- 

*  Bunsen  weighed  the  antimony  as  pentasulphide  after  washing  with  car- 
bon bisulphide.  As,  however,  antimony  pentasulphide  is  likely  to  be  changed 
to  the  trisulphide  on  treating  with  carbon  bisulphide,  the  above  procedure 
is  better.  According  to  Braun,  Sb2S3  is  reduced  to  Sb2S2  on  long-continued 
treatment  vrith  CS2. 


SEPARATION  OF  ARSENIC  FROM  ANTIMONY.  243 

oxide  *  and  then  to  precipitate  the  arsenic  with  magnesia  mix- 
ture as  magnesium  ammonium  arsenate,  as  described  on  p.  206, 
weighing  it  as  Mg2As2C>7. 

Remark. — The  method  gives  very  accurate  results,  but  con- 
sumes considerable  time. 


(6)    Method  of  Fred.  Neherj 

This,  in  the  author's  estimation,  the  best  method  for  the  separa- 
tion of  arsenic  and  antimony,  depends  upon  the  fact  that  arsenic 
is  precipitated  from  a  solution  strongly  acid  with  hydrochloric 
acid  by  a  rapid  stream  of  hydrogen  sulphide,  while  antimony  is 
not. 

Procedure. — Starting  with  a  precipitate  consisting  of  the  tri- 
sulphides  of  arsenic  and  antimony,  this  is  dissolved  in  caustic 
potash  solution  and  oxidized  exactly  as  described  under  the 
previous  method.  When  free  from  chlorate,  the  acid  solution 
is  washed  into  an  Erlenmeyer  flask  and  cooled  by  surround- 
ing the  flask  with  ice.  In  another  flask  some  concentrated 
hydrochloric  acid  (sp.  gr.  1.2)  is  likewise  cooled.  When  both 
solutions  are  at  0°  C.,  the  arsenic  antimony  solution  is  diluted 
with  twice  its  volume  of  the  strong  hydrochloric  acid.  Into 
this  cold  solution  a  rapid  stream  of  hydrogen  sulphide  is  passed 
for  one  and  one-half  hours.  The  flask  is  stoppered  up  and  allowed 
to  stand  one  to  two  hours.  The  As2S5  is  filtered  through  a  Gooch 
crucible  and  washed  with  hydrochloric  acid  (1  vol.  water,  2  vols.  con- 
centrated hydrochloric  acid)  until  1  c.c.  of  the  filtrate  after  being 
considerably  diluted  with  water  and  tested  with  hydrogen  sul- 
phide shows  no  precipitation.  It  is  then  washed  with  water,  and 


*  For  this  purpose  as  much  of  the  precipitate  as  possible  is  placed  in  a 
beaker,  the  portion  adhering  to  the  filter  is  dissolved  by  hot  ammonia  into 
the  same  beaker,  and  this  is  warmed  until  the  precipitate  has  entirely  dis- 
solved. After  this,  for  every  0.1  gm.  of  AszSj,  30-50  c.c.  of  pure  3  per  cent. 
H2O2  are  added,  the  solution  heated  for  some  time  on  the  water-bath  and 
then  boiled  ten  minutes. 

t  Z.  anal.  Chem.,  32,  45. 


244  GRAVIMETRIC  ANALYSIS. 

finally  with  hot  alcohol.    After  drying  at  110°  C.,  the  precipitate 
is  weighed  as  As2Ss.  * 

The  filtrate  from  the  arsenic  sulphide  is  diluted  largely  with 
water  and  saturated  with  hydrogen  sulphide.  The  Sb2S5  is  filtered 
through  a  Gooch  crucible,  dried  at  280°  C.  in  a  current  of  carbon 
dioxide  and  weighed. 

(c)  The  Tartaric  Acid  Method. 

Principle. — The  separation  is  based  upon  the  fact  that  if 
magnesia  mixture  is  added  to  a  solution  of  an  alkali  arsenate 
and  antimonate  containing  tartaric  acid,  only  arsenic  will  be  pre- 
cipitated. 

Procedure. — Th&  sulphides  are  oxidized  as  described  under 
(a)  by  solution  in  aqueous  caustic  potash  and  introduction  of 
chlorine.  The  solution  thus  obtained  is  made  acid,  treated  with 
tartaric  acid  and  an  excess  of  ammonia  added.  This  should 
not  cause  any  turbidity.  If  a  precipitate  is  formed,  it  shows  that 
an  insufficient  amount  of  tartaric  acid  is  present.  In  this  case 
the  clear  solution  is  decanted  off,  the  precipitate  is  dissolved  by 
warming  with  tartaric  acid,  and  the  two  solutions  are  mixed. 
To  the  clear,  ammoniacal  solution,  magnesia  mixture  is  added 
slowly  with  constant  stirring  (cf.  p.  206.  foot-note).  After  stand- 
ing twelve  hours,  the  precipitate  of  magnesium  ammonium 
arsenate  is  filtered  off  (it  usually  contains  a  little  basic  mag- 
nesium tartrate),  washed  a  few  times  with  2J  per  cent,  ammonia, 
dissolved  in  hydrochloric  acid,  and  reprecipitated  by  the  addi- 
tion of  an  excess  of  ammonia.  After  standing  for  twelve  hours 
more,  the  precipitate  is  filtered,  washed  with  2J  per  cent, 
ammonia,  and  weighed  as  magnesium  pyroarsenate  as  described 
on  p.  206, 

*  If  the  solution  was  not  cold,  some  arsenic  trisulphide  will  be  found 
in  the  precipitate.  The  results  are  scarcely  affected,  however,  when  the 
precipitate  is  merely  washed  with  water  and  alcohol,  because  the  free  sulphur 
is  weighed  with  the  sulphide  of  arsenic.  If,  however,  the  precipitate  is 
washed  with  CS2,  it  is  evident  that  the  results  will  be  too  low.  For  the 
highest  degree  of  accuracy,  it  is  advisable  to  dissolve  the  precipitated  sulphide 
in  ammoniacal  hydrogen  peroxide,  or  in  fuming  nitric  acid,  and  to  deter- 
mine the  arsenic  as  Mg2As2O7  as  described  on  page  206. 


SEPARATION   OF  ARSENIC  FROM  ANTIMONY.  245 

Remark. — Arsenic  can  also  be  separated  from  tin  according 
to  the  above  method,  except  that  more  tartaric  acid  is  necessary 
to  prevent  the  precipitation  of  the  tin  than  is  the  case  when  an- 
timony alone  is  present  (cf.  p.  255). 


(d)  Method  of  E.  Fischer* 

Principle. — This  separation  depends  upon  the  ready  vola- 
tility of  arsenic  trichloride  in  a  current  of  hot  hydrochloric  acid 
gas,  under  which  conditions  antimony  chloride  is  not  volatile.  If 
the  arsenic  is  present  as  arsenic  acid,  which  is  usually  the  case, 
the  distillation  must  take  place  in  the  presence  of  -some  reducing 
agent,  t 

Procedure. — The  apparatus  shown  in  Fig.  47  is  used  for  this 
determination.  In  the  course  of  analysis,  the  arsenic  and  anti- 
mony, as  a  rule,  are  obtained  first  in  the  form  of  the  sulphides,  and 
these  are  dissolved,  as  described  under  (a),  in  caustic  potash 
solution  and  oxidized  by  chlorine.  Instead  of  using  chlorine, 
the  alkaline  solution  may  be  boiled  with  hydrogen  peroxide  or 
potassium  percarbonate.  If  the  latter  method  is  used  for  the 
oxidation,  the  boiling  must  be  continued  until  there  is  no  further 
evolution  of  oxygen. 

The  oxidized  solution  is  transferred,  by  means  of  a  long- 
stemmed  funnel,  to  the  500-c.c.  distilling  flask,  A,  in  which  has 
been  placed  1.5  gms.  of  potassium  bromide ;{  the  solution  is 
diluted  in  the  flask  with  fuming  hydrochloric  acid  to  a  volume  of 
about  200  c.c.  The  receiver,  V,  consists  of  a  large  flask  of  from 

*  Z.  anal.  Chem.,  21,  266.  The  process  as  described  is  the  modification 
of  M.  Rohmer,  Ber.,  34,  33  and  1565  (1901). 

f  Fischer  used  a  ferrous  salt,  O.  Piloty  and  A.  Stock  used  hydrogen 
sulphide  (Ber.,  30,  1649),  and  Friedheim  and  Michaelis  used  methyl  alcohol 
(Ber.,  28,  1414). 

J  Instead  of  the  potassium  bromide,  hydrogen  bromide  may  be  used 
which  has  previously  been  prepared  by  treating  1  gm.  of  bromine  with 
sulphurous  acid.  It  is  not  permissible  to  introduce  the  bromine  into  the 
flask,  A,  in  order  to  convert  it  to  hydrogen  bromide  by  introducing  sulphur 
dioxide  gas  into  the  flask,  because  it  is  then  possible  for  bromine  vapors 
to  get  into  the  receiver  by  means  of  the  air  which  is  first  expelled  from 
the  apparatus,  and  the  bromine  would  oxidize  the  volatilized  AsCl3,  and 
thus  interfere  with  the  subsequent  determination  of  the  arsenic  by  pre- 
cipitation as  the  trisulphide,  or  by  titration. 


246 


GRAVIMETRIC  ANALYSIS. 


1.5-2  liters  capacity;  it  is  kept  surrounded  by  a  current  of  cold 
water  coming  from  the  condenser  and  contains  at  the  start,  800  c.c. 
of  cold  distilled  water.  Then,  with  the  apparatus  all  connected 
as  shown  in  the  drawing,  the  distilling  flask  is  heated  and  its 
contents  partially  distilled  in  a  current  of  hydrogen  chloride,* 
meanwhile  constantly  passing  a  little  sulphur  dioxide  into  the 
flask,  until  at  the  end  of  about  forty-five  minutes,  the  volume  of 


.CH-N»(Am.)Cl 


FIG.  47. 


liquid  in  A  is  reduced  to  about  40  c.c.  The  flame  is  then  removed 
and  the  T-tube  between  the  two  evolution  flasks  removed  in  order 
to  prevent  liquid  from  backing  up  into  the  wash  bottles.  The 
adapter  tube  which  connects  the  condenser  with  the  receiver  is 
rinsed  off  and  the  receiver  removed. 

A  new  receiver  is  now  placed  at  the  end  of  the  apparatus  and 
a  seccond  distillation  is  made  in  order  to  make  sure  that  all  of 
the  arsenic  has  been  volatilized. t  Then,  for  the  determination  of 
the  arsenic,  the  contents  of  the  two  receivers  are  each  diluted  to  a 
volume  of  about  1250  c.c.,  and  the  excess  of  sulphurous  acid  is 
removed  by  heating  to  boiling  and  passing  a  stream  of  carbon 
dioxide  through  the  liquid  as  is  shown  in  Fig.  48.  When  the 
sulphur  dioxide  has  all  been  expelled  (as  can  be  shown  by  insert- 
ing a  stopper  with  delivery  tube  into  the  flask  so  that  the  escaping 
vapors  can  be  led  into  a  dilute  sulphuric  acid  solution  of  deci- 
ttormal  permanganate  which  will  be  decolorized  by  sulphur 
dioxide) ,  the  solution  is  allowed  to  cool  and  the  arsenic  determined 
as  trisulphide  according  to  the  directions  on  p.  205  and  weighed 

*  If  there  is  any  tendency  to  suck  back,  a  little  more  sulphur  dioxide 
should  be  introduced. 

t  Rohmer  found  that  as  much  as  0.15  gm.  arsenic  was  volatilized  com- 
pletely by  one  distillation. 


DETERMINATION   OF  ARSENIC  BY  TITRATION. 


247 


as  As2Sa  after  treatment  with  CS2  (pp.  170,  223),  or  it  may  be 
titrated  with  iodine. 

Determination  of  Arsenic  by  Titration. 

The  solution  is  treated  with  a  few  drops  of  phenolphthalem 
and  solid  potassium  hydroxide  is  introduced  until  a  permanent 
pink  color  is  imparted  to  the  liquid.  Tho  solution  is  then  decolor- 
ized by  the  addition  of  a  few  drops  of  hydrochloric  acid;  5  gms. 
of  sodium  bicarbonate  are  added,  and  the  solution  titrated  with 
decinormal  iodine  solution  as  described  on  p.  688.* 

The  antimony  is  determined  by  treating  the  contents  of  the 
distilling  flask  with  2  or  3  gms.  of  tartaric  acid,  washing  the 


FIG.  48. 

solution  into  an  Erlenmeyer  flask,  expelling  the  sulphur  dioxide 
as  above, f  and  determining  the  antimony  gravimetrically  by 
precipitating  as  the  trisulphide  according  to  the  directions  on 
p.  218,  or  it  is  estimated  volumetrically  by  titration  with  iodine 
as  described  on  p.  688. 

*  A  blank  determination  should  be  made  with  all  the  reagents  that  are 
to  be  used,  and  the  iodine  solution  must  be  standardized  in  a  solution  as 
dilute  as  that  in  which  the  analysis  is  made. 

f  The  escaping  gas  will  not  decolorize  a  solution  of  2-3  c.c.  dilute  sulphuric 
acid  and  one  drop  of  0.01X.  KMnO4,  when  all  the  SO2  is  expelled. 


248  GRAVIMETRIC  ANALYSIS. 


Determination  of  Arsenic  in  Commercial  Sulphuric  Acid. 

About  30  c.c.  of  concentrated  hydrochloric  acid  and  a  little 
potassium  bromide,  or  hydrogen  bromide,  are  placed  in  the  dis- 
tilling flask  A  (Fig.  47),  whereupon  50  to  100  gms.  of  the  acid  to 
be  tested  (the  weight  is  determined  by  difference)  is  introduced 
through  a  funnel  that  is  fastened  by  means  of  rubber  tubing  to 
the  upper  end  of  the  delivery  tube  which  enters  the  flask;*  the 
funnel  is  rinsed  with  concentrated  hydrochloric  acid,  and  the 
distillation  begun. 

When  the  contents  of  the  distilling  flask  have  been  concen- 
trated so  that  concentrated  sulphuric  acid  remains,  the  acid  is 
kept  hot  by  means  of  a  small  flame  until  all  of  the  arsenic  has 
been  expelled.  On  account  of  the  high  temperature,  1  gm.  of 
arsenic  will  be  driven  over  in  about  fifteen  minutes.  The  analysis 
is  finished  as  described  above. 


Separation  of  Antimony  from  Tin. 
(a)  F.'W.  Clarke' s^  Method. 

Of  all  the  present  known  methods  for  the  separation  of  anti- 
mony from  tin  this  is  probably  the  most  accurate.  It  depends 
upon  the  fact  that  antimony  is  completely  precipitated  from  a 
solution  containing  oxalic  acid,  while  stannic  salts  are  not.  Stan- 
nous  sulphide,  however,  is  decomposed  by  oxalic  acid,  forming  an 
insoluble  crystalline  stannous  oxalate,  so  that  the  tin  must  be  in 
the  stannic  form. 


*  When  the  concentrated  sulphuric  acid  runs  into  the  flask,  it  often  happens 
that  distillation  begins  to  take  place  and  some  of  the  arsenic  would  be  lost 
if  the  flask,  A,  were  left  open. 

t  Chem.  News,  Vol.  21,  p.  124.  Cf.  also  Rossing,  Zeitschr.  fiir  anal.  Chem., 
XLI,  1.  F.  Henz,  Z.  anorg.  Chem.,  37,  18  (1903).  Vortmann  and  Metzl, 
Z.  anal.  Chem.,  44,  525  (1905). 


SEPARATION  OF  ANTIMONY  FROM   TIN.  249 

Procedure. — In  the  majority  of  cases  it  is  a  question  of  sepa- 
rating antimony  from  tin  after  these  metals  have  been  separated 
from  the  members  of  the  copper  group  by  means  of  alkaline  poly- 
sulphide;  i.e.,  the  tin  and  the  antimony  are  in  the  form  of  their 
soluble  sulpho-salts. 

The  solution  of  the  sulpho-salts,  containing  not  more  than 
0.3  gm.  of  the  two  metals,  is  placed  in  a  500-c.c.  Jena  beaker  and 
treated  with  a  solution  of  6  gms.  of  the  purest  caustic  potash 
(one-third  the  sum  of  the  weights  of  tartaric  and  oxalic  acids  to 
be  added)  and  3  gms.  of  tartaric  acid  (ten  times  the  maximum 
weight  of  the  two  metals).  To  this  mixture  30  per  cent,  hydro- 
gen peroxide  is  added  slowly  until  the  yellow  solution  is  com- 
pletely decolorized;  then  1  c.c.  in  excess  is  added  and  the 
solution  is  boiled  for  a  few  minutes  to  change  any  thiosulphate 
to  sulphate  and  to  decompose  the  greater  part  of  the  excess 
peroxide.  As  soon  as  the  evolution  of  oxygen  ceases,  the  solu- 
tion is  cooled  somewhat,  the  beaker  covered  with  a  watch-glass, 
and  a  hot  solution  of  15  gms.  pure  recrystallized  oxalic  acid 
is  cautiously  added  (5  gms.  for  0.1  gm.  of  the  mixed  metals). 
This  causes  the  evolution  of  considerable  gas  (CO2-f  O2).  Now, 
in  order  to  completely  remove  the  excess  of  hydrogen  peroxide, 
the  solution  is  boiled  vigorously  for  ten  minutes.  The  volume 
of  the  liquid  should  amount  to  from  80  to  100  c.c.  After  this 
a  rapid  stream  of  hydrogen  sulphide  is  conducted  into  the  boiling 
solution,  and  for  some  time  there  will  be  no  precipitation,  but 
only  a  white  turbidity  formed.  At  the  end  of  five  or  ten  minutes 
the  solution  becomes  orange-colored  and  the  antimony  begins 
to  precipitate,  and  from  this  point  the  time  is  taken.  At  the 
end  of  fifteen  minutes  the  solution  is  diluted  with  hot  water  to 
a  volume  of  250  c.c.,  at  the  end  of  another  fifteen  minutes  the 
flame  is  removed,  and  ten  minutes  later  the  current  of  hydrogen 
sulphide  is  stopped.  The  precipitated  antimony  pentasulphide 
is  filtered  off  through  a  Gooch  crucible  which,  before  weighing 
and  after  drying,  has  been  heated  in  a  stream  of  carbon  dioxide 
at  300°  C.  for  at  least  one  hour.  The  precipitate  is  washed 
twice  by  decantation  with  1  per  cent,  oxalic  acid  and  twice 
with  very  dilute  acetic  acid  before  bringing  it  in  the  crucible. 


250  GRAVIMETRIC 

Both  of  these  wash  liquids  should  be  boiling  hot  and  saturated 
with  hydrogen  sulphide. 

The  crucible  is  heated  at  280°-300°  in  a  current  of  carbon 
dioxide  (free  from  air)  to  constant  weight  and  its  contents 
weighed  as  Sb2S3. 

To  determine  the  tin,  the  nitrate  is  evaporated  to  a  volume 
of  about  150  c.c.,  the  excess  of  oxalic  acid  nearly  neutralized 
with  ammonia  and  the  tin  deposited  electrolytic  ally  as  described 
on  p.  234. 

According  to  Vortmann  and  Metzl,*  antimony  may  be  sepa- 
rated from  tin  by  passing  hydrogen  sulphide  into  a  solution 
containing  hydrochloric  and  phosphoric  acids  of  the  proper 
concentration. 

(6)  Method  of  H.  Rose. 

Principle. — This  method  is  based  upon  the  insolubility  of 
sodium  metantimonate  and  the  solubility  of  sodium  stannate 
in  dilute  alcohol. 

Procedure. — Both  metals  are  assumed  to  be  present  in  the 
form  of  an  alloy.  The  alloy  is  treated  with  nitric  acid,  whereby 
the  antimony  and  tin  are  left  in  the  form  of  their  oxides  (cf.  pp. 
228,  231,  and  236).  The  residue  is  filtered  off,  washed  with  am- 
monium nitrate  water,  dried,  transferred  as  completely  as  possible 
to  a  large  silver  crucible  and  the  ash  of  the  filter  added,  after 
which  the  precipitate  is  gently  ignited.  From  ten  to  twelve 
times  as  much  solid  sodium  hydroxide  and  a  little  sodium  nitrate, 
or  better,  sodium  peroxide,  are  added  and  the  silver  crucible 
is  placed  within  a  larger  porcelain  one  in  order  to  protect  it 
from  the  flame:  the  contents  are  fused  and  kept  liquid  for 
twenty  minutes.  After  cooling,  the  crucible  is  placed  in  a 
large  porcelain  dish  and  its  contents  treated  with  hot  water 
until  the  melt  has  disintegrated,  leaving  the  insoluble  part  in 
the  form  of  a  fine  meal.  One-third  of  the  solution's  volume 
of  alcohol  (sp.  gr.  0.833)  is  now  added,  the  mixture  is  well  stirred 
and  filtered  after  standing  twelve  hours.  The  residue  remaining 

*  Z.  anal.  Chem.,  44,  533  (1905). 


SEPARATION   OF  ANTIMONY  FROM   TIN.  251 

LJ£ 

on  the  sides  of  the  dish  is  washed  onto  the  filter  with  dilute 
alcohol  (1  vol.  alcohol +  2  vols.  water).  The  sodium  metanti- 
monate  is  washed  first  with  a  mixture  of  1  vol.  alcohol +  2  vols. 
water,  then  with  1  vol.  alcohol +1  vol.  water,  and  finally  with 
3  vols.  alcohol +  1  vol.  water,*  until  the  filtrate  when  acidified 
with  hydrochloric  acid  and  tested  with  hydrogen  sulphide  no 
longer  gives  a  yellow  coloration  (tin  sulphide). 

If  considerable  tin  and  little  antimony  were  originally  present, 
a  single  fusion  of  the  oxides  with  caustic  soda  does  not  afford  a 
complete  separation,  as  the  residue  of  sodium  pyroantimonate 
always  contains  some  tin.  It  is,  therefore,  dried,  separated  from 
the  filter  and  placed  hi  a  silver  crucible.  The  filter  is  treated 
repeatedly  in  a  porcelain  crucible  with  fuming  nitric  acid  until 
the  paper  is  completely  destroyed  and  the  excess  of  acid  is  then 
removed  by  heating  in  an  air-bath.  The  contents  of  the  porce- 
lain crucible  are  subsequently  dissolved  in  a  little  caustic  soda 
solution  and  washed  into  the  silver  crucible;  the  water  is  then 
removed  by  heating  the  silver  crucible  at  first  on  the  water- 
bath  and  finally  in  an  air-bath.  Ten  grams  of  solid  caustic  soda 
are  now  added,  the  mixture  fused,  and  the  melt  treated  in  the 
same  way  as  before. 

The  second  residue  of  sodium  metantimonate  is  free  from 
tin.  It  is  dissolved  from  off  the  filter  by  a  mixture  of  hydro- 
chloric and  tartaric  acids, f  in  which  it  is  readily  soluble.  From 
this  solution  the  antimony  is  precipitated  by  hydrogen  sulphide 
and  determined  as  described  on  p.  218.  For  the  tin  determina- 
tion, the  alcoholic  filtrate  is  gently  heated  to  remove  the  alcohol, 
acidified  slightly  with  hydrochloric  acid,  and  the  tin  precipitated 
as  sulphide  by  hydrogen  sulphide  and  determined  according  to 
p.  233,  3. 

Remark. — If  the  oxide  residue  which  was  first  fused  with 
sodium  hydroxide  and  nitre  consisted  solely  of  tin  and  antimony 
oxides)  this  method  gives  very  good  results.  As  a  rule,  however, 

*  A  few  drops  of  sodium  carbonate  solution  should  be  added  to  all  the 
alcoholic  wash  liquids. 

t  A  mixture  consisting  of  equal  volumes  dilute  hydrochloric  acid  (1:4) 
and  5-10  per  cent,  tartaric  acid  is  used. 


252  GRAVIMETRIC  ANALYSIS. 

most  antimony  and  tin  alloys  contain  lead  and  other  metals  whose 
oxides  remain  to  small  extent  with  the  tin  and  antimony  on 
treatment  of  the  alloy  with  nitric  acid,  so  that  the  sodium  metan- 
timonate  is  subsequently  rendered  impure  by  the  presence  of  these 
metals.  The  antimony  determination  therefore  gives  too  high  re- 
sults. In  this  case  the  method  of  W.  Hampe  *  should  be  used. 

The  alloy  is  dissolved  in  aqua  regia  (as  described  below  in 
the  analysis  of  bearing  metal)  and  the  tin  and  antimony  sepa- 
rated from  the  remaining  metals  by  means  of  colorless  sodium 
sulphide.  From  the  solution  of  the  sulpho-salts  the  tin  and  anti- 
mony are  precipitated  by  making  barely  acid  with  dilute  sulphuric 
acid;  the  precipitate  is  washed  and  dissolved  in  a  little  warm 
sodium  sulphide.  After  cooling,  sodium  peroxide  is  added  to 
the  concentrated  solution  in  small  amounts  until  the  liquid 
becomes  colorless,  and  when  treated  with  more  sodium  peroxide 
a  distinct  evolution  of  oxygen  takes  place.  By  this  treatment 
sodium  antimonate  is  formed;  this  separates  out  to  some  extent, 
while  the  tin  remains  in  solution.  In  order  to  completely  pre- 
cipitate the  antimony  from  the  solution,  one-third  as  much  alcohol 
(sp.  gr.  0.833)  is  added,  after  which  the  precipitate  is  filtered  off 
and  treated  as  above  described. 


Analysis  of  Bearing  Metal. 

This  alloy  contains  tin,  antimony,  lead  and  a  little  copper 
and  usually  small  amounts  of  iron,  bismuth  and  zinc. 

One  gram  of  borings  are  placed  in  a  400-c.c.  beaker  and 
dissolved  in  15  c.c.  concentrated  hydrochloric  acid,  to  which 
3  c.c.  of  concentrated  nitric  acid  are  immediately  added.  The 
alloy  will  usually  dissolve  in  the  cold  after  standing  a  while; 
when  rich  in  lead,  however,  it  will  be  necessary  to  heat  for  some 
time  on  the  water-bath.  As  soon  as  all  has  dissolved,  the  solu- 
tion (it  should  be  yellow,  or  greenish  yellow  if  much  copper  is 

*  Chem.  Ztg.,  18,  p.  1900. 


ANALYSIS    OF  BEARING   METAL.  253 

present)   is  diluted  with   15  times  as  much  alcohol,   added   in 
small  portions  with  constant  stirring.* 

After  standing  for  twelve  hours,  and  stirring  frequently,  the 
precipitated  lead  chloride  is  filtered  into  a  weighed  Gooch 
crucible,  washed  with  absolute  alcohol,  dried  at  150°  and  weighed. 
In  the  nitrate  will  be  found  a  few  milligrams  of  lead  in  the 
presence  of  antimony,  tin,  copper,  bismuth,  iron  and  zinc.f 
The  alcoholic  nitrate  is  poured  into  a  large,  deep  porcelain  dish  t 
and  the  alcohol  is  evaporated  off  at  as  low  a  temperature  as 
possible.  It  is  necessary  to  avoid  evaporating  the  solution  to 
dryness  as  in  that  case  some  SnCl4  will  be  volatilized.  When 
the  alcohol  is  all  gone,  0.1  gm.  of  potassium  chlorate  is  added, 
the  solution  is  evaporated  to  a  small  volume  and  then  there 
is  added  one  gm.  of  tartaric  acid  and  enough  caustic  potash 
to  make  the  solution  barely  alkaline.  It  is  now  treated,  as 
recommended  by  Finkener,  with  freshly  prepared  hydrogen 
sulphide  water  until  no  further  precipitation  takes  place.  In 
this  way  all  the  Cu,  Bi,  Fe,  Zn  and  the  last  of  the  Pb  are  pre- 
cipitated as  sulphides  (precipitate  a)  while  all  the  Sn  and  Sb 
remain  in  solution  (solution  &).§ 


*  This  stirring  is  indispensable  because  lead  chloride  separates  out 
very  slowly  from  a  supersaturated  alcoholic  solution  containing  other 
chlorides.  The  complete  precipitation  is  best  recognized  by  the  fact  that 
no  mark  is  left  upon  the  sides  of  the  beaker  when  the  stirring  rod  is  rubbed 
against  it. 

t  Alloys  low  in  lead  are  not  treated  with  alcohol  in  this  way.  In  such 
cases  it  is  best  to  decompose  the  alloy  with  chlorine  gas,  as  described  in  the 
analysis  of  tetrahedrite  on  page  359. 

I  In  evaporating  off  the  alcohol  there  is  a  tendency  for  the  solution  to 
creep  over  the  edges  of  the  dish  so  that  it  is  advisable  to  employ  a  deep 
dish  and  to  evaporate  the  liquid  in  small  portions. 

§  The  separation  is  complete  only  when  all  the  tin  is  in  the  quadrivalent 
condition.  In  driving  off  the  alcohol  there  is  always  some  stannous 
chloride  formed  which  must  be  subsequently  oxidized  by  means  of  KC1OS 
and  HC1. 


254  GRAVIMETRIC  ANALYSIS. 

Treatment  of  Precipitate  a. 

The  precipitate  is  filtered  off,  washed  with  hydrogen  sulphide 
water,  dissolved  in  nitric  acid  (sp.  gr.  1.2)  and  evaporated 
with  hydrochloric  acid  to  remove  the  nitric  acid,  and  the  solution 
of  chlorides  diluted  so  that  its  acidity  corresponds  to  1  part 
HC1  (sp.  gr.  1.12)  to  25  parts  water.  The  Cu,  Pb,  and  Bi  are 
precipitated  as  sulphides  by  hydrogen  sulphide,  filtered  and 
washed  with  water  containing  H2S.  The  filtrate  contains  the 
iron  and  zinc  (Filtrate  c). 

The  precipitate  is  dissolved  in  nitric  acid,  evaporated  with 
the  addition  of  4  or  5  drops  of  concentrated  sulphuric  acid, 
and  the  last  of  the  lead  determined  as  sulphate  according  to 
p.  174.  FromTthis  filtrate  the  bismuth  is  precipitated  with  an 
excess  of  ammonia  and  determined  as  Bi2O3  according  to  p.  179. 

In  the  ammoniacal  filtrate  from  the  bismuth  precipitation, 
the  copper  is  determined  electrolytically,  after  acidifying  with 
sulphuric  acid,  according  to  p.  187,  or  as  cuprous  sulphide, 
according  to  p.  183. 

To  determine  the  iron  and  zinc,  the  Filtrate  c  is  oxidized  by 
boiling  with  a  few  drops  of  concentrated  HNOa  and  the  iron 
precipitated  by  an  excess  of  ammonia  and  weighed  as  Fe2Os, 
p.  87.  The  zinc  is  determined  in  this  last  filtrate  by  acidifying 
with  acetic  acid,  precipitating  as  sulphide  and  weighing  as  such, 
according  to  p.  143. 

Treatment  of  Solution  b. 

To  determine  the  antimony  and  tin,  the  alkaline  solution 
is  diluted  to  exactly  250  c.c.  in  a  measuring  flask,  and  after 
thoroughly  mixing,  100  c.c.  is  withdrawn  in  a  pipette,  trans- 
ferred to  a  400-c.c.  beaker,  acidified  with  acetic  acid,  and  boiled 
to  expel  the  hydrogen  sulphide.  Then  3  gms.  of  tartaric  acid 
and  6  gms.  of  purest  potassium  hydroxide  are  added,  whereby 
any  precipitated  sulphide  is  redissolved.  At  this  point  some 
30  per  cent,  hydrogen  peroxide  is  allowed  to  run  slowly  into  the 
solution,  until  the  yellow  color  disappears,  then  2  or  3  c.c.  in 
excess  are  added  and  the  solution  boiled  a  few  minutes.  Then, 


SEPARATION  OF  ARSENIC  FROM    TIN.  255 

for  each  0.1  gm.  of  metal  present  (Sb-j-Sn),  5  gins,  of  pure  oxalic 
acid  are  added,  the  solution  boiled  ten  minutes  and  the  antimony 
separated  from  the  tin  as  described  on  p.  248.  From  the  nitrate 
the  tin  is  determined  electrolytically.  For  this  purpose  the 
oxalic  acid  solution  is  evaporated  to  a  volume  of  about 
200  c.c.  and  electrolyzed  with  a  gauze  electrode.  At  the  end 
of  six  hours  the  deposition  is  complete.  The  electrodes  are 
washed  as  described  on  p.  136,  dried  and  weighed. 

Remark. — Rossing's  method,*  which  was  recommended  in 
the  earlier  editions  of  this  book,  is  not  altogether  satisfactory. 
Usually  the  lead  results  are  too  high  and  the  tin  too  low,  on 
account  of  the  lead  sulphide  precipitate  being  contaminated 
with  tin. 

Separation  of  Arsenic  from  Tin. 

(a)  Method  of  Fred.  Neher.^ 

The  moist  sulphides  are  dissolved  in  freshly-prepared  ammo- 
nium sulphide,  evaporated  in  an  Erlenmeyer  flask  nearly  to  dry- 
ness  and  oxidized  with  hydrochloric  acid  and  potassium  chlorate. 
From  this  solution  the  arsenic  is  precipitated  as  sulphide  under  the 
conditions  described  on  p.  243.  In  the  nitrate  from  the  arsenic 
pentasulphide  all  of  the  tin  is  found  and  can  be  precipitated  as  sul- 
phide after  diluting  largely  with  water  and  passing  hi  hydrogen  sul- 
phide. It  is  finally  changed  to  the  oxide  as  described  on  p.  233,  /?. 

(6)  Method  of  W.  Hampe.  J 

The  precipitated  sulphides  are  dissolved  as  soon  as  possible  in 
freshly-prepared  ammonium  sulphide,  the  solution  is  evaporated 
almost  to  dryness  and  oxidized  with  hydrochloric  acid  and  potassium 
chlorate  in  a  flask  connected  with  a  return-flow  condenser. §  Tar- 
taric  acid  and  ammonia  are  then  added  and  the  arsenic  precipi- 
tated with  magnesia  mixture  as  magnesium  ammonium  arsenate,  as 

*  Z.  anal.  Chem.,  41,  1  (1902). 

t  Ibid.  (1893),  32,  p.  45. 

J  Chem.  Ztg.  (1894),  18,  p.  1900. 

§  So  that  no  arsenic  trichloride  will  be  lost  by  volatilization. 


256  GRAVIMETRIC  ANALYSIS. 

described  on  p.  206.  After  standing  twelve  hours,  the  precipitate 
is  filtered  off,  washed  with  2J  per  cent,  ammonia,  and,  in  order 
to  remove  a  little  magnesia,  the  precipitate  is  dissolved  in  hydro- 
chloric acid  and  reprecipitated  by  the  addition  of  ammonia.  After 
standing  another  twelve  hours,  the  precipitate  is  filtered  off  and 
again  washed  with  2£  per  cent,  ammonia. 

This  precipitate  can  be  converted  into  magnesium  pyroar- 
senate  and  weighed  in  this  form  as  described  on  p.  207.  This 
transformation  is  somewhat  tiresome,  however,  so  that  Hampe 
prefers  to  dissolve  the  precipitate  in  hydrochloric  acid  once  more, 
to  precipitate  the  arsenic  by  means  of  hydrogen  sulphide,  and 
then  to  determine  the  magnesium  in  the  evaporated  filtrate  as 
magnesium  pyrophosphate  according  to  p.  66  or  p.  67.  From 
the  weight  of  the  latter  the  amount  of  arsenic  can  be  computed 
as  follows: 


2As 

x  = 


or 

x  =0.6734  -p  gm.  arsenic. 

Separation  of  Antimony  from  Arsenic  and  Tin. 

(a)  Method  of  Rose. 

If  the  metals  are  present  in  solution,  they  are  precipitated  as 
sulphides  with  hydrogen  sulphide,  heated  with  fuming  nitric  acid 
in  a  large  covered  beaker  until  the  sulphur  is  completely  oxidized, 
washed  into  a  porcelain  dish,  and  the  excess  of  acid  removed  by 
evaporation  on  the  water  -bath.  The  almost-dry  residue  is  treated 
with  concentrated  sodium  hydroxide  solution  and  the  contents 
of  the  dish  are  transferred  to  a  silver  crucible,  after  which  a  little 
solid  sodium  hydroxide  is  added  and  the  contents  of  the  crucible 
dried  in  an  air-bath.  It  is  then  fused  *  and  kept  liquid  for  about 
twenty  minutes  by  heating  over  a  Teclu  burner.  After  cooling, 
the  melt  is  disintegrated  with  water,  one-third  as  much  alcohol 
(sp.  gr.  0.833)  is  added  in  order  to  completely  precipitate  the 
sodium  metantimonate,  and  after  standing  twelve  hours  the 

*  The  silver  crucible  is  placed  in  a  larger  porcelain  one  so  as  to  avoid 
contact  with  the  flame. 


GOLD  IS  PRESENT  IN  SOLUTION.  257 

precipitate  is  filtered  and  subjected  to  the  treatment  described  on 
p.  251.  The  nitrate  containing  all  the  arsenic  and  tin  is  acidified 
with  hydrochloric  acid,  whereby  stannic  arsenate  is  precipitated. 
Without  filtering,  hydrogen  sulphide  is  conducted  into  the  liquid, 
the  precipitated  sulphides  of  tin  and  arsenic  are  filtered  off,  oxi- 
dized with  hydrochloric  acid  and  potassium  chlorate,  and  the 
arsenic  separated  from  the  tin  as  described  on  p.  255. 

(b)  Method  of  Hampe. 

The  moist  sulphides  are  oxidized  as  described  on  p.  255,  &.  and 
the  arsenic  determined  in  the  same  way. 

In  the  combined  filtrates  from  the  magnesium  ammonium 
arsenate  the  antimony  and  tin  are  precipitated  by  hydrogen 
sulphide,  after  making  the  solution  acid.  These  are  separated 
either  according  to  the  method  of  Clarke  (p.  248)  or  that  of  Rose 
(p.  250). 

SUPPLEMENT  TO  THE  HYDROGEN  SULPHIDE  GROUP. 

GOLD,    PLATINUM,  SELENIUM,  TELLURIUM,    VANADIUM, 
MOLYBDENUM,  TUNGSTEN. 

GOLD,  Au.    At.  Wt.  197.2. 

Gold  is  always  determined  as  the  metal  itself.  We  have  three 
cases  to  distinguish: 

1.  The  gold  is  present  in  solution. 

2.  The  gold  is  alloyed  with  copper  and  silver. 

3.  The  gold  is  present  in  an  ore. 

i.  Gold  is  Present  in  Solution. 

In  almost  all  cases  gold  is  deposited  as  metallic  gold  from 
its  solutions  and  weighed  after  filtering  and  washing. 

For  the  deposition  of  gold  the  following  reducing  agents  are 
to  be  considered:  ferrous  sulphate,  oxalic  acid,  formaldehyde, 
and  hydrogen  peroxide.  If  the  gold  is  to  be  precipitated  by  means 
of  either  ferrous  sulphate  or  oxalic  add,  there  must  be  no  free 
nitric  acid  present  in  the  solution.  If  some  is  present,  it  must 
be  removed  by  repeated  evaporation  with  concentrated  hydro- 
chloric acid  and  the  solution  then  diluted  with  water.  To  this 


258  GRAVIMETRIC  ANA 'LYSIS. 

dilute  solution  n  large  excess  of  clear  ferrous  sulphate  solution 
is  added,  the  beaker  is  covered  and  its  contents  are  heated  for 
several  hours  on  the  water-bath.  The  precipitate  is  then  filtered 
off,  washed  first  with  water  containing  hydrochloric  acid  until  the 
iron  is  completely  removed,  and  then  with  pure  water.  The  pre- 
cipitate is  dried,  transferred  as  completely  as  possible  to  a  porce- 
lain crucible,  the  ash  of  the  filter  added,  and  the  gold  is 
ignited  and  weighed.  In  this  way  gold  can  be  separated  from 
almost  all  metals,  even  platinum,  but  not  from  silver.  If  silver 
is  present,  which  is  of  course  never  the  case  in  a  dilute  hydrochloric 
acid  solution,  it  is  first  removed  by  the  addition  of  hydrochloric 
acid,  the  precipitated  silver  chloride  filtered  off,  and  the  filtrate 
treated  as  above  described. 

For  the  precipitation  of  gold  by  means  of  oxalic  acid,  the 
slightly  acid  solution  is  diluted  with  water,  oxalic  acid  or  ammo- 
nium oxalate  is  added  with  a  little  sulphuric  acid,  and  the  covered 
beaker  is  allowed  to  stand  forty-eight  hours  in  a  warm  place. 

The  yellow  scales  of  the  deposited  gold  are  filtered  off  and 
washed,  as  above  described,  with  hydrochloric  acid  and  then  with 
water.  It  is  then  ignited  and  weighed. 

Deposition  of  Gold  by  Means  of  Hydrogen  Peroxide  (L.  Vanino  and 

L.  Seemari)* 

If  a  gold  solution  is  treated  with  potassium  or  sodium  hy- 
droxide solution  and  then  with  formaldehyde,  or,  better  still, 
hydrogen  peroxide,  the  gold  is  soon  precipitated  quantitatively, 
even  in  the  cold.  By  boiling,  the  finely-divided  gold  collects 
together  and  assumes  a  reddish-brown  color.  The  reaction  takes 
place  according  to  the  following  equation: 

2  AuCl3 + 3H2O2  +  6KOH  -  6KC1  +  6H2O  +  3O2  +  Au2. 

If  the  gold  is  deposited  by  this  method  from  very  dilute  solu- 
tions it  is  obtained  in  such  a  finely-divided  condition  that  it 
passes  through  the  filter.  If,  however,  the  solution  is  boiled 
until  the  excess  of  hydrogen  peroxide  is  completely  destroyed, 
and  it  is  then  acidified  with  hydrochloric  acid,  the  gold  can 
be  readily  filtered.  Gold  can  be  separated  from  platinum  by 
this  method. 

*  Berichte  (1899),  32,  p.  1968. 


COLD  ALLOYED   WITH  COPPER  AND  SILVER.  259 

2.  The  Gold  is  Alloyed  with  Copper  and  Silver. 

When  gold  is  present  in  alloys  it  is  most  rapidly  and  most 
accurately  determined  in  the  dry  way.  The  principle  of  the 
method  is  very  simple. 

If  a  gold-silver  alloy  is  melted  in  the  air  with  lead  upon  a 
" cupel"  (a  very  porous  vessel  made  of  bone-ash)*  the  lead  and 
copper  are  oxidized,  the  oxides  fuse  and  are  absorbed  by  the 
cupel,  while  all  the  gold  and  silver  are  left  behind  in  the  form  of 
a  metallic  button,  whose  weight  is  obtained.  The  silver  is  after- 
wards separated  from  the  gold  by  the  action  of  nitric  acid  which 
dissolves  the  silver  but  leaves  the  gold  behind.  If  the  weight 
of  the  gold  that  is  left  undissolved  is  deducted  from  the  weight 
of  the  gold-silver  button  the  weight  of  the  silver  is  obtained. 

In  order  to  obtain  accurate  results  a  number  of  precautions 
must  be  taken.  By  the  cupellation  of  the  alloy  some  noble 
metal  is  always  lost  and  the  amount  lost  increases  in  proportion 
to  the  amount  of  lead  used  and  the  higher  the  temperature. 
Furthermore,  small  amounts  of  the  noble  metal  are  absorbed  by 
the  cupel  and  this  amount  is  greater  the  smaller  the  amount  of 
lead  used.  This  second  loss  amounts  to  much  less  than  the  former 
one  occasioned  by  the  use  of  too  much  lead.  Consequently,  in 
every  gold  cupellation  an  unnecessary  excess  of  lead  must  be  avoided. 

Experience  has  shown  that  the  richer  a  gold-silver  alloy  is 
in  base  metal  the  more  lead  is  necessary  for  the  cupellation. 
Furthermore,  in  the  separation  of  gold  from  silver  by  means  of 
nitric  acid  it  is  necessary  to  remember  that  the  separation  is  only 
quantitative  when  the  alloy  consists  of  three  or  more  parts  of 
silver  to  one  part  of  gold.  If  less  than  three  parts  of  silver  are 
originally  present  for  one  part  of  gold,  it  is  necessary  to  add  pure 
silver  until  this  proportion  is  reached.  This  operation  is  known 
as  quartation  or  inquartation.  The  separation  of  the  silver  from, 
the  gold  by  means  of  nitric  acid  is  spoken  of  as  parting.  If  a 
gold-silver  alloy,  in  the  form  of  foil,  which  consists  of  three  parts, 
of  silver  to  one  of  gold,  is  treated  with  nitric  acid,  the  latter  metal 
remains  behind  as  a  brownish  scale;  if  more  silver  is  present, 
it  is  left  as  a  fine  powder,  unless  the  acid  is  made  extremely  dilute. 

*  According  to  R.  Gruncl,  Oesterr.  Z.  Berg-Huttenw.,  57,  681,  magnesite 
is  better  than  bone-ash. 


260 


GRAVIMETRIC  ANALYSIS. 


From  what  has  been  said,  it  is  clear  that  accurate  results  can 
be  obtained  only  when  the  correct  amount  of  lead  is  present  in 
the  alloy  that  is  cupelled,  and  when  the  gold  and  silver  are  present 
in  the  proper  proportion;  i.e.,  it  is  necessary  to  know  the  approxi- 


FIG.  49. 

mate  composition  of  the  alloy  before  an  accurate  determination 
can  be  made.     This  is  determined  by 

The  Preliminary  Assay. 

For  this  purpose  the  muffle  shown  in  Fig.  49  is  heated  to  a 
cherry-red  heat,  a  cupel  weighing  from  6  to  7  gms.*  is  placed  in 
the  back  part  of  it,  the  muffle  door  is  closed,  and  the  cupel  heated 
until  it  has  acquired  the  same  color  as  the  muffle.  After  this 
5  gms.  of  lead  are  placed  upon  the  cupel,  the  muffle  is  closed  until 

*  A  good  cupel  will  absorb  its  own  weight  of  litharge.  During  the  cupel- 
lation  about  one-tenth  of  the  litharge  formed 
is  lost  by  volatilization,  so  that  the  weight  of 
litharge  absorbed  by  the  cupel  is  practically 
that  of  the  original  lead  button.  Fig.  50  repre- 
sents a  cupel,  together  with  its  cross-section.  FIG.  50. 


THE  PRELIMINARY  ASSAY.  261 

the  lead  is  melted  and  then  0.25  gm.  of  the  accurately-weighed 
alloy  is  enveloped  in  a  small  piece  of  lead-foil,  placed  in  the 
molten  lead  (with  the  help  of  a  pair  of  tongs),  and  the  muffle  closed 
until  the  alloy  has  melted  and  shows  a  bright  upper  surface.  With 
the  help  of  an  iron  hook  the  cupel  is  now  carefully  advanced  to 
about  the  middle  of  the  muffle  and  the  door  should  be  left  open 
so  that  there  is  a  ready  access  of  air  into  the  muffle. 

After  about  twenty  minutes  the  lead  will  be  all  absorbed, 
which  is  shown  by  the  "blick."  *  The  hot  cupel  is  then  removed 
from  the  muffle  and  after  cooling,  the  color  of  the  button  is  ob- 
served. 

(a)  //  the  button  is  greenish  yellow  or  darker,  it  contains  less 
than  three  parts  of  silver  to  one  part  of  gold,  in  which  case  from 
four  to  six  parts  of  "fine  silver"  are  added  (the  proper  amount 
can  be  usually  told  by  the  practised  eye)  and  the  button  is  cupelled 
in  a  new  cupel  with  1  gm.  of  lead.     The  button  now  obtained  is 
treated  with  nitric  acid  and  the  residual  gold  weighed. 

(b)  If  .the  button  is  pure  white,  then  three  or  more  parts  of 
silver  are  present  to  one  part  of  gold.      In  this  case  it  is  imme- 
diately "  parted "  and  the  residual  gold  weighed. 

1  After  the  approximate  amount  of  gold  present  has  been  ascer- 
tained ,f  the  analysis  proper  is  made,  using  the  amount  of  lead  as 
indicated  in  the  following  table: 


LEAD  TABLE. 
1 

1000  thousandths 0.25  gm. 


Amount  of  Gold  Amount  of  Lead  Necessary  for  the 

Present  in  the  Alloy.  Cupellation  of  0.25  gm.  of  Alloy. 


900 

u 

2  .  50  | 

Tms. 

800 

It 

4  00 

u 

700 

it 

5.50 

n 

600 

u 

6.00 

M 

500 

ii 

6  50 

U 

400  or  less 

u 

8  50 

tl 

*  The  blick  is  the  brightening  of  the  metal  which  appears  when  the  outer 
layer  of  lead  oxide  that  is  constantly  becoming  thinner  finally  bursts  and  the 
bright  noble  metal  shines  through.  Just  before  the  blick  there  is  a  distinct 
iridescence,  so  that  the  point  can  never  be  mistaken. 

f  In  assay  laboratories  the  approximate  gold  contents  of  the  alloy  is  deter- 
mined by  its  streak.  A  fine-grained  piece  of  silicate  is  blackened  with  char- 
coal The  alloy  to  be  tested  is  rubbed  upon  it  and  the  color  produced  com- 


262  GRAVIMETRIC  ANALYSIS.  '] 

\ 
The  Final  Assay. 

For  the  definite  determination  of  the  gold  and  silver,  two 
portions  weighing  exactly  0.25  gm.  are  taken;  the  one  to  serve 
for  the  silver  determination  and  the  other  for  the  gold.  The 
former  is  cupelled  with  the  correct  amount  of  lead  and  the  weight 
of  the  gold-silver  button  is  determined. 

If  the  original  alloy  was  very  white,  it  contains  more  than 
500  thousandths  fine  of  silver. 

If  the  alloy  was  greenish  yellow,  it  contains  550-750  thou- 
sandths of  noble  metal,  and  silver  is  present  to  a  considerable 
extent. 

If,  however,  the  alloy  was  a  beautiful  yellow  or  reddish  yellow, 
it  contains  more  than  700  thousandths  of  noble  metal  and  the 
gold  predominates. 

If,  therefore,  the  alloy  was  white,  once  again  as  much  pure  silver 
is  weighed  out  as  the  amount  of  gold  found  to  be  present  by  the 
preliminary  assay  (inquartated  with  one  part  of  silver),  and  this 
mixture  is  cupelled  with  the  same  amount  of  lead  as  the  first  portion. 

If  the  original  alloy  was  greenish  yellow,  it  is  inquartated  * 
with  two  parts  of  silver;  if  it  was  distinctly  yellow  or  reddish 
yellow  it  is  inquartated  with  2J  parts  of  silver. 

Treatment  of  the  Quartered  Gold-Silver  Button. 

The  gold-silver  button  is  removed  from  the  cupel  with  xhe 
"button  tongs,"  cleaned  with  a  stiff  brush  ("button  brush"), 
and  hammered  upon  an  anvil  to  a  round  disk  about  1  mm.  thick 
(Fig.  51,  a).  This  is  heated  upon  a  fresh  cupel  and  quickly  cooled 
by  placing  it  upon  a  piece  of  brass  foil  and  rolling  it  between  two 
steel  rollers  to  a  long  strip  (Fig.  51,  6);  it  is  again  heated  and 
rolled  f  up  as  shown  in  Fig.  51,  c.  This  little  roll  is  placed  in  a 

pared  with  that  obtained  from  alloys  containing  known  amounts  of  gold. 
Afterwards  these  streaks  are  tested  with  dilute  aqua  regia;  alloys  containing 
the  same  amounts  of  gold  are  attacked  equally  readily. 

*  Cf.  p.  259. 

t  By  hammering  the  gold-silver  alloy,  the  metal  becomes  so  brittle  that 
it  cannot  be  converted  to  a  smooth-margined  roll,  and  on  the  subsequent 
treatment  with  nitric  acid,  little  pieces  would  probably  drop  off.  By  again 
heating  the  metal  and  then  quickly  cooling,  it  regains  its  original  softness. 


DETERMINATION  OF  GOLD  IN  ORES. 


263 


little  flask  (Fig.  52,  7),  covered  with  30-40  c.c.  of  nitric  acid  (sp. 
gr.  1.188)  free  from  chloride,  heated  to  boiling  and  kept  so  for 
ten  minutes.  The  acid  is  then  poured  off  and  replaced  by  the 


FIG.  51. 

same  amount  of  stronger  acid  (sp.  gr.  1.295)  and  the  above  treat- 
ment repeated.  After  this  acid  is  poured  off,  the  button  is 
washed  by  decanting  three  times  with  distilled  water.  The  flask 
is  filled  with  water,  covered  with  an  annealing  cup  (or  lack- 
ing this  an  ordinary  porcelain  crucible  may  be  used),  and  is 
then  quickly  inverted  (Fig.  51,  //),  when  the  gold  will  pass 
into  the  cup.  The  flask  is  removed  by  first  raising  its  mouth 
to  the  level  of  the  water  in  the  crucible  and  then  sliding  it 
off  at  right  angles  and  skilfully  turning  the  flask  right  side  up. 
The  water  is  poured  off  from  the  gold  and  the  crucible  is  placed 
in  the  back  part  of  the  muffle  for  a  short  time,  whereby  the  gold 
is  dried  and  is  changed  from  its  former  brown  and  soft  condition 
into  a  harder,  beautiful  yellow  substance.  After  cooling,  it  is 
weighed.  By  subtracting  the  weight  of  the  gold  from  the  weight 
of  the  gold  and  silver  together,  the  amount  of  silver  is  obtained. 

Determination  of  Gold  in  Ores. 

Principle. — The  very  finely  ground  and  sifted  ore  is  mixed  in 
a  No.  9  French  crucible  with  lead  oxide,  charcoal,  and  some  suit- 
able slag-forming  material.  The  charcoal  reduces  a  part  of  the 
lead  oxide  to  metal  which  alloys  with  the  noble  metal  and 
sinks  to  the  bottom  in  the  form  of  a  button,  while  the  foreign  sub- 


,264  GRAVIMETRIC  ANALYSIS. 

stances  should  pass  into  the  slag.  After  cooling,  the  crucible  is 
broken,  the  slag  is  hammered  off,  the  lead  button  cupelled  and 
the  silver-gold  button  parted  in  the  same  way  as  before.  The 
noble  metal  should  be  extracted  with  as  little  lead  as  possible, 
for  with  an  unnecessarily  large  amount  of  lead  some  gold  is  lost 
during  the  cupellation. 

The  amount  of  lead  reduced  from  the  litharge  depends  largely 
upon  the  nature  of  the  ore.  Sulphide  ores  act  strongly  reducing, 
as  is  shown  by  the  following  equations: 


PbS+2PbO  =  S 

FeS2  +  5PbO  =  2SO2  +  FeO  +  5Pb. 

In  such  cases  less  charcoal  (or  in  some  cases  none  at  all)  should 
be  added  than  would  be  otherwise  necessary  to  produce  the  right 
amount  of  lead,  or  in  case  considerable  sulphide  is  present,  it  is 
sometimes  necessary  to  neutralize  its  action  by  the  addition  of 
oxidizing  agents. 

Reducing  ores  are  recognized  by  their  color:  they  are  gray, 
bluish-black,  or  yellow  (pyrite,  etc.).  Reddish-brown  ores  (Fe2O3) 
usually  act  oxidizing: 


in  which  case  more  charcoal  must  be  added  to  the  charge. 

The  best  results  are  obtained  when  the  lead  button  weighs 
about  18  gms.  when  obtained  from  30  gms.  of  ore.*  In  order 
that  such  a  button  may  be  obtained,  it  is  usually  necessary  to 
make  a  preliminary  assay  of  the  ore.  But  above  all,  it  is  neces- 
sary that  the  purity  of  the  reagents  used  should  be  tested. 

Testing  the  Reagents. 
The  ordinary  reagents  necessary  for  a  gold  assay  are: 

i.  Litharge  (PbO). 

Litharge,  the  most  important  reagent,  is  a  basic  flux,  for  it 
forms  with  the  silicic  acid  of  the  ore  a  readily  fusible  silicate; 

*  This  amount  is  usually  sufficient;  with  very  rich  gold  ores  10-15  gms. 
is  enough,  while  with  very  poor  ores  as  much  as  120  gms.  may  be  used  to 
advantage.  Cf.  Ricketts  and  Miller,  Notes  on  Assaying,  New  York,  1897. 


REAGENTS  FOR   GOLD-SILVER  ASSAY.  265 

at  the  same  time,  however,  it  is  a  desulphurizing  agent,  as  is  shown 
by  the  above  reaction. 

The  litharge  used  must  be  dry  and  free  from  minium,  for 
the  latter  oxidizes  silver,  carrying  it  into  the  slag,  so  that  low 
results  would  be  obtained  in  the  silver  determination.  The 
litharge  should  be  free  from  silver  (which  is  almost  never  the  case), 
or  its  silver  contents  must  be  known;  this  is  determined  once  for 
all  by  the  following  experiment: 

Litharge 120  gms. 

Sodium  bicarbonate  (NaHCOJ 60    " 

Argols  (crude  KHC4H4O6) 2    " 

are  mixed  thoroughly  upon  a  sheet  of  glazed  paper  and  the  mix- 
ture placed  in  a  No.  9  French  crucible  and  covered  with  a  layer 
of  finely-powdered,  dry  common  salt.  The  covered  crucible  is 
placed  in  a  glowing  coke-oven. 

As  soon  as  the  contents  of  the  crucible  have  reached  the  state 
of  quiet  fusion,  the  crucible  is  removed  from  the  fire,  its  walls  are 
gently  tapped  by  the  tongs,  and  it  is  lightly  tapped  upon  its 
bottom  in  order  to  knock  down  any  small  particles  of  lead  adher- 
ing to  the  sides  and  to  make  all  of  the  free  metal  collect  together 
on  the  bottom  in  the  form  of  a  button. 

After  cooling  the  crucible  is  broken,  the  slag  removed  from 
the  lead  button  by  hammering  it  upon  an  anvil,  and  it  is  cupelled 
upon  a  cupel  weighing  only  a  few  grams  more  than  the  button 
itself.  The  resulting  silver  button  is  weighed.  The  amount  of 
silver  obtained  must  be  deducted  whenever  the  corresponding 
amount  of  litharge  is  used  in  an  assay. 

2.  Sodium  Bicarbonate  (NaHCO,). 

3.  Anhydrous  Borax  (Na^O,). 
2  and  3  require  no  testing. 

4.  Charcoal. 

The  reducing  power  is  determined  as  follows: 

Litharge 60  gms. 

Sodium  bicarbonate 15    " 

Charcoal 1  gm. 


266  GRAVIMETRIC  ANALYSIS. 

are  mixed,  as  in  the  testing  of  litharge,  in  a  French  crucible  No.  9 
with  a  cover  of  ordinary  common  salt  and  fused.     After  cooling, 
the  weight  of  the  lead  button  obtained  is  determined  and  this  ex- 
presses in  terms  of  lead  the  reducing  power  of  the  charcoal. 
1  gm.  of  charcoal  should  reduce  about  30  gms.  lead. 

5.  Nitre  (KNO3) 

serves  as  an  oxidizing  agent.  Its  oxidizing  power  expressed  in 
terms  of  lead  is  determined: 

Nitre 3  gms. 

Litharge  .  . 60    " 

Charcoal 1  gm. 

Sodium  bicarbonate 15  gms. 

are  mixed  and  fused  as  before  and  the  weight  of  the  lead  button  de- 
termined. If  under  (4)  it  was  found  that  1  gm,  charcoal  would 
reduce  P  gm.  lead,  and  if  p  gm.  of  lead  were  obtained  in  this  ex- 
periment, then  the  difference  P—p  shows  the  amount  of  lead  that 
was  oxidized  by  3  gms.  nitre,  or  the  oxidizing  power  of  the  nitre. 
1  gm.  nitre  oxidizes  about  4  gms.  lead. 

6.  Common  Salt. 

Ordinary  table  salt  is  heated  in  a  large  Hessian  crucible  until 
it  melts,  and  the  contents  of  the  crucible  are  poured  into  a  shal- 
low iron  mould  with  a  raised  edge.  The  solidified  crust  is  finely 
powdered  and  preserved  in  a  stoppered  flask. 

After  the  reagents  have  all  been  tested  the  next  step  is  the 

Preliminary  Assay. 

Five  grams  of  the  finely-powdered  and  sifted  ore  are  weighed 
out  and  mixed  with: 

Litharge 80  gms. 

Sodium  bicarbonate 20    " 

Borax 5    " 

placed  in  a  crucible  and  covered  with  a  layer  of  common  salt.  After 
fusing,  cooling,  and  hammering  off  the  slag,  the  lead  button  ob- 
tained is  weighed. 

Since  in  an  ordinary  assay  we  start  with  30  gms.  of  ore,  the 


GOLD-SILVER  ASSAY.  267 

weight  of  the  lead  button  now  obtained  multiplied  by  6  will  give 
the  weight  of  the  button  from  the  real  assay.  We  will  distinguish 
four  cases : 

(1)  The  lead  button  weighs  3  gms. 

Consequently  the  button  obtained  from  30  gms.  of  ore  would 
weigh  18  gms.  In  this  case  the  ore  is  assayed  with  the  following 
proportions  of  flux: 

Ore 30  gms. 

Litharge 80    " 

Sodium  bicarbonate 20    " 

Borax 5     " 

(2)  The  lead  button  weighs  less  than  3  gms. 

Evidently  the  ore  acts  reducingly,  but  not  enough  so  to  yield  a 
button  weighing  18  gms.  when  30  gms.  of  ore  are  used;  it  is, 
therefore,  necessary  to  add  charcoal  to  the  flux. 

Example. — Let  us  assume  that  the  lead  button  obtained  by  the 
preliminary  assay  weighed  1  gm.,  then  the  button  obtained  from 
30  gms.  of  ore  would  weigh  6  gms.  In  order  to  obtain  a  button 
weighing  18  gms.  it  is  necessary  to  add  enough  charcoal  to  supply 
12  gms.  of  lead.  If  1  gm.  of  charcoal  was  found  to  reduce  30  gms. 
of  lead,  then  it  is  necessary  to  add  12-r-SO  gms.  =  0.4  gms.  of 
charcoal. 

(3)  The  lead  button  weighs  more  than  3  gms. 

In  this  case  the  ore  has  a  strong  reducing  power,  and  to  obtain 
the  lead  button  of  the  right  weight  it  is  necessary  to  add  some  nitre. 

Example. — Suppose  the  button  to  weigh  6  gms.;  this  would 
mean  a  36-gm.  button  when  30  gms.  of  ore  were  used;  i.e.  18  gms. 
too  much  lead  would  be  produced.  We  must  add,  therefore, 
enough  nitre  to  oxidize  this  18  gms.  of  lead.  If  the  oxidizing  power 
of  1  gm.  of  nitre  was  found  to  be  4  gms.  of  lead,  then  18 -=-4  =  4.5 
gms.  of  nitre  must  be  added  to  the  flux. 

Remark. — Ores  which  have  a  very  strong  reducing  power  would 
frequently  require  the  addition  of  enough  nitre  to  cause  the  con- 
tents of  the  crucible  to  boil  over.  In  such  a  case,  about  40-50  gms. 
are  placed  in  a  "  roasting-dish "  and  roasted  in  a  muffle,  and  from 
this  roasted  ore  the  portions  are  taken  for  the  preliminary  and 


268  GRAVIMETRIC  ANALYSIS. 

final  assays.     The  results,  however,  must  be  expressed  in  terms  of 
the  unroasted  ore. 

(4)  There  is  no  lead  button  formed. 

The  ore  is  either  neutral  or  possesses  an  oxidizing  action.  The 
assay  is  repeated,  using  1  gm.  of  charcoal,  and  from  the  results 
now  obtained  the  final  assay  is  based. 

Final  Assay. 

For  the  final  assay  from  30-120  gms.  of  ore  *  are  taken  (accord- 
ing to  the  amount  of  gold  present)  and  the  corresponding  amount 
of  sodium  bicarbonate  is  added.  The  amount  of  litharge  also  varies 
with  the  amount  of  ore,  and  in  some  cases  as  much  as  240  gms.  are 
necessary,  although  as  a  rule  80  gms.  are  sufficient.  Otherwise  the 
procedure  is  exactly  the  same  as  in  the  preliminary  assay.  The  lead 
button  is  cupelled  and  the  weighed  silver-gold  button  is  parted  as 
described  on  p.  263. 

PLATINUM,  Pt.  At.  Wt.  195.0. 

Platinum  is  best  determined  as  metallic  platinum. 

The  following  three  cases  will  be  considered : 

1.  The   platinum  is    present    in  a  hydrochloric   acid  solution 
either   alone  or  together  with  other  metals,  but  other  platinum 
metals  are  absent. 

2.  The  platinum  is  present  alloyed  with  gold  and  silver. 

3.  The  platinum  is  alloyed  with  small  amounts  of  the  plati- 
num metals  together  with  small  amounts  of  base  metals. 

I.  The  Platinum  is  Present  in  Hydrochloric  Acid  Solution  Either 
Alone  or  Together  with  Other  Metals. 

The  platinum  is  either  precipitated  from  the  solution  as  ammo- 
nium chloroplatinate,  (NH4)2PtCl6,  which  is  decomposed  by  ignition 
and  the  residual  platinum  weighed;  or  the  platinum  is  precipi- 
tated as  metal  by  the  addition  of  reducing  agents  to  the  solution ; 
or  finally  the  platinum  is  precipitated  as  sulphide  by  conducting 
hydrogen  sulphide  into  the  hot  solution  and  changed  to  platinum 
by  ignition.  The  two  former  methods  serve  to  separate  platinum 

*  Usually  "  assay  tons "  are  used  as  units  in  weighing  out  the  ore,  and 
the  weights  are  calibrated  in  terms  of  this  unit  instead  of  the  gram.  An 
"assay  ton"  contains  the  same  number  of  milligrams  that  there  are  ounces 
troy  to  a  ton,  so  that  by  weighing  the  button  obtained  in  milligrams,  it  is 
at  once  known  how  many  ounces  per  ton  the  ore  carries. 


PLATINUM.  269 

from  most  other  metals,  while  the  latter  serves  to  separate  plati- 
num only  from  the  metals  of  the  alkali,  alkaline  earth,  and  ammo- 
nium sulphide  groups,  and  not  from  members  of  the  hydrogen  sul- 
phide group. 

(a)  Precipitation  of  Platinum  as  Ammonium  Chloroplatinaie. 

The  solution,  concentrated  as  much  as  possible,  is  nearly  neu- 
tralized with  ammonia,  an  excess  of  ammonium  chloride  and  consid- 
erable alcohol  are  added,  and  the  mixture  allowed  to  stand  twelve 
hours  under  a  glass  bell-jar.  It  is  then  filtered  through  an  asbestos 
filter  tube  10-15  cm.  long,  washed  with  80  per  cent,  alcohol  until 
a  few  drops  of  the  filtrate  leave  no  residue  on  being  evaporated 
to  dryness  on  a  platinum  foil.  The  precipitate  is  dried  by  con- 
ducting a  stream  of  air  warmed  to  about  90°  C.  through  the  tube. 
After  cooling  the  tube  is  weighed,  a  plug  of  ignited  asbestos  *  is 
introduced,  and  the  tube  is  again  weighed;  in  this  way  the  weight 
of  the  asbestos  plug  is  found.  A  stream  of  dry  hydrogen  is  now 
conducted  through  the  tube,  and  the  latter  is  heated  at  as  low  a 
temperature  as  possible  until  no  more  hydrochloric  acid  is  evolved 
and  all  the  ammonium  chloride  has  been  driven  off,  after  which  the 
tube  is  cooled  in  a  desiccator  and  weighed. 

Instead  of  filtering  the  precipitate  upon  asbestos  an  un- 
weighed  paper-filter  may  be  used.  The  moist  precipitate  is 
placed  together  with  the  filter  in  a  large  porcelain  crucible 
so  that  the  apex  of  the  filter-paper  points  upward,  and  the 
covered  crucible  is  then  ignited.  This  ignition  must  be  performed 
with  great  care,  as  otherwise  there  can  be  a  considerable  loss  dur- 
ing the  process.  At  first  the  precipitate  is  dried  by  gently  wann- 
ing the  covered  crucible,  and  when  the  odor  of  alcohol  has  disap- 
peared, the  temperature  is  raised  very  slowly  until  the  crucible  is  at 
a  strong  red  heat.  During  the  whole  operation  there  must  be  no 
visible  escape  of  vapors  from  the  crucible.  The  decomposition  is 
complete  when  there  is  no  longer  a  penetrating  odor  arising  from 
the  covered  crucible.  When  this  point  is  reached,  the  cover  (whose 
under  side  will  be  covered  with  carbon)  is  removed  for  the  first 
time  and  leaned  against  the  crucible  and  the  contents  of  the  latter 

*  Ammonium  chloroplatinate  decrepitates  during  the  heating.  To  pre- 
vent loss  of  substance  it  is  heated  between  two  asbestos  plugs. 


270  GRAVIMETRIC  ANALYSIS. 

are  ignited  with  free  access  of  air  until  the  carbon  from  the  filter- 
paper  is  completely  burned.  Often  a  slight  deposit  of  platinum* 
will  be  found  in  the  upper  part  of  the  crucible  and  upon  the  cover, 
so  that  the  latter  must  always  be  weighed  with  the  crucible. 

Remark. — If  it  seems  likely  that  the  precipitate  of  ammonium 
chloroplatinate  is  contaminated  with  other  substances  (e.g.  so- 
dium chloride,  etc.)  the  precipitate  can  be  dissolved  in  water  after 
It  has  been  washed  with  alcohol  and  dried.  The  platinum  may 
then  be  determined,  as  described  on  p.  50,  by  precipitating  with 
mercury,  washing  with  dilute  hydrochloric  acid  and  then  with 
water,  and  finally  weighing. 

The  results  obtained  by  this  method  are  satisfactory  but  some- 
what lower  than  the  true  values;  the  following  process  is  more 
accurate: 

(6)  Precipitation  of  Platinum  by  Reducing  Agents. 

The  solution  is  freed  from  any  excess  of  acid  by  evaporation, 
placed  in  an  Erlenmeyer  flask  into  the  neck  of  which  is  ground 
to  fit  a  return-flow  condenser.  The  solution  is  neutralized  with  am- 
monia, an  excess  of  formic  acid  and  a  little  ammonium  acetate  are 
added,  and  the  contents  of  the  flask  after  being  diluted  to  a  volume 
of  200  c.c.  are  heated  to  about  80°  C.  on  the  water-bath  until  the 
evolution  of  carbon  dioxide  has  nearly  ceased.  The  flask  is  now 
connected  with  the  return-flow  condenser,  and  its  contents  boiled  for 
twenty-four  hours.  The  precipitated  metal  is  filtered  off,  washed 
with  dilute  hydrochloric  acid,  then  with  water,  dried,  ignited,  and 
weighed. 

2.  The  Platinum  is  Alloyed  with  Gold  and  Silver. 

An  alloy  is  seldom  found  which  contains  only  the  above  three 
noble  metals ;  usually  copper  is  also  present.  The  first  step,  then, 
is  to  separate  the  noble  metals  from  the  others  by  cupellation  with 

*  By  means  of  the  dry  distillation  of  the  filter,  carbon  monoxide  is  formed, 
and  by  the  decomposition  of  the  ammonium  chloroplatinate  chlorine  is  set 
free.  These  two  gases  act  upon  the  metallic  platinum  and  form  volatile  com- 
pounds (RClrCO,  PtClr2CO,  and  2PtCl2.3CO),  which,  however,  are  later 
decomposed  by  the  aqueous  vapor.  This  causes  the  deposit  of  platinum  in 
the  upper  part  of  the  crucible.  In  order  to  avoid  loss,  a  large  crucible  should 
be  used. 


PLATINUM.  271 

lead  as  described  on  p.  259,  after  which  the  hammered  and  rolled 
button  is  treated  with  pure  concentrated  sulphuric  acid.  [Nitric 
acid  cannot  be  used,  for  some  platinum  would  be  dissolved  with 
the  silver.]  After  boiling  for  ten  minutes,  the  silver  will  be  com- 
pletely dissolved,  provided  at  least  two  parts  of  silver  are  present 
for  each  part  of  platinum,  which  is  usually  the  case.  If  more 
platinum  is  probably  present  than  corresponds  to  the  above  ratio, 
pure  silver  should  be  added,  and  the  mixture  cupelled  once  more 
with  1  gm.  of  lead. 

After  the  alloy  has  been  boiled  for  ten  minutes  with  sulphuric 
acid  it  is  allowed  to  cool,  filtered,  and  the  treatment  with  sulphuric 
acid  repeated  once  again.  The  metal  remaining  behind  (in  the 
form  of  a  roll  or  as  a  powder)  is  washed  three  times  by  decantation 
with  water,  ignited,  and  weighed  as  described  under  gold.  This 
gives  the  weight  of  the  gold  and  the  platinum  together,  and  by  sub- 
tracting this  amount  from  the  original  weight  of  the  noble  metals 
obtained  after  cupellation,  the  weight  of  the  silver  is  obtained. 

Separation  of  Gold  from  Platinum. 

Principle. — If  an  alloy  of  gold  and  platinum  is  treated  with 
nitric  acid,  neither  metal  is  attacked.  If,  however,  the  alloy  con- 
tains three  parts  of  silver  to  one  of  gold  and  platinum  taken  to- 
gether, and  the  alloy  is  treated  at  first  with  acid  of  sp.gr.  1.16  and 
then  with  acid  of  sp.  gr.  1.28,  the  platinum  gradually  goes  into 
solution  with  the  silver. 

Procedure. — The  gold-platinum  alloy  is  cupelled  with  three  times 
its  weight  of  pure  silver  and  1  gm.  of  lead,  the  resulting  button  is 
hammered  and  rolled,  after  which  it  is  treated  with  nitric  acid  (of 
the  strength  stated  above),  and  the  residual  metal  weighed.  It  is 
again  cupelled  with  three  parts  of  pure  silver,  and  the  same  proc- 
ess repeated.  This  is  continued  until  a  constant  weight  is  finally 
obtained  for  the  residual  gold ;  the  third  operation  usually  accom- 
plishes this. 

Instead  of  effecting  the  separation  of  the  gold  from  the  platinum 
in  this  way,  the  two  metals  may  be  dissolved  in  aqua  regia,  and 
the  gold  precipitated  by  means  of  ferrous  sulphate,  as  described  on 
p.  257.  This  is  a  good  method. 


272  GRAVIMETRIC  ANALYSIS. 

According  to  Vanino,  and  Seemann,*  the  separation  is  effected 
much  more  quickly  by  precipitating  the  gold  from  an  alkaline 
solution  by  means  of  hydrogen  peroxide.  In  order  to  determine 
the  platinum,  it  is  precipitated  from  the  boiling  acid  nitrate  by 
hydrogen  sulphide  and  weighed  as  metal  after  ignition  in  a  porce- 
lain crucible. 

Analysis  of  Commercial  Platinum,  according  to  Deville  and  Stas. 

Five  grams  of  the  platinum  alloy  f  are  heated  for  five  hours 
at  a  temperature  of  about  1000°  G.  with  ten  times  as  much  lead 
in  a  crucible  of  purified  gas-carbon;  this  crucible  is  embedded  in 
one  of  clay  which  is  filled  with  charcoal.  After  cooling,  the  lead 
button  is  treated  with  very  dilute  nitric  acid  until  there  is  no 
longer  any  gas  evolved. 

In  this  way  a  solution,  A,  is  obtained,  containing  about  98.4 
per  cent,  of  the  lead  used,  all  the  palladium  and  copper,  and  small 
amounts  of  platinum,  rhodium,  and  iron,  and  a  residue,  B,  consist- 
ing of  a  black  metallic  powder,  which  is  filtered  off,  and  will  contain 
the  remainder  of  the  platinum  and  rhodium  with  all  of  the  iridium 
and  ruthenium. 

1.  Treatment  of  the  Nitric  Acid  Solution  A. 

The  lead  is  precipitated  by  the  addition  of  slightly  more  than  the 
theoretical  amount  of  sulphuric  acid,  and  filtered.  If  the  lead  sul- 
phate is  pure  white,  it  is  washed  with  dilute  sulphuric  acid.  If  it 
is  not  absolutely  white,  it  is  washed  with  a  solution  of  ammonium 
carbonate  until  it  becomes  so ;  small  amounts  of  lead  are  dissolved 
by  this  operation.  This  last  wash  liquid,  therefore,  is  concen- 
trated, to  precipitate  the  lead  carbonate,  filtered,  and  after  making 
acid  with  hydrochloric  acid,  added  to  the  main  filtrate. 

The  solution  is  evaporated  to  about  100  c.c.,  and  when  cold  is 
poured  into  a  saturated  solution  of  ammonium  chloride.  The 
mixture  is  heated  to  boiling  and  allowed  to  cool  again.  The  am- 
monium chloroplatinate  is  filtered  off  and  washed  with  a  saturated 
solution  of  ammonium  chloride;  in  this  way,  the  greater  part  of 
the  platinum  is  obtained. 

*  Berichte  1899,  p.  1971. 

f  All  commercial  platinum  contains  other  platinum  metals,  especially 
iridium. 


PLATINUM.  273 

The  filtrate  from  the  platinum  precipitate  is  boiled  with  formic 
acid  and  ammonium  acetate  as  described  on  p.  270,  b.  In  this  case 
the  remainder  of  the  platinum,  the  palladium,  and  the  rhodium  will 
be  precipitated.  These  metals  are  filtered  off,  and  the  copper  and 
iron  are  determined  in  the  nitrate  in  the  usual  way.  The  formic 
acid  precipitate  (consisting  of  a  black  metallic  powder)  is  dried  and 
fused  with  potassium  bisulphate  in  a  porcelain  crucible.  The  melt 
is  treated  with  water,  the  solution  decanted  from  the  unattacked 
platinum  and  washed  alternately  with  ammonium  carbonate  and 
nitric  acid  (to  remove  traces  of  lead  sulphate),  then  with  dilute 
hydrofluoric  acid,  and  finally  with  water;  it  is  then  dried  and 
weighed.  The  filtrate  from  the  platinum  contains  palladium  and 
rhodium.  The  former  is  precipitated  by  the  addition  of  mercuric 
cyanide,  and  boiling  until  the  odor  of  hydrocyanic  acid  has  disap- 
peared. The  voluminous,  yellowish- white  precipitate  of  palladouo 
cyanide  is  washed  first  by  decantation,  then  upon  the  filter,  dried,  and 
ignited  at  first  cautiously  and  then  strongly  over  the  blast  until  the 
paracyanide  is  completely  destroyed;  finally  heating  in  a  current 
of  hydrogen  (as  in  the  case  of  copper  sulphide,  p.  183)  in  order  to 
reduce  any  palladium  that  has  been  oxidized  by  the  previous  treat- 
ment. As  soon  as  the  flame  is  removed,  the  supply  of  hydrogen  is 
at  once  cut  off  in  order  to  prevent  its  being  absorbed  by  the  metal. 
The  palladium  is  weighed  after  cooling. 

The  rhodium  is  precipitated  from  the  filtrate  by  means  of  formic 
acid,  as  before,  and  the  deposited  metal  is  dried,  ignited  in  a  stream 
of  hydrogen,  allowed  to  cool  in  the  gas,  and  then  weighed. 

2.  Treatment  of  the  Residue  B. 

The  washed  residue  is  warmed  with  dilute  aqua  regia  (in  this 
case  2  vol.  nitric  acid,  8  vol.  hydrochloric  acid,  and  90  vol.  water), 
and  in  this  way  solution  C  is  obtained,  which  contains  the  rest  of 
the  lead,  platinum,  and  rhodium,  and  residue  D,  consisting  of 
lamella?  of  iridium  and  ruthenium. 

3.  Treatment  of  the  Solution  C. 

After  evaporating  to  a  small  volume,  the  lead  is  removed  by  sul- 
phuric acid,  the  solution  again  evaporated,  taken  up  in  hydro- 
chloric acid,  and  the  platinum  present  is  precipitated  by  pouring 


274  GRAVIMETRIC  ANALYSIS. 

into  a  cold  saturated  solution  of  ammonium  chloride  exactly  as 
described  under  1,  p.  272.  The  platinum  precipitate  contains  a 
little  rhodium,  and  after  washing  it  with  a  saturated  solution  of 
ammonium  chloride,  it  is  placed  at  one  side  for  the  time  being. 

The  nitrate,  together  with  the  wash  water,  is  evaporated  until 
more  platinum  and  rhodium  separate  out  on  cooling,  and  this  pre- 
cipitate is  filtered  off  and  washed  as  before. 

Both  filters,  together  with  the  precipitates,  are  now  placed  in  a 
small,  weighed  porcelain  dish,  dried,  and  reduced  at  as  low  a  tem- 
perature as  possible,  in  a  stream  of  illuminating  gas,  and  heated 
somewhat  in  a  muffle  so  as  to  remove  the  carbon  from  the  filter. 
The  metal  thus  obtained  (platinum  +  rhodium)  is  weighed.  For  the 
separation  of  the  rhodium  from  the  platinum,  the  spongy  metal  is 
heated  in  the  same  dish  with  potassium  bisulpnate,  gradually  rais- 
ing the  temperature  until  a  dull-red  heat  is  obtained.  After  cool- 
ing, the  melt  is  extracted  with  water,  the  unattacked  platinum  (it 
may  still  contain  small  amounts  of  rhodium)  is  filtered  off,  washed, 
and  again  fused  with  potassium  bisulphate.  This  operation  is 
repeated  until  the  rhodium  is  completely  extracted,  which  is  known 
by  the  melt  showing  no  yellow  color  after  ten  minutes. 

The  platinum  is  washed,  ignited,  and  weighed  as  described 
under  1. 

The  Combined  filtrates  from  the  platinum  contain  rhodium  and 
still  a  little  platinum.  Ammonia,  acetic  and  formic  acids,  there- 
fore, are  added  once  more,  and  the  solution  boiled  for  a  long  time. 
The  precipitated  metal  is  filtered  off,  ignited,  weighed,  afterward 
fused  at  a  distinct  red  heat  with  potassium  bisulphate,  and  the 
cold  melt  extracted  with  water.  If  a  residue  remains  after  this 
treatment,  it  is  filtered  off,  weighed,  and  treated  with  dilute  aqua 
regia.  If  it  dissolves,  it  is  platinum ;  if  it  does  not,  it  is  rhodium. 

The  filtrate  from  the  ammonium  chloroplatinate,  which  con- 
tained some  rhodium,  is  diluted,  formic  acid  and  ammonium  ace- 
tate are  added,  and  it  is  gently  boiled  for  two  or  three  days  in  an 
Erlenmeyer  flask  connected  with  a  return-flow  condenser.  .The 
liquid  evaporates  somewhat  in  spite  of  the  condenser,  and  the  evap- 
orated part  is  replaced  from  time  to  time  with  a  dilute  solution  of 
ammonium  formate.  In  this  way  small  amounts  of  platinum  and 
rhodium  are  precipitated,  which  are  filtered  off  and  separated  by 


PLATINUM.  275 

fusion  with  potassium  bisulphate  as  before.     In  the  filtrate  there 
are  likely  to  be  present  traces  of  platinum,  rhodium,  and  iron. 

The  iron  is  first  removed  by  the  addition  of  chlorine  water  and 
afterward  ammonia;  the  ferric  hydroxide  is  filtered  off,  ignited,  and 
weighed.  In  order  to  remove  the  last  traces  of  platinum  and  rho- 
dium, this  last  filtrate  is  evaporated  to  dryness,  the  residue  heated 
with  nitric  acid  in  order  to  remove  the  ammonium  chloride  com- 
pletely, and  then  boiled  for  a  long  time  with  formic  acid  and  am- 
monium acetate.  The  traces  of  metal  thus  obtained  are  washed 
with  hydrofluoric  acid  and  added  to  the  main  portion  of  platinum 
and  rhodium. 

4.  Treatment  of  the  Residue  D. 

The  undissolved,  gray  lamellae  consisting  of  iridium,  ruthenium, 
and  small  amounts  of  iron  obtained  by  the  action  of  dilute  aqua 
regia,  are  filtered  off,  dried,  ignited  in  an  atmosphere  of  hydrogen 
or  illuminating  gas,  and  weighed. 

The  weighed  metal  is  then  fused  in  a  pure  gold  crucible  with 
potassium  nitrate  and  carbonate.  For  this  purpose,  a  previously 
melted  mixture  of  3  gms.  potassium  nitrate  and  10  gms.  potassium 
carbonate  is  placed  in  the  crucible,  the  metal  added,  and  the  mix- 
ture heated  for  two  hours  at  a  dull-red  temperature.  In  this  way 
the  ruthenium  is  changed  completely  into  water-soluble  potassium 
ruthenate,  K2RuO4,  and  the  iridium  is  oxidized  to  Ir2O3 ;  the  latter 
forms,  to  some  extent,  a  soluble  compound  with  the  alkali. 

The  melt  is  treated  with  water,  and  the  solution,  together  with 
the  suspended  Ir2O3,*  is  poured  into  a  stoppered  cylinder,  the  pre- 
cipitate allowed  to  settle,  and  the  clear  liquid  decanted  off  into  a 
retort. 

The  residue  remaining  in  the  cylinder  is  covered  repeatedly  with 
a  dilute  solution  of  sodium  hypochlorite  and  sodium  carbonate, 
until  the  yellow  color  is  completely  removed.  The  decanted  liquid 
is  added  to  the  main  solution  in  the  retort.  This  solution  con- 
tarns  all  the  ruthenium  and  a  part  of  the  iridium.  It  is  saturated 
with  chlorine  in  the  cold,  distilled,  and  the  distillate  received  in  a 
mixture  of  alcohol  (distilled  over  potassium)  and  pure  hydro- 
chloric acid. 

*  Cf.  W.Palmaer,  Z.  anorg.  Chem.,  10,  332  (1896). 


276  GRAVIMETRIC  ANALYSIS. 

After  the  distillation  is  complete,  the  alcoholic  distillate  is  evap- 
orated to  dryness  and  the  ruthenium  chloride  thus  obtained  is 
reduced  to  metal  by  heating  in  a  stream  of  hydrogen.  After 
weighing,  the  purity  of  the  ruthenium  is  tested.  It  must  dissolve 
completely  in  a  concentrated  solution  of  sodium  hypochlorite. 

The  liquid  remaining  in  the  retort  is  evaporated  to  a  small  vol- 
ume, the  insoluble  residue  remaining  in  the  cylinder  (that  was 
washed  with  sodium  hypochlorite  and  sodium  carbonate)  is  added, 
and  the  mixture  boiled  with  caustic  soda  solution,  with  the  addi- 
tion of  a  little  alcohol,  until  all  of  the  iridium  is  precipitated. 

The  dark-blue  precipitate,  consisting  of  iridinm  oxide  and  small 
amounts  of  ferric  hydroxide,  is  filtered  off,  washed,  and  strongly 
ignited.  The  ferric  oxide  contained  in  it  is  then  extracted  with 
hydrochloric  acid  containing  some  ammonium  iodide,  and  the  re- 
sidual iridium  oxide  is  washed  successively  with  water,  chlorine 
water,  and  hydrofluoric  acid  in  order  to  remove  gold  that  came 
from  the  crucible  and  silicic  acid  from  the  caustic  soda.  It  is  then 
ignited  in  hydrogen  and  the  iridium  weighed. 

The  iron  present  in  the  hydrochloric  acid  extract  is  precipitated 
as  ferric  hydroxide,  ignited,  and  weighed.  Its  purity  is  tested  by 
heating  in  a  stream  of  hydrogen  and  hydrochloric  acid,  to  see  if  it 
can  be  completely  changed  to  ferrous  chloride  and  volatilized  as 
such. 

F.  Mylius  and  F.  Forster*  have  recommended  that  platinum 
be  tested  for  small  amounts  of  impurity  by  taking  three  separate 
portions  each  weighing  10  gms.  The  first  portion  is  tested  for 
palladium,  iridium,  and  ruthenium  according  to  the  lead  pro- 
cedure just  described  of  Deville  and  Stas.  The  second  portion 
serves  for  the  iron  determination;  the  metal  is  dissolved  in  aqua 
regia,  the  platinum  metals  precipitated  by  formic  acid,  and  the 
iron  determined  in  the  filtrate.  In  the  third  portion,  rhodium, 
silver,  copper,  and  lead  are  determined  by  volatilizing  the  plati- 
num as  PtCl2CO  at  238°  C.  (temperature  of  boiling  quinolin)  in 
a  stream  of  carbon  monoxide  and  chlorine,  and  determining  the 
above  substances  in  the  residue. 

Remark. — The  determination  of  the  iron  in  a  separate  portion  is 
to  be  recommended,  for  in  the  lead  procedure  some  iron  is  always 
obtained  from  the  carbon  crucible. 

*  Berichte  1892,  p.  665. 


SELENIUM.  277 

SELENIUM,  Se.    At.  Wt.  79.2. 

Selenium  is  usually  determined  as  the  element  itself. 
Three  cases  are  to  be  considered: 

I.  The  selenium  is  present  as  alkali  selenite  or  as  selenious  acid. 
II.  The  selenium  is  present  as  alkali  selenate  or  as  selenic  acid. 
III.  The  selenium  is  present  as  potassium  selenocyanide. 

I.  The  selenium  is  present  as  selenite  or  as  free  seleinous  add. — 
The  solution  is  acidified  with  hydrochloric  acid,  saturated  with 
sulphur  dioxide  gas,  boiled,  filtered  through  a  Gooch  crucible,  and 
washed  first  with  water,  then  with  alcohol.     The  residue  is  dried 
at  105°  C.  and  weighed. 

Remark. — The  precipitation  of  selenium  by  sulphur  dioxide  is 
always  quantitative  whether  the  solution  is  concentrated  or  dilute, 
whether  it  contains  much  or  little  free  acid.  This  latter  fact  is  of 
importance  in  the  separation  of  selenium  from  tellurium,  for  the 
latter  element  is  not  precipitated  by  sulphur  dioxide  when  consid- 
erable hydrochloric  acid  is  present  (cf.  p.  279). 

Phosphorous  acid  does  not  precipitate  selenium  from  cold, 
dilute,  strongly  acid  solutions;  this  fact  is  made  use  of  in  the 
separation  of  selenium  from  mercury  (cf.  p.  281). 

II.  The  selenium  is  present  as  alkali  selenate  or  as  free  selenif- 
acid. — As  selenium  in  the  form  of  selenic  acid  is  not  precipitated 
by  sulphur  dioxide,  phosphoric  acid,   or  hydrogen  sulphide,  it 
must  be  first  reduced  to  selenous  acid  by  long-continued  boiling 
with  hydrochloric  acid  (cf .  Vol.  1) ;    the  above  procedure  is  then 
followed. 

III.  The  selenium  is  present  as  potassium  selenocyanide. — The 
solution,  concentrated  as  much  as  possible,  is  treated  with  hydro- 
chloric acid,  boiled,  allowed  to  settle,  and  the  precipitate  filtered 
through  a  Gooch  crucible,  dried  at  105°  C.,  and  weighed. 

Remark. — From  very  dilute  solutions  of  potassium  selenocy- 
anide, selenium  separates  out  only  very  slowly  according  to  this 
method;  it  is  therefore  advisable  to  concentrate  the  solution  as 
much  as  possible,  but  when  this  cannot  be  done,  the  boiling  with 
hydrochloric  acid  should  be  continued  for  some  time  and  the  liquid 
allowed  to  stand  before  filtering. 


278  GRAVIMETRIC  ANALYSIS. 

In  practice,  selenium  is  obtained  usually  in  none  of  the  above 
forms,  but  as  impure  selenium  (selenium  sponge)  or  as  selenide, 
and  by  the  treatment  of  these  substances  one  or  the  other  of  the 
above  selenium  compounds  is  obtained. 

If  the  selenium  or  selenide  is  acted  upon  by  concentrated  nitric 
acid,*  or  aqua  regia,  all  of  it  is  dissolved  in  the  form  of  selenous  acid 
(not  selenic  acid).  After  evaporating  the  solution  several  times 
with  hydrochloric  acid  in  order  to  remove  the  excess  of  nitric 
acid,  the  selenium  is  precipitated  by  sulphur  dioxide  as  described 
under  1. 

If  the  finely  powdered  selenium  or  selenide  is  intimately  mixed 
with  two  parts  sodium  carbonate  and  one  part  potassium  ni- 
trate, placed  in  a  nickel  crucible,  covered  with  a  layer  of 
sodium  carbonate  and  potassium  nitrate  and  heated  gradually 
until  it  fuses,  all  the  selenium  forms  alkali  selenate  and  on  ex- 
tracting the  melt  with  water  it  goes  into  solution;  in  this  way  it 
is  separated  from  most  of  the  remaining  oxides.  The  solution, 
however,  often  contains  small  amounts  of  lead.  In  order  to  re- 
move the  latter,  the  filtrate  is  treated  with  hydrogen  sulphide, 
and  again  filtered;  the  solution  is  freed  from  hydrogen  sulphide 
by  boiling,  strongly  acidified  with  hydrochloric  acid,  boiled  until 
no  more  chlorine  is  evolved  and  the  selenium  is  precipitated  by 
sulphur  dioxide  according  to  II. 

Remarks.  — The  mixture  must  be  heated  very  slowly,  as  other- 
wise some  selenium  is  likely  to  be  lost  by  volatilization. 

Selenium  and  very  many  selenium  compounds  may  be  satisfac- 
torily determined  as  follows:  The  dry,  finely  powdered  sponge  is 
fused  at  as  low  a  temperature  as  possible  in  a  current  of  hydrogen  f 
with  twelve  times  as  much  potassium  cyanide.  After  the  mass 
has  fused  quietly  for  about  fifteen  minutes  it  is  allowed  to  cool  in 
hydrogen.  It  is  then  extracted  with  water,  the  solution  is  heated 
to  boiling,  and  analyzed  according  to  III. 

*  Mercury  cyanide  is  unacted  upon  by  nitric  acid,  but  is  dissolved  by 
aqua  regia. 

j-  A  Rose  crucible  (Fig.  37,  p.  185)  is  used,  or  a  round-bottomed  flask 
with  a  long  neck  made  of  difficultly  fusible  glass,  from  which  the  air  is  replaced 
by  hydrogen.  In  the  latter  case  the  delivery-tube  must  be  so  wide  that  the 
Beck  of  the  flask  is  nearly  filled  with  it. 


SELENIUM  AND    TELLURIUM.  279 

It  is  necessary  to  boil  the  solution  of  potassium  selenocyanide 
before  acidifying  it,  for  small  amounts  of  potassium  selenide 
(K2Se)  are  almost  always  present,  and  on  acidifying  with  hydro- 
chloric acid  this  is  decomposed  with  evolution  of  hdyrogen  selenide. 
On  boiling,  the  potassium  selenide  is  changed  to  potassium 
selenocyanide  according  to  the  equation: 

K2Se  +  KCN  +  H20  +  O  =  2KOH  +  KCNSe. 

TELLURIUM,  Te.    At.  Wt.    127.5. 

Tellurium   is  usually  determined  as  the  element  itself. 

If  sulphur  dioxide  is  conducted  into  a  hydrochloric  acid  solu- 
tion containing  tellurous  acid,  black  tellurium  is  quantitatively 
precipitated,  provided  the  solution  does  not  contain  too  muck  acid. 
If  tellurous  acid  is  dissolved  in  200  c.c.  of  hydrochloric  acid,  sp.  gr. 
1.175,  no  tellurium  will  be  precipitated  on  passing  sulphur  dioxide 
into  the  cold  solution.  If,  however,  the  solution  is  diluted  with 
an  equal  volume  of  water  and  sulphur  dioxide  is  passed  into  the 
boiling  solution,  all  the  tellurium  will  be  precipitated.  The  pre- 
cipitate is  filtered  off,  washed  with  water  until  free  from  chlorides, 
then  with  alcohol,  dried  at  105°  C.  and  weighed.  The  oxidation  of 
the  tellurium  during  the  drying  is  so  slight  that  it  can  be  disre- 
garded.* 

Separation  of  Selenium  and  Tellurium  from  the  Metals  of 
Groups  III,  IV,  and  V. 

By  conducting  sulphur  dioxide  into  the  solution  fairly  acid  with 
hydrochloric  acid,  the  selenium  and  tellurium  will  be  quantita- 
tively precipitated  while  the  other  metals  remain  in  solution. 

*  The  presence  of  nitric  acid  prevents  the  complete  precipitation  of  the 
tellurium  by  means  of  sulphur  dioxide  and  similarly  the  presence  of  sulphuric 
acid  is  harmful.  To  remove  nitric  acid,  sodium  chloride  is  added  and  the 
solution  evaporated  to  dryness  repeatedly  with  hydrochloric  acid.  According 
to  Brauner  the  addition  of  sodium  chloride  is  absolutely  necessary,  as  other- 
wise an  appreciable  amount  of  tellurium  will  be  volatilized  as  chloride. 
A.  Gutbier  (Ber.  34,  2724  (1901)  )  reports  that  all  these  difficulties  are  over- 
come by  precipitating  tellurium  from  a  hot  solution  by  means  of  hydrazine 
hydrate  or  hydrazine  hydrochloride,  but  not  the  sulphate.  See  also  P. 
Jannasch  and  M.  Miiller;  Ber.  31,  2393  (1898). 


280  GRAVIMETRIC  ANALYSIS. 

Separation  of  Selenium  and  Tellurium  from  the  Metals  of  Group  II. 
(a)  From  Copper,  Bismuth,  and  Cadmium. 

Sulphur  dioxide  is  passed  into  the  boiling  solution,  acid  with 
hydrochloric  acid,  whereby  all  of  the  selenium  and  tellurium  and 
usually  some  of  the  bismuth  are  precipitated.  The  precipitate 
after  being  washed  is  dissolved  in  nitric  acid,  the  solution  evapo- 
rated to  dry  ness,  taken  up  in  concentrated  hydrochloric  acid, 
diluted  with  a  little  water  and  precipitated  with  hydrogen  sul- 
phide. The  precipitate,  consisting  of  the  three  sulphides,  is  washed 
and  then  treated  with  sodium  sulphide  solution  whereby  selenium 
and  tellurium  pass  into  solution  while  the  bismuth  remains  behind 
as  its  brown  sulphide  and  is  filtered  off. 

The  solution  containing  the  selenium  and  tellurium  is  acidified 
with  nitric  acid,  carefully  evaporated  to  dryness  and  the  residue 
boiled  with  200  c.c.  of  hydrochloric  acid,  sp.  gr.  1.175,  until  there 
is  no  longer  any  evolution  of  chlorine.  The  deposited  sulphur  is 
then  filtered  off  through  a  Gooch  crucible,  and  the  filtrate  satu- 
rated with  sulphur  dioxide  gas ;  all  the  selenium  is  in  this  way  pre- 
cipitated. The  latter  is  filtered  off  through  a  Gooch  crucible  and 
washed  successively  with  a  mixture  of  90  vol.  HC1  (sp.  gr.  1.175) 
and  10  vol.  water,  dilute  hydrochloric  acid,  and  finally  absolute 
alcohol.  The  precipitate  is  dried  at  105°  C.  and  weighed.  The 
filtrate  is  diluted  with  an  equal  volume  of  water  and  the  tellurium 
precipitated  by  passing  sulphur  dioxide  into  the  boiling  solution. 
This  precipitate  is  washed  with  water  until  free  from  chlorides, 
then  with  absolute  alcohol,  after  which  it  is  dried  at  105°  C.  and 
weighed. 

Remark. — The  above  method  is  suitable  for  the  separation  of 
selenium  and  tellurium  from  small  amounts  of  bismuth,  but  does 
not  effect  the  separation  of  selenium  (and  tellurium)  from  copper. 
In  this  case,  more  or  less  copper  selenide  is  formed  according  to 
the  conditions,  and  this  compound  is  not  decomposed  quantita- 
tively by  sodium  sulphide.*  In  this  case,  the  method  of  B. 
Brauner  and  B.  Kuzmaf  may  be  used. 

*  Cf.  E.  Keller,  J.  Am.  Chem.  Soc.,  19,  771. 
t  Berichte,  1907,  3362. 


SELENIUM  AND    TELLURIUM.  281 

The  tellurium  and  selenium  are  precipitated  in  a  pressure 
flask,  by  means  of  SO2,  the  precipitate,  which  is  contaminated 
with  copper,  antimony  and  bismuth,  is  filtered  (using  a  Gooch 
crucible)  washed,  dissolved  in  nitric  acid,  the  solution  evaporated 
to  dryness  and  the  residue  taken  up  in  caustic  potash  solution 
(1:5).  The  alkaline  solution  is  placed  in  an  Erlenmeyer  flask 
upon  a  water-bath,  and  little  by  little  4-6  gm.  of  ammonium 
persulphate  are  added,  whereby  the  potassium  tellurite  is  oxidized 
to  potassium  tellurate  and  the  selenite  to  selenate.  When  all 
the  persulphate  has  been  introduced,  the  solution  is  heated  to 
boiling  to  decompose  the  excess  of  persulphate,  then  acidified 
with  sulphuric  acid  and  allowed  to  cool.  Now,  100  c.c.  of  H2S- 
water  are  added,  the  excess  of  the  H2S  expelled  by  passing  C02 
through  the  solution,  and  the  precipitated  CuS  (Bi2S3,  Sb2S^) 
filtered  off,  and  treated  as  described  on  p.  235.  The  filtrate  is 
boiled  with  hydrochloric  acid  to  reduce  the  telluric  acid  to 
tellurous  acid,  and  the  solution  is  reduced  by  means  of  SO2  and 
analyzed  as  described  above. 

The  first  filtrate  from  the  impure  Te  and  Se  will  contain  the 
greater  part  of  the  Cu,  Bi,  etc. 

(6)  From  Antimony,  Tin  and  Arsenic. 

If  considerable  antimony  is  present,  tartaric  acid  is  added  to 
the  solution,  and  the  selenium  and  tellurium  are  then  precipitated 
by  boiling  with  sulphur  dioxide. 

According  to  Muthmann  and  Schroder  *  this  method  of 
separating  tellurium  from  antimony  is  not  quantitative;  some 
antimony  is  always  precipitated  with  the  tellurium.  A.  Gutbier,f 
however,  finds  that  a  perfect  separation  can  be  accomplished  by 
means  of  hydrazine  hydrochloride  (not  the  sulphate). 

(c)  From  Mercury. 

The  mercury  selenide,  or  telluride,  is  dissolved  in  aqua  regia,  chlo- 
rine water  is  added,  and  the  solution  is  diluted  largely  with  water. 
Phosphorous  acid  is  added, t  and  after  twenty-four  hours  standing, 

*Z.  anorg.  Chem.,  14,  433  (1897). 

t  Z.  anorg.  Chem.,  32,  263  (1902). 

}  Selenous  and  tellurous  acids  are  not  precipitated  by  phosphorous  acid 
from  dilute  hydrochloric  acid  solution,  but  are  precipitated  from  hot  con- 
centrated solutions. 


282  GRAVIMETRIC  S.N4LYSIS. 

the  mercury  is  precipitated  completely  as  mercurous  chloride,  and 
is  determined  as  such  according  to  p.  170. 

The  filtrate  containing  selenium  and  tellurium  is  concentrated, 
taken  up  in  water,  and  the  selenium  separated  from  the  tellurium 
according  to  the  method  of  Keller  (see  below.) 

(d)  From  Gold  and  Silver. 

The  separation  of  selenium  and  tellurium  from  silver  offers  no 
difficulty,  inasmuch  as  the  latter  can  be  precipitated  by  hydro- 
chloric acid  and  determined  as  the  chloride. 

The  gold  is  precipitated  as  described  on  p.  257  by  oxalic  acid 
and  the  selenium  and  tellurium  in  the  filtrate  by  means  of  sulphur 
dioxide.  The  three  metals  may  also  be  precipitated  together  by 
sulphur  dioxide,  weighed,  and  the  selenium  and  tellurium  after- 
ward volatilized  by  roasting,  leaving  the  gold  behind. 

Tellurium  may  be  separated  from  gold  by  precipitating  the  lat- 
ter with  ferrous  sulphate.  In  the  case  of  selenium,  however,  it  is 
also  precipitated  quantitatively  by  ferrous  sulphate  from  solutions 
strongly  acid  with  hydrochloric  acid. 

Separation  of  Selenium  from  Tellurium. 
A.    Method  of  E.  Keller.* 

Keller's  method  is  based  upon  the  fact  that  tellurous  acid  is  not 
precipitated  from  solutions  strongly  acid  with  hydrochloric  acid 
while  selenium  is  precipitated  quantitatively. 

Procedure. — The  mixture  of  the  two  elements  precipitated 
by  sulphur  dioxide  is  dissolved  in  nitric  acid  and  carefully 
evaporated  to  dryness.  The  dry  mass  is  treated  with  200  c.c.  of 
hydrochloric  acid  (sp.  gr.  1.175),  boiled  to  remove  the  nitric  acid 
and  saturated  with  sulphur  dioxide.  The  precipitated  selenium 
is  filtered  through  a  Gooch  crucible,  washed  first  with  a  mixture 
of  90  vol.  HC1  (sp.  gr.  1.175)  and  10  vol.  water,  then  with  dilute 
hydrochloric  acid,  then  with  water  until  free  from  chloride,  finally 
with  absolute  alcohol.  The  selenium  is  then  dried  at  105°  C.  and 
weighed.  The  filtrate  is  diluted  with  an  equal  volume  of  water, 

*  Jour.  Amer.  Chem.  Soc.,  19,  771. 


SEPARATION  OF  SELEHIUM  FROM    TELLURIUM.  283 

heated  to  boiling,  and  the  tellurium  precipitated  by  sulphur  diox- 
ide and  treated  in  exactly  the  same  way  us  the  selenium. 

Accc  rding  to  Keller,  this  method  gives  thoroughly  satisfactory 
results,  as  long  as  the  amount  of  tellurium  present  does  not  exceed 
5  gms.  Even  then  the  separation  can  be  effected  by  increasing  the 
amount  of  acid  to  450  c.c. 

B.    The  Potassium  Cyanide  Method. 

The  precipitate  of  selenium  and  tellurium  produced  by  sulphur 
dioxide  is  fused  with  twelve  times  as  much  of  pure  (98  per  cent.) 
potassium  cyanide,  in  an  atmosphere  of  hydrogen,  as  described  on 
p.  278.  -The  tellurium  is  almost  wholly  changed  to  potassium 
telluride,  K2Te  (a  small  amount  of  potassiuift  tellurocyanide  is 
probably  formed),  while  the  selenium  is  changed  for  the  most  part 
into  potassium  selenocyanide,  and  to  a  slight  extent  into  potassium 
selenide. 

The  brown  melt  is  dissolved  in  water,  and  a  slow  current  of  air  is 
conducted  through  the  solution  whereby  the  K2Te  is  quantitatively 
decomposed  according  to  the  equation 

K2Te+H20+O  =  2KOH+Te. 

After  standing  twelve  hours  the  tellurium  is  filtered  off  through  a 
Gooch  crucible,  washed  with  water,  then  with  absolute  alcohol, 
dried  at  105°  and  weighed. 

The  colorless  nitrate  is  heated  to  boiling  *  in  order  to  change  any 
potassium  selenide  into  the  double  cyanide;  it  is  then  acidified 
with  hydrochloric  acid  under  a  good  hood  (hydrocyanic  acid!}, 
filtered,  and  the  selenium  determined  according  to  p.  277. 

Remark. — This  method  gives  slightly  low  results  for  tellurium 
and  high  values  for  selenium.  This  is  due  to  the  fact  that  a  little 
potassium  tellurocyanide  is  formed  by  the  fusion  and  this  com- 
pound is  not  decomposed  by  the  current  of  air,  but  is  subsequently 
precipitated  with  the  selenium  on  acidifying  the  solution. 

*  Cf.  p.  27'.». 


284  GRAVIMETRIC  ANALYSIS. 

Determination  of  Selenium  and  Tellurium  in  Crude  Copper. 

Many  copper  ores  contain  selenium  and  tellurium,  so  that  the 
crude  copper  obtained  from  such  ores  always  contains  these  ele- 
ments. The  amount  present  may  be  determined,  according  to 
Keller,*  as  follows:  According  to  the  amounts  of  selenium  and 
tellurium  present,  from  5  to  100  gm.  of  the  copper  are  taken  for 
the  analysis  The  sample  is  dissolved  in  nitric  acid  and  an  excess 
of  ammonia  is  added  whereby  the  phosphorus,  arsenic,  antimony, 
tin,  bismuth,  selenium,  and  tellurium  are  precipitated  together  with 
the  ferric  hydroxide,  while  the  copper  is  held  in  solution  by  the 
excess  of  ammonia.  The  precipitate  is  filtered  off  and  washed  with 
dilute  ammonia- wattr  until  the  copper  is  completely  removed.  The 
precipitate  is  dissolved  in  hydrochloric  acid  and  this  solution  satu- 
rated with  hydrogen  sulphide  in  the  cold,  whereby  selenium  and 
tellurium  together  with  arsenic,  antimony,  tin,  and  bismuth  are 
thrown  down  as  sulphides  and  are  separated  by  nitration  from  the 
iron  and  phosphorus.  The  precipitate  thus  obtained  is  treated 
with  sodium  sulphide  and  filtered.  The  filtrate  containing  all  the 
selenium  and  tellurium  in  the  presence  of  arsenic,  antimony,  and 
tin  as  sulpho  salts  is  acidified  with  nitric  acid  and  carefully  evapo- 
rated to  dryness.  The  residue  is  dissolved  in  200  c.c.  of  hydro- 
chloric acid  (sp.  gr.  1.175)  and  treated  as  described  on  p.  282,  A. 

MOLYBDENUM,  Mo.    At.  Wt.  96.0. 
Form:  Molybdenum  Trioxide,  MoO3. 

If  the  molybdenum  is  present  as  ammonium  molybdate,  a 
weighed  portion  is  heated  in  a  spacious  porcelain  or  platinum 
crucible,  at  first  carefully  and  later  to  a  dull  red  heat;  this  leaves 
the  molybdenum  trioxide  behind  in  the  form  of  a  dense  powder, 
appearing  yellow  when  hot  and  almost  white  when  cold. 

There  is  no  danger  of  losing  any  of  the  molybdenum  by  volatili- 
zation, provided  the  dull  red  heat  is  not  exceeded. 

If  the  molybdenum  is  present  as  alkali  molybdate,  it  is  changed 
to  mercurous  molybdate  or  to  its  sulphide,  and  then  analyzed  as 
described  below. 

*  Jour.  Amer.  Chem.  Soc.,  22,  241. 


PRECIPITATION  OF  MOLYBDENUM.  285 

Precipitation  of  Molybdenum  as  Mercurous  Molybdate. 

In  the  course  of  analysis  it  is  frequently  necessary  to  determine 
molybdenum  in  alkali  molybdates  obtained  by  fusion  with  an  alkali 
carbonate. 

The  greater  part  of  the  alkali  is  neutralized  with  nitric  acid, 
and  to  the  slightly  alkaline  solution  a  barely  acid  solution  of  mer- 
curous  nitrate  is  added  until  no  further  precipitation  is  effected. 
The  liquid  is  then  heated  to  boiling,  the  black  precipitate,  consisting 
of  mercurous  carbonate  and  mercurous  rnolybdate,  is  allowed  to  set- 
tle, is  filtered  and  washed  with  a  dilute  solution  of  mercurous 
nitrate.  The  precipitate  is  dried,  transferred  as  completely  as  pos- 
sible to  a  watch-glass,  and  the  precipitate  remaining  on  the  filter  is 
dissolved  in  hot  dilute  nitric  acid  into  a  large  porcelain  crucible. 
The  solution  is  then  evaporated  to  dryness,  the  main  portion  of  the 
precipitate  added  to  the  residue,  and  the  whole  is  heated  very  care- 
fully over  a  low  flame  until  the  mercury  is  completely  volatilized, 
after  which  the  residual  molybdenum  trioxide  is  weighed. 

Remark. — It  was  formerly  customary  to  add  a  slight  excess  of 
mercurous  nitrate  solution  and  then  to  add  mercuric  oxide  to  neu- 
tralizD  the  excess  of  nitric  acid  (the  solution  of  mercurous  nitrate 
contains  free  nitric  acid).  According  to  the  above  procedure  of 
Hilbbrand,  the  addition  of  mercuric  oxide  is  wholly  superfluous, 
for  th?  basic  mercurous  carbonate  suffices  to  remove  the  slight 
amount  of  free  nitric  acid. 

Precipitation  of  Molybdenum  as  Molybdenum  Sulphide. 

The  precipitation  of  molybdenum  as  the  sulphide  can  take  place 
in  two  different  ways:  either  the  acid  solution  may  be  precipitated 
by  hydrogen  sulphide  gas,  or  the  solution  of  ammonium  sulpho- 
molybdate  may  be  acidified  with  dilute  acid. 

(a)  Precipitation  of  Molybdenum  Sulphide  from  Acid  Solutions. 

The  molybdenum  solution,  slightly  acid  with  sulphuric  acid,*  is 
placed  in  a  small  pressure-flask  and  saturated  in  the  cold  with 
hydrogen  sulphide.  The  flask  is  closed,  heated  on  the  water- 

*  In  many  cases,  e.g.,  for  the  separation  of  Mo  from  Ba,  Sr,  and  Ca,  it  is 
necessary  to  effect  the  separation  in  a  hydrochloric  acid  solution. 


286  GRAVIMETRIC  ANALYSIS. 

bath  until  the  precipitate  has  completely  settled,  and  filtered 
after  it  has  become  cold.  The  precipitate  is  washed  with  very 
dilute  sulphuric  acid  and  finally  with  alcohol  until  the  acid  has 
been  completely  removed.  The  moist  filter  is  placed  in  a  large 
porcelain  crucible  and  dried  upon  the  water-bath.  The  crucible 
is  then  covered  and  very  carefully  heated  over  a  small  flame  until 
no  more  hydrocarbons  are  expelled.  The  cover  is  then  removed, 
the  carbon  burned  from  the  sides  of  the  crucible  at  as  low  a  tem- 
perature as  possible,  and,  by  raising  the  temperature  gradually, 
the  sulphide  is  changed  to  oxide.  The  operation  is  finished  when 
no  more  sulphur  dioxide  is  formed.  After  cooling,  a  little  mercuric 
oxide  suspended  in  water  is  added  to  the  contents  of  the  cr^ible, 
the  mixture  is  well  stirred,  evaporated  to  dryness  on  the  water- 
bath,  the  mercuric  oxide  is  removed  by  gentle  ignition,  and  the  resi- 
due of  molybdenum  trioxide  is  weighed.  The  mercuric  oxide  is 
added  in  order  to  remove  particles  of  unburned  carbon. 

It  is  much  easier  to  transform  the  molybdenum  trisulphide  into 
the  oxide  as  follows :  The  sulphide  is  filtered  through  a  Gooch  cru- 
cible, washed  with  water  containing  sulphuric  acid,  and  then  with 
alcohol,  and  dried  at  100°  C.  The  crucible  is  placed  within  a  larger 
nickel  one,  covered  with  a  watch-glass, •*  and  carefully  heated  over 
a  small  flame  whereby  the  sulphide  is  for  the  most  part  changed 
to  the  oxide.  As  soon  as  the  odor  of  sulphur  dioxide  can  no  longer 
be  detected,  the  watch-glass  is  removed  and  the  open  crucible 
heated  until  it  is  brought  to  a  constant  weight.  The  molybdenum 
oxide  thus  obtained  always  contains  traces  of  SO3,  and  consequently 
has  a  bluish  appearance.  The  results,  nevertheless,  are  excellent. 

(6)  Hydrogen   Sulphide  is    passed    into    the   Ammoniacal  Molyb- 
denum Solution 

until  it  assumes  a  bright-red  color,  when  it  is  acidified  with  sul- 
phuric acid  and  the  precipitate  treated  as  described  under  (a). 

The  Separation  of  Molybdenum  from  the  Alkalies 

can  take  place  by  precipitation  as  mercurous  molybdate  or  as  sul- 
phide, as  described  above. 

*  To  avoid  loss  by  decrepitation. 


SEPARATION  Of  MOLYBDENUM.  28? 

Separation  of  Molybdenum  from  the  Alkaline  Earths. 

The  substance  is  fused  with  sodium  carbonate,  the  meh  ex- 
tracted with  water  and  filtered.  The  solution  contains  all  the 
molybdenum  as  alkali  molybdate,  while  the  alkaline  earths  remain 
undissolved  as  carbonates.  From  the  aqueous  solution  the  molyb- 
denum is  determined  as  previously  described. 

Separation  of  Molybdenum  from  the  Metals  of  the  Ammonium 
Sulphide  Group. 

The  molybdenum  is  precipitated  as  sulphide  (preferably  from  a 
sulphuric  acid  solution)  by  treatment  with  hydrogen  sulphide  under 
pressure  (see  p.  286) .  If  the  solution  contains  titanium,  it  is  better 
to  first  add  ammonia  and  ammonium  sulphide,  whereby  the  metals 
of  Group  III  will  be  precipitated  and  the  molybdenum  will  remain 
in  solution  in  the  form  of  its  sulpho  salt.  After  filtration,  the 
molybdenum  is  precipitated  as  sulphide  by  the  addition  of  acid 
(see  p.  286,  6). 


Separation  of  Molybdenum  from  the  Metals  of  Group  II. 
(a)  From  Lead,  Copper,  Cadmium,  and  Bismuth. 

The  solution  is  treated  with  caustic  soda  and  then  with  sodium 
sulphide,  digested  some  time  in  a  closed  flask,  and  filtered.  The 
molybdenum  remains  in  solution  as  its  sulpho  salt,  while  the  other 
metals  are  precipitated  as  sulphides.  After  filtering,  the  solution 
is  acidified  with  sulphuric  acid  and  heated  in  a  pressure-flask  until 
the  precipitate  has  settled  and  the  supernatant  liquid  appears  col- 
orless. After  allowing  to  cool,  the  molybdenum  sulphide  is  fil- 
tered off  and  convened  to  oxide,  as  described  on  p.  286. 

(6)  From  Arsenic. 

The  solution,  which  must  contain  the  arsenic  as  arsenic  acid,  is 
treated  with  ammonia,  the  arsenic  precipitated  by  magnesia  mix- 
ture (see  p.  206)  and  filtered  off.  The  filtrate  is  acidified  with  sul- 
phuric acid  and  the  molybdenum  precipitated  as  sulphide  by  mean? 
of  hvdrogen  sulphide. 


288  GRAVIMETRIC  ANALYSIS. 

Separation  of  Molybdenum  from  Phosphoric  Acid. 

The  phosphoric  acid  is  precipitated  from  the  ammoniacal  solu- 
tion as  magnesium  ammonium  phosphate  (see  phosphoric  acid) 
and  the  molybdenum  is  precipitated  as  sulphide  from  the  filtrate 
(cf.  p.  285,  a).  Another  way  is  to  saturate  the  ammoniacal 
solution  with  hydrogen  sulphide,  acidify  with  hydrochloric  acid, 
and  then  precipitate  the  molybdenum  as  sulphide.  In  the  filtrate 
the  phosphoric  acid  is  precipitated  as  magnesium  ammonium 
phosphate  under  the  customary  conditions. 

TUNGSTEN,  W.    At.  Wt.  184.0. 


Tungsten  is  determined  as  its  trioxide, 

If  the  tungsten  is  present  as  ammonium  tungstato.  as  mercu- 
rous  tungstate,  or  as  tungstic  acid,  it  is  readily  changed  by  ignition 
in  the  air  to  yellow  tungsten  trioxide.  Since  the  trioxide  is  not 
volatile  there  is  no  loss  to  be  feared  during  its  ignition.  It  is  even 
advisable  to  heat  the  crucible  finally  over  a  good  Teclu  burner  or 
over  the  blast-lamp. 

If  the  tungsten  is  present  as  alkali  tungstate.  the  tungstic  acid 
may  be  precipitated  as  such,  o^  by  means  of  mercurous  nitrate  as 
mercurous  tungstate;  by  ignition  the  yellow  trioxide  is  obtained 
and  weighed. 

Precipitation  of  Tungstic  Acid. 

The  aqueous  solution  of  the  alkali  tungstate  is  treated  with  an 
equal  volume  of  concentrated  hydrochloric  acid,  evaporated  to 
dryness  on  the  water-bath  and  heated  for  an  hour  in  the  hot 
closet  at  120°.  Tlie  residue  is  moistened  with  a  little  hydro- 
chloric acid,  diluted,  boiled,  filtered  and  washed  with  G  per  cent. 
hydrochloric  acid,  or  with  10  per  cent,  ammonium  nitrate  solu- 
tion. The  yellow  tungstic  acid  is  ignited  and  weighed.  In  this 
way  the  greater  part  of  the  tungstic  acid  is  precipitated,  but 
there  remains  in  the  filtrate  a  weighable  amount  which  can  be 
recovered  by  repeated  evaporations  with  hydrochloric  or  nitric 
acid. 


TUNGSTEN.  289 

Remark.  —  The  reason  why  the  tungstic  acid  is  not  removed 
completely  by  the  first  treatment  with  nitric  or  hydrochloric 
acid  is  that  there  is  always  a  small  amount  of  an  acid  tungstate 
formed  : 

•Na2WO4  +3WO3=  Na2W4Ol3, 

which  it  is  hard  to  decompose  by  acids.  If,  however,  the  filtrate 
is  evaporated  to  dryness  with  ammonia,  then,  according  to 
Philipp,*  the  acid  tungstate  is  converted  into  ordinary  tungstate, 


and  the  latter  is  decomposed  by  the  nitric  acid  treatment. 

This  is  perfectly  true,  but  on  evaporating  with  acid  some  more 
of  the  acid  tungstate  is  formed,  which  can  be  transformed  into 
insoluble  tungstic  acid  only  by  repeatedly  evaporating  the  filtrates 
to  dryness  with  hydrochloric  or  nitric  acid. 

The  washing  of  the  precipitate  with  acid,  or  with  ammonium 
nitrate  solution,  is  necessary  in  order  to  prevent  the  formation 
of  hydrosol,  which  would  result  if  pure  water  were  used. 

Precipitation  of  Tungsten  as  Mercurous  Tungstate,  according  to 

Berzelius.f 

In  the  majority  of  cases  it  is  a  question  of  separating  tungstic 
acid  from  a  solution  obtained  after  fusing  with  sodium  carbonate. 
The  concentrated  solution  is  treated  with  a  few  drops  of  methyl 
orange,  nitric  acid  is  added  until  the  indicator  turns  pink,  the 
solution  is  boiled  to  expel  all  the  carbonic  acid,  allowed  to  cool, 
and  then  an  excess  of  mercurous  nitrate  solution  is  added.  The 
yellow  precipitate  settles  quickly  and  the  supernatant  liquid 
should  appear  clear  as  water.  After  standing  three  or  four  hours, 
the  precipitate  is  filtered  off,  washed  with  water  containing  mer- 
curous nitrate  (5  c.c.  saturated  mercurous  nitrate  solution 
diluted  with  water  to  100  c.c.)  dried,  ignited  in  a  porcelain  crucible 
under  a  good  hood,  using  the  flame  of  a  Bunsen  burner,  and 
weighed  as  W03. 

Remark.  —  The  tungstic  acid  is  quantitatively  precipitated  by 

*  Ber.,  15,  501  (1882). 

t  Jahresber.,  21,  II,  143.     Cf.  O.  v.  der  Pfordten,  Ann.  222,  152  (1883). 


2go  GRAVIMETRIC  A NA LYSIS. 

a  single  treatment  with  mercurous  nitrate.  The  process  is  pref- 
erable to  the  above,  therefore,  when  nothing  else  is  present  which 
precipitates  under  the  same  conditions. 

Precipitation  of  Tungsten  as  Benzidine  Tungstate,  according  to 

G.  v.  Knorre.* 

If  a  neutral  solution  of  sodium  tungstate  is  treated  with 
benzidine  hydrochloride,  a  white  flocculent  precipitate  of  ben- 
zidine  tungstate  is  formed  and  the  precipitate  is  insoluble  in  water 
containing  benzidine  hydrochloride;  when  formed  in  the  cold 
it  is  hard  to  filter  and,  on  being  washed  with  pure  water,  tends 
to  run  through  the  filter.  If  the  precipitate  is  formed  from  a 
hot  solution,  however,  it  comes  down  in  a  more  compact  condi- 
tion and  after  cooling  |  can  be  easily  filtered  and  washed  without 
loss  with  water  containing  benzidine  hydrochloride. 

The  moist  precipitate  is  ignited  in  a  platinum  crucible,  heated 
to  800°  in  an  electric  oven,  and  the  residue  of  WO3  is  weighed. 

The  precipitation  can  also  take  place  satisfactorily  from  a 
cold  solution  if,  before  adding  the  precipitant,  a  little  dilute 
sulphuric  acid  or  alkali  sulphate  is  added  to  the  solution.  In  this 
case  te  mixture  of  crystalline  benzidine  sulphate  and  amorphous 
benzidine  tungstate  is  formed  which  can  be  filtered  after  stand- 
ing five  minutes.  The  benzidine  sulphate  is  entirely  volatile, 
so  that  equally  good  results  are  obtained  by  either  of  the  above 
two  procedures. 

If  the  tungsten  is  present  as  tungstate  after  fusing  with 
sodium  carbonate,  the  melt  is  dissolved  in  water,  a  little  methyl 
orange  is  added  to  the  clear  solution,  then  hydrochloric  acid 
until  the  pink  color  is  obtained,  and  finally  10  c.c.  of  0.1  N. 
sulphuric  acid.  Benzidine  hydrochloride  gives  a  precipitate 
which  can  be  filtered  in  five  minutes.  The  washing  with  dilute 
benzidine  hydrochloride  is  continued  until  the  evaporation 
of  a  few  drops  of  the  filtrate  on  platinum  foil  leaves  no  weighable 
residue.  The  precipitate  is  ignited  wet  as  described  above. 

*Ber.,  38,  783  (1905). 

t  Since  benzidine  tungstate  is  appreciably  soluble  in  hot  water  contain- 
ing benzidine  hydrochloride,  it  is  necessary  in  all  cases  to  postpone  the 
nitration  until  the  solution  is  cold. 


TUNGSTEN.  291 

Preparation  of  the  Benzidine  Solution. 

Twenty  grams  of  commercial  benzidine  are  triturated  in  a 
mortar  with  water,  washed  with  about  400  c.c.  of  water  into  a 
beaker,  treated  with  25  c.c.  hydrochloric  acid  (sp.  gr.  1.2),  heated 
until  solution  is  completed  and  a  brown  liquid  is  formed,  which 
is  filtered  and  diluted  to  a  volume  of  one  liter.  *  Of  this  solu- 
tion, 5.6  c.c.  are  sufficient  to  precipitate  0.1  gm.  of  WO. 

If  the  analysis  is  carried  out  with  the  aid  of  sulphuric  acid, 
it  is  necessary  to  add  at  least  one  cubic  centimeter  of  the  benzi- 
dine solution  for  10  c.c.  of  0.1  X  sulphuric  acid  added. 

Preparation  of  the  Wash  Liquid. 

Ten  c.c.  of  the  above  solution  are  diluted  with  distilled  water 
to  a  volume  of  300  c.c. 

Remark. — The  method  is  excellent.  W.  Kunz  found  0.1028 
gm.,  0.1029  gm.,  0.1026  gm.  and  0.1033  gm.  in  aliquot  parts  of 
a  solution  supposed  to  contain  0.1029  gm.  of  WO3. 

Determination  of  Tungsten  in  Tungsten  Steel. 
(a)  Method  of  G.  v.  Knorre.f 

V.  Knorre  observed  that  on  dissolving  tungsten  steel  in 
hydrochloric  or  dilute  sulphuric  acid,  out  of  contact  with  the  air, 
all  the  tungsten  remains  behind  as  metal,  contaminated  with 
more  or  less  iron.  If  the  finely  divided  tungsten  is  filtered  off, 
it  oxidizes  quickly  in  the  air,  forming  grayish-yellow  tungstic 
acid,  which  on  further  washing  with  water  gives  a  turbid  filtrate; 
the  tungstic  acid  does  not  pass  through  the  filter  if,  instead  of 
using  water,  the  washing  is  accomplished  with  a  dilute  solution 
of  benzidine  hydrochloride.  By  the  treatment  of  the  tungsten 
steel  with  acids,  therefore,  the  greater  part  of  the  iron  is  separated 
from  the  tungsten.  To  complete  the  separation,  the  washed 
precipitate  is  ignited  wet  in  a  platinum  crucible,  the  residue  is 
fused  with  four  times  as  much  sodium  carbonate,  the  melt 

*  Instead  of  dissolving  the  benzidine  in  hydrochloric  acid,  28  gms.  of 
benzidine  hydrochloride  may  be  dissolved  in  1  liter  of  water  to  which  6  c.c. 
of  hydrochloric  acid  have  been  added. 

t  Her.,  38,  783  (1905). 


29 2  GRAVIMETRIC  ANA LYSIS. 

extracted  with  a  water,  the  ferric  oxide  filtered  off,  and  the  tung- 
sten determined  as  described  on  p.  289.  It  is  probable  that 
aluminium-tungsten  alloys  can  be  analyzed  in  a  similar  manner. 

(6)  L.  Welter's  Method.* 

L.  Wolter,  who  has  carefully  studied  the  determination  of 
tungsten  in  tungsten  steel,  found  that  most  of  the  methods 
hitherto  recommended  are  not  practical  because  they  required 
the  metal  to  be  in  too  fine  a  condition.  Now  a  steel  with  a  high 
percentage  of  tungsten  is  so  hard  that  it  is  practically  impossible 
to  get  borings  or  filings  without  the  steel  tools  becoming  badly 
worn;  in  such  cases  the  sample  to  be  analyzed  becomes  con- 
taminated with  foreign  material  and  the  accuracy  of  the  analysis 
is  affected.  For  this  reason  if  it  is  required  to  analyze  a  steel 
with  a  high  percentage  of  tungsten,  it  should  suffice  to  hammer 
the  sample  in  a  steel  mortar  until  a  coarse  powder  is  obtained. 
Unfortunately  a  coarse  powder  dissolves  extremely  slowly  in 
dilute  or  concentrated  acids  and  fusion  with  sodium  carbonate  and 
potassium  nitrate,  or  with  sodium  peroxide,  has  little  effect  upon 
large  particles;  it  is  otherwise  with  a  potassium  bisulphate  fusion, 
whereby  even  large  particles  of  tungsten  steel  are  readily  attacked. 

Procedure. — From  0.2-0.5  gm.  of  the  coarse  material,  which 
may  be  in  the  form  of  coarse  pieces,  is  fused  in  a  40-45  c.c. 
platinum  crucible  with  thirty  times  as  much  potassium  bisul- 
phate. To  prevent  too  violent  effervescence,  at  first  only 
one-third  of  the  bisulphate  is  added  and  the  well-covered,  inclined 
crucible  is  heated  over  a  small  flame  until  white  vapors  are  evolved. 
The  flame  is  then  removed  for  half  a  minute  because  a  fairly 
violent  reaction  is  now  taking  place  within  the  crucible.  In 
fact  when  this  point  is  reached,  a  strong  effervescence  can  be 
heard.  After  cooling  somewhat,  the  remainder  of  the  bisulphate 
is  added  in  two  separate  portions.  The  reaction  now  takes 
place  more  slowly  and  the  temperature  is  raised  until  the  bottom 
of  the  crucible  begins  to  redden  and  dense  white  vapors  are 
evolved.  The  heating  is  now  carefully  continued  until  finally 

*  Chem.  Ztg.,  1910,  2. 


TUNGSTEN.  293 

the  whole  crucible  is  heated  to  redness.  After  about  fifteen 
minutes  the  reaction  will  be  finished.  The  molten  mass  in  the 
crucible  should  show  merely  a  gentle  evolution  of  gas  and  there 
should  be  no  black  particles  of  unattacked  steel  visible.  When 
this  point  is  reached,  the  mass  is  allowed  to  cool,  the  crucible 
and  cover  boiled  with  water,  carefully  washed,  and  the  water 
added  is  measured  so  that  the  volume  of  the  solution  will  amount 
to  about  60  or  75  c.c.  The  liquid,  which  is  somewhat  turbid  with 
tungstic  acid,  is  treated  with  20  c.c.  of  concentrated  hydro- 
chloric acid  and  boiled  until  the  precipitated  tungstic  acid  has 
become  a  pure  yellow.  After  standing  half  an  hour  on  the  water- 
bath,  the  solution  is  filtered  through  a  small  filter  and  the  pre- 
cipitate washed  with  10  per  cent,  ammonium  nitrate  solution. 
The  lemon -yellow  precipitate  is  dissolved  on  the  filter  in  hot, 
dilute  ammonia,  the  solution  caught  in  a  weighed  platinum 
crucible,  and  the  filter  washed  with  ammonium  nitrate.  The 
solution  of  ammonium  tungstate  in  the  crucible  is  evaporated 
to  dryness  on  the  water-bath,  then  covered  with  a  watch-glass, 
and  very  cautiously  heated  over  a  free  flame  until  no  more 
ammonium  salts  are  evolved.  After  igniting  the  residue  over 
the  full  flame  of  a  Bunsen  burner,  it  is  weighed  as  WO3.  In 
this  way  the  greater  part  of  the  tungsten  is  obtained.  The 
filtrate  from  the  tungstic  acid  precipitate  still  contains  tungsten. 
In  order  to  recover  the  later,  it  is  treated  as  described  on  paga  288. 
If  the  steel  contained  silicon,  the  weighed  tungstic  acid  will  always 
contain  silica;  it  is  covered  with  a  few  drops  of  hydrofluoric 
acid,  evaporated  to  dryness,  again  treated  with  hydrofluoric 
acid,  evaporated  once  more  and  finally  ignited  over  the  flame 
of  a  Bunsen  burner.  The  crucible  will  now  contain  no  silica, 
but  only  tungstic  acid. 

Separation  of  Molybdenum  from  Tungsten. 

(a)    The  Sulphuric  Acid  Method. 

This  method,  proposed  by  M.  Ruegenberg  and  E.  F.  Smith,* 
depends  upon  the  fact  that  unignited  molybdic  acid  is  readily  dis- 

*  J.  Am.  Chem.  Soc.,  22,  772. 


294  GRAVIMETRIC  ANALYSIS. 

solved  by  warming  with  sulphuric  acid  (sp.  gr.  1.378),  while  tung- 
stic  acid  is  not. 

W.  Hommel*  tested  this  method  in  the  author's  laboratory,  and 
could  not  obtain  good  results  except  by  digesting  the  moist  oxide3 
with  concentrated  sulphuric  acid,  and  afterward  diluting  with  three 
times  as  much  water. 

Procedure. — (a)  Both  acids  are  present  in  a  moist,  freshly 
precipitated  state. 

The  mixture  is  covered  with  concentrated  sulphuric  acid  in  a 
porcelain  dish  and  heated  over  a  free  flame.  By  this  means,  usu- 
ally a  small  amount  of  the  tungstic  acid  is  oxidized  to  the  blue 
oxide,  so  that  the  yellow  precipitate  of  tungstic  acid  is  tinted  with 
green.  On  adding  one  or  two  drops  of  dilute  nitric  acid,  the  green 
color  disappears  and  the  tungstic  acid  is  of  a  pure  yellow  color. 
After  digesting  for  half  an  hour,  the  separation  is  complete. 
After  cooling,  the  liquid  is  diluted  with  three  times  its  volume  of 
water,  filtered,  washed  with  water  containing  sulphuric  acid,  then 
two  or  three  times  with  alcohol,  ignited  (after  burning  the  filter  by 
itself)  in  a  porcelain  crucible,  and  weighed  as  WO3. 

The  molybdenum  is  precipitated  from  the  filtrate  by  passing 
hydrogen  sulphide  into  the  sulphuric  acid  solution  in  a  pressure- 
flask,  and  the  precipitate  is  treated  as  described  on  p.  286. 

If  only  a  little  sulphuric  acid  is  used  for  the  separation,  the 
filtrate  from  the  tungstic  acid  can  be  evaporated  in  a  platinum 
dish,  the  sulphuric  acid  driven  off  for  the  most  part,  and  the  residue 
washed  into  a  weighed  platinum  crucible  with  ammonia,  and  then 
evaporated,  ignited,  and  weighed.  In  case  large  amounts  of  molyb- 
denum are  present,  however,  it  is  always  safer  to  precipitate  the 
molybdenum  as  sulphide. 

(/?)  Tungsten  and  Molybdenum,  are   Present  in  the  Form  of  their 

Ignited  Oxides. 

These  ignited  oxides  cannot  b?  separated  by  treatment  with 
sulphuric  acid.  According  to  W.  Hommel,  they  can  readily  be 
brought  into  solution  by  heating  for  half  an  hour  on  the  water-bath 
with  concentrated  ammonia  in  a  pressure-flask,  shaking  frequently. 

After   cooling,  the  contents  of  the  flask,  whether  dissolved  or 

*  Inaus;.  Dissert.,  Giessen,  1902. 


TUNGSTEN,  295 

not,  are  washed  into  a  porcelain  dish,  evaporated  to  dryness,  and 
treated  as  described  under  (a). 

It  is  still  better  to  fuse  the  ignited  oxides  with  four  times  as 
much  sodium  carbonate,  and  treat  the  melt  as  described  under  (a). 

(6)  Sublimation  Method* 

If  a  mixture  of  the  trioxides  of  tungsten  and  molybdenum, 
or  of  their  alkali  salts,  is  heated  at  250-270°  C.  in  a  current  of 
dry  hydrochloric  acid,  the  molybdenum  is  volatilized  completely 
as  MoO3.2HCl,  which  collects  on  the  cooler  parts  of  the  tube  as  a 
beautiful,  white,  woolly  sublimate,  while  the  tungsten  trioxide  re- 
mains behind  in  the  boat. 

Procedure. — The  oxides  of  the  two  elements,  or  their  sodium 
salts,  are  weighed  into  a  porcelain  boat,  and  the  latter  is  placed  in  a 
tube  made  of  difficultly  fusible  glass,  of  which  one  end  is  bent  verti- 
cally downward  and  is  connected  with  a  Peligot  tube  containing  a 
little  water.  The  horizontal  arm  of  the  tube  is  passed  through  a 
drying-oven  (to  serve  as  an  air-bath)  (see  Fig.  19,  p.  3c),  and  is 
connected  with  apparatus  for  generating  hydrochloric  acid  gas. 
The  hydrochloric  acid  before  reaching  the  tube  is  slowly  passed 
through  a  flask  containing  concentrated  hydrochloric  acid,  and 
then  through  sulphuric  acid.  As  soon  as  the  temperature  has 
reached  about  200°  C.  the  sublimation  of  the  molybdenum  begins. 
From  time  to  time  the  sublimate  collecting  in  the  tube  is  driven 
toward  the  Peligot  tube  f  by  carefully  heating  with  a  free  flame,  so 
that  it  will  be  possible  to  see  whether  any  more  molybdenum  is  being 
volatilized.  After  heating  for  an  hour  and  one-half  or  two  hours, 
the  operation  is  usually  complete.  The  boat,  now  containing  tung- 
sten trioxide,  or  the  latter  with  sodium  chloride,  is  removed,  and  in 
case  only  the  former  is  present,  it  is  weighed  after  drying  in  a  desic- 
cator over  caustic  potash.  In  case,  however,  sodium  chloride  is 
present  (when  the  tungsten  was  originally  present  as  sodium  tung- 
state)  this  is  removed  by  treatment  with  water,  and  the  filtered 
WO3  is  weighed. 

*  Pochard,  Comptes  rendus,  114,  p.  173,  and  46,  p.  1101. 

t  By  the  absorption  of  the  MoO3.2HCl  in  the  water  of  the  Peligot  tube, 
the  brick-red  acid  chloride,  Mo3O5Cl8,  is  often  formed: 

3[MoO.,.2HCl]  +  2HCI  =  4H2O  +  Mo3O5Cl3. 
This  substance  is  insoluble  in  hydrochloric  acid,  but  readily  soluble  in  nitric  acid 


296  GRA  yiME  TRIC  ANAL  YSIS. 

For  the  determination  of  the  molybdenum,  the  sublimate  in  the 
tube  is  washed  out  by  means  of  water  containing  a  little  nitric  acid, 
and  finally  the  nitric  acid  solution  of  the  entire  sublimate  is  care- 
fully evaporated  to  dryness  in  a  porcelain  dish.  The  residue  is 
dissolved  in  ammonia,  washed  into  a  porcelain  crucible,  evaporated 
to  dryness,  and  changed  to  the  oxide  by  gentle  ignition. 

(c)    The  Tartaric  Acid  Method  of  H.  Rose. 

The  alkali  salts  of  the  two  metals  are  dissolved  in  considerable 
tartaric  acid,  an  excess  of  sulphuric  acid  is  added,  and  the  molyb- 
denum precipitated  according  to  p.  285,  by  hydrogen  sulphide  in 
a  pressure-flask.  The  molybdenum  sulphide  is  filtered  off  and 
changed  by  roasting  in  the  air  to  the  trioxide.  For  the  determi- 
nation of  the  tungsten,  the  tartaric  acid  is  first  destroyed  by  re- 
peated evaporation  with  nitric  acid,  and  the  precipitated  tungstic 
acid  is  finally  filtered  off  and  changed  by  ignition  to  the  trioxide. 

Remark. — This  method  gives  correct  results,  but  is  not  so  satis- 
factory as  the  preceding  one  on  account  of  the  time  consumed  in 
removing  the  tartaric  at 


Analysis  of  Wolframite  (Wolfram). 

The  monoclinic  Wolframite  is  an  isomorphous  mixture  of 
Ferberite,  FeWO4,  and  Hiibnerite,  MnWC>4,  but  often  contains 
small  amounts  of  silicic,  niobic,  tantalic,  and  stannic  acids, 
besides  calcium  and  magnesium. 

About  1  gm.  of  the  extremely  finely-powdered  mineral  is  fused 
with  4  gms.  sodium  carbonate  in  a  platinum  crucible  over  a  good 
burner  for  from  one-half  to  three-quarters  of  an  hour.  After  cool- 
ing, the  melt  is  boiled  with  water  and  filtered.  The  residue  con- 
tains iron,  manganese,  calcium,  and  magnesium,  and  sometimes 
small  amounts  of  niobic  and  tantalic  acids.  The  solution  contains 
all  the  tungstic  acid,  and  silicic  acid  (stannic  acid). 

The  tungstic  acid  is  separated,  as  above  described,  either  by 
evaporating  with  nitric  acid  or  by  precipitating  with  mercurous 
nitrate.  The  precipitate  is  ignited,  and  weighed  as  impure  WO3. 

The  oxide  obtained  in  this  way  almost  always  contains  silicic 
acid  and  sometimes  stannic  acid.  To  remove  the  former  the  resi- 


TUNGSTEN.  297 

due  is  heated  with  hydrofluoric  and  sulphuric  acids,  first  on  the 
water-bath  and  finally  over  a  free  flame,  and  the  residue  is  weighed. 
The  difference  shows  the  amount  of  silicic  acid  present.  Stannic 
acid  is  usually  present  in  such  small  amounts  that  it  is  not  usually 
determined. 

The  separation  of  tungsten  from  tin,  however,  may  be  effected 
(according  to  Rammelsberg)  by  repeated  ignition  with  pure,  dry 
ammonium  chloride.  The  tin  is  volatilized  as  stannic  chloride, 
while  the  tungsten  remains  behind. 

This  last  operation  is  conducted  as  follows :  The  residue  obtained 
after  the  treatment  with  hydrofluoric  acid  is  mixed  with  six  to  eight 
times  as  much  ammonium  chloride,  the  crucible  is  placed  within 
a  second  larger  crucible,*  and  the  latter  is  covered  and  ignited 
until  the  ammonium  chloride  is  completely  expelled.  This  opera- 
tion is  repeated  three  times.  The  inner  crucible  is  then  heated 
with  ready  access  of  air  until  its  contents  become  of  a  pure-yellow 
color,  after  which  it  is  cooled  and  weighed.  The  ignition  with 
ammonium  chloride  and  weighing  of  the  residue  is  repeated  until 
a  constant  weight  is  obtained. 

To  determine  the  iron,  manganese,  calcium  and  magnesium, 
the  insoluble  residue  from  the  sodium  carbonate  fusion  is  dis- 
solved in  hydrochloric  acid,  the  solution  evaporated  to  dry- 
ness,  the  dry  residue  moistened  with  concentrated  hydrochloric 
acid  which  is  allowed  to  act  for  ten  minutes,  then  diluted  with 
water,  boiled  and  any  silica  filtered  off.  In  the  filtrate,  the  iron 
is  separated  from  the  other  metals  by  the  basic  acid  acetate 
process  described  on  p.  152.  The  manganese  is  precipitated 
by  heating  with  bromine  (see  p.  123)  and  the  calcium  and 
magnesium  separated  as  described  on  p.  77. 

Remark. — For  the  determination  of  niobium  and  tantalum,  a 
larger  portion  of  the  substance,  about  5  gms.,  must  be  taken.  The 
finely-powdered  material  is  treated  with  hydrochloric  acid  to  which 
about  one-fourth  of  its  volume  of  nitric  acid  has  been  added,  and 
digested  on  the  water-bath  until  the  residue  is  colored  a  pure  yel- 
low. The  latter  is  filtered  off,  washed  with  water  containing  acid 

*  This  is  to  prevent  any  stannic  oxide  from  collecting  on  the  outside  of 
the  crucible;  the  oxide  is  formed  when  tin  chloride  comes  in  contact  with 
moist  air. 


298  GRAVIMETRIC  ANALYSIS. 

until  the  iron  reaction  can  no  longer  be  obtained,  when  the  residue 
is  taken  up  in  ammonia  and  filtered ;  in  this  way  the  tungstlc  acid 
is  removed.  The  residue  is  usually  dark-colored  and  consists  of 
enclosed  mineral  as  well  as  silicic,  stannic,  niobic,  and  possibly 
tantalic  acids.  It  is  treated  with  aqua  regia  again,  water  is 
added,  and  the  filtered  residue  is  once  more  treated  with  ammonia. 
The  final  residue  is  now  free  from  tungsten;  it  is  ignited,  weighed, 
and  freed  from  silica  by  treatment  with  sulphuric  and  hydrofluoric 
acids.  The  residue  cf  tin  dioxide  and  niobium  pentoxide  (and 
perhaps  tantalum  pentoxide)  is  placed  in  a  porcelain  boat  and 
ignited  in  a  current  of  hydrogen.  The  metallic  tin  is  ex- 
tracted with  hydrochloric  acid,  and  the  residue,  consisting  of 
Nb2O5(Ta2O5),  is  weighed. 

Iron  alloys  rich  in  tungsten  are  acted  upon  only  slowly  by 
aqua  regia.  If,  however,  they  are  first  roasted  in  the  air,  they  are 
comparatively  easily  brought  into  solution.*  Decomposition 
by  a  potassium  bisulphate  fusion  is  still  better  (see  p.  292). 


Analysis  of  Tungsten  Bronzes. 

The  analysis  of  these  alkali  salts  of  complex  tungsten  acids, 
discovered  by  Wohler  in  1824,f  offered  for  a  long  time  considerable 
difficulty  on  account  of  the  fact  that  acids  do  not  decompose  them 
very  readily. 

By  fusion  with  alkalies  in  the  air,  or  better  still  in  the  presence 
of  potassium  nitrate,  the  tungsten  bronzes  can  be  converted 
without  difficulty  into  normal  alkali  tungstate,  and  the  tungsten 
determined  by  one  of  the  methods  already  described.  It  is 
obvious  that  the  alkalies  cannot  be  determined  in  the  same 
sample,  so  that  PhilippJ  proceeded  as  follows: 

The  bronze  is  treated  with  ammoniacal  silver  nitrate  solution, 
whereby  the  W(>2  is  oxidized  to  WOs  with  the  precipitation  of  an 
equivalent  amount  of  silver,  whereas  the  whole  of  the  tungsten 
remains  in  solution  in  the  form  of  alkali  and  ammonium  tungstates. 
In  the  filtrate  obtained  after  filtering  off  the  deposited  silver,  the 

*  Preusser.     Zeit.  f.  anal.  Chem.,  1889,  p.  173. 

t  Pogg.  Ann.,  2,  350. 

%  Berichte,  15,  500  (1882). 


TUNGSTEN.  299 

tungstic  acid  is  precipitated  by  treatment  \vith  nitric  acid  and 
determined  as  WO$.  After  removing  the  excess  of  silver,  by 
precipitating  it  as  the  chloride,  the  filtrate  is  evaporated  to  dryness 
with  the  addition  of  sulphuric  acid,  and  the  alkali  weighed  as 
sulphate. 

Although  the  above  method  affords  satisfactory  results  in  the 
analysis  of  bronzes  containing  comparatively  little  tungsten,  it- 
is  wholly  inadequate  in  the  case  of  bronzes  rich  in  tungsten.  The 
method  of  Brunner,*  which  follows,  is  applicable  in  all  cases. 
It  is  based  upon  the  fact  that  the  bronzes  can  be  transformed  very 
easily,  and  without  loss  of  alkali,  into  normal  tungstates  by  heat- 
ing them  with  ammonium  persulphate,  or  ammonium  acid 
sulphate. 

Procedure. — About  0.5  gm.  of  the  finely-powdered  bronze  is 
treated  in  a  porcelain  crucible  with  2  gms.  of  alkali-free  ammonium 
sulphate  and  2  c.c.  of  concentrated  sulphuric  acid  f  and  carefully 
heated  over  a  very  small  flame.  During  the  heating  the  contents 
of  the  crucible  are  frequently  shaken  about  a  little  by  cautiously 
moving  the  crucible.  The  escape  of  gases  from  the  crucible  soon 
ceases  and  when  sulphuric  acid  vapors  begin  to  be  evolved,  the 
decomposition  of  the  bronze  results. 

In  the  case  of  sodium  and  lithium  bronzes,  the  fused  mass 
appears  greenish,  whereas  with  a  potassium  bronze  the  color  is 
yellowish  white.  After  a  part  of  the  ammonium  sulphate  has 
been  volatilized,  the  mass  in  the  crucible  is  allowed  to  cool, 
another  gram  of  ammonium  sulphate  is  added  and  1  c.c.  of  con- 
centrated sulphuric  acid,  whereupon  the  contents  "of  the  crucible 
are  once  more  heated  as  before  until  sulphuric  acid  fumes  come  off 
thickly;  the  crucible  is  then  allowed  to  cool. 

The  greenish  or  yellowish-white  fusion  is  softened  by  treat- 
ment with  water  and  rinsed  into  a  porcelain  dish.  After  adding 
50  c.c.  of  concentrated  nitric  acid,  the  contents  of  the  evaporating 
dish  are  digested  on  the  water-bath  for  three  or  four  hours,  and 

*  Inaug.  Dissert,  Zurich,  1903. 

t  If  ammonium  persulphate  is  used  the  addition  of  sulphuric  acid  is 
unnecessary.  The  only  objection  to  the  use  of  the  persulphate  lies  in  the 
fact  that  the  commercial  salt  often  contains  some  potassium  persulphate. 


300  GRAVIME TRIG  ANAL  YSIS. 

then,  after  diluting  with  water,  the  residue  of  pure  yellow  tungstic 
acid  is  filtered  off. 

In  order  to  recover  the  small  amount  of  tungstic  acid  remain- 
ing in  the  nitrate,  it  is  evaporated  as  far  as  possible  on  the  water- 
bath,  allowed  to  cool,  diluted  with  a  little  water,  carefully 
treated  with  an  excess  of  ammonia,  again  evaporated  on  the 
water-bath  and  treated  as  described  on  p.  288. 

The  final  filtrate  from  the  tungstic  acid  determination  is 
evaporated  to  dryness,  the  ammonium  salts  expelled  by  ignition 
and  the  residue  of  alkali  sulphate  weighed  (cf.  pp.  41  and  42). 

Separation  of  Tungsten  from  Tin.    Augenot's  Method.* 

One  gram  of  the  finely  powdered  mineral  is  intimately  mixed 
in  an  iron  crucible  with  8  gms.  of  sodium  peroxide,  and  the 
mixture  is  carefully  fused  over  the  Bunsen  burner  for  about 
fifteen  minutes.  After  cooling,  the  melt  is  softened  with  water 
and  transferred  into  a  250-c.c.  flask  (if  lead  is  present,  carbon 
dioxide  is  conducted  into  the  solution  for  a  few  minutes),  the 
solution  is  diluted  to  the  mark,  well  mixed  and  filtered  through  a 
dry  filter,  rejecting  the  first  few  c.c.  of  the  filtrate. 

For  the  determination  of  the  tungstic  acid,  Augenot  proceeds 
according  to  H.  Borntrager.f  100  c.c.  of  the  filtrate  are  allowed 
to  flow  into  a  mixture  of  15  c.c.  concentrated  nitric  acid  and  45  c.c. 
of  concentrated  hydrochloric  acid,  evaporated  to  dryness  in  a 
porcelain  dish,  the  dry  residue  treated  with  50  c.c.  of  a  solution 
1000  c.c.  water,  100  g.  concentrated  hydrochloric  acid  and  100  g. 
ammonium  chloride,  filtered  and  washed  with  the  same  solution. 
The  precipitate,  which,  besides  the  tungstic  acid,  also  contains 
silicic  acid  and  stannic  oxide,  is  dissolved  in  warm  ammonia 
water,  the  filter  well  washed  with  the  same  reagent,  and  the 
ammoniacal  solution  allowed  to  flow  into  15  c.c.  of  con- 
centrated nitric  acid  and  45  c.c.  of  concentrated  hydrochloric 
acid.  This  time  the  liquid  is  evaporated  to  complete  dryness, 
is  taken  up  with  the  above  mixture  of  hydrochloric  acid  and 
ammonium  chloride,  filtered  and  washed  finally  with  dilute  nitric 

*  Z.  angew.  Chem.,  19,  140  (1906). 
fZ.  anal.  Chem.,  39,  361  (1900). 


TUNGSTEN.  301 

acid  (using  preferably  a  Munroe  crucible),  and  ignited  in  an  electric 
oven;  or  in  case  the  latter  is  not  available,  the  crucible  is  placed 
inside  a  larger  one  of  platinum  and  ignited  over  a  Teclu  burner 
to  constant  weight.  The  resulting  tungstic  acid  is  said  to  be 
free  from  silica  and  stannic  oxide. 

For  the  determination  of  the  tin,  a  second  100  c.c.  of  the  original 
alkaline  solution  is  used.  It  is  treated  with  45  c.c.  of  concentrated 
hydrochloric  acid,  whereby  tungstic  acid  and  stannic  acid  are 
precipitated.  At  this  point  2  or  3  gms.  of  pure  zinc  are  added 
whereby  the  tungstic  acid  is  changed  to  the  blue  oxide  and  the 
stannic  acid  is  reduced  to  metallic  tin.  The  mixture  is  allowed 
to  stand  quietly  for  an  hour  at  a  temperature  between  50° 
and  60°.  The  tin  then  goes  into  solution  as  stannous  chloride, 
and  the  greater  part  of  the  tungsten  remains  undissolved  in  the 
form  of  the  blue  oxide.  The  latter  is  filtered  off,  and  washed. 
In  this  way  the  whole  of  the  tin  is  obtained  in  an  acid  solution, 
in  the  presence  of  a  small  amount  of  tungsten,  which  does  no 
harm.  The  blue  oxide  on  the  filter,  however,  is  dissolved  in  hot, 
dilute  ammonia  solution  in  order  to  make  sure  that  it  contains 
no  trace  of  metallic  tin.  If  this  should  be  the  case,  the  small 
particle  of  tin  is  dissolved  in  a  little  hydrochloric  acid  and  the 
resulting  solution  added  to  the  main  solution  of  the  tin. 

The  solution  is  now  diluted  with  water  and  the  tin  precipitated 
as  stannous  sulphide  by  the  introduction  of  hydrogen  sulphide 
gas.  The  precipitate  is  filtered,  ignited  in  a  porcelain  crucible 
and  weighed  as  SnC>2  (see  p.  233).  Or,  the  moist  precipitate  of 
stannous  sulphide  may  be  dissolved  in  potassium  hydroxide 
solution  and  the  tin  determined  by  electrolysis  (see  p.  234). 

According  to  Donath  and  Miiller  *  a  mixture  of  stannic  oxide 
and  tungstic  acid  may  be  separated  as  follows:  The  mixture  is 
ignited  with  powdered  zinc  for  fifteen  minutes  in  a  covered 
porcelain  crucible.  After  cooling,  the  spongy  contents  of  the 
crucible  are  heated  in  a  beaker  with  hydrochloric  acid  (1:2)  until 
there  is  no  more  evolution  of  hydrogen  perceptible.  The  solution 
is  then  allowed  to  cool  somewhat  and  some  powdered  potassium 


*  Wiener  Monatshefte,  8,  647  (1887). 


302  GRAVIMETRIC  ANALYSIS. 

chlorate  is  added  little  by  little  until  the  blue  tungsten  oxide  is 
completely  transformed  to  yellow  tungstic  acid,  when  the  liquid 
no  longer  shows  any  blue  tinge.  It  is  diluted  with  one  and  one- 
half  times  as  much  water,  and  after  standing  twenty-four  hours 
the  tungstic  acid  is  filtered  off,  washed  first  with  dilute  nitric  acid 
and  then  with  one  per  cent,  solution  of  ammonium  nitrate,  dried, 
ignited,  and  weighed  as  WO3. 

The  tin  is  determined  in  the  filtrate  as  above. 


Separation  of  Tungstic  Acid  from  Silica. 

When  a  mixture  of  tungstic  and  silicic  acid  is  at  hand,  such  as 
is  obtained  by  evaporation  with  nitric  acid,  the  silicic  acid  may  be 
removed  by  treating  the  ignited  residue  with  hydrofluoric  acid 
and  a  large  excess  of  sulphuric  acid.  The  separation  does  not 
succeed,  however,  in  the  mixture  of  oxides  as  obtained  after 
precipitation  with  mercurous  nitrate;  for  in  this  case  the  silicic 
acid  is  so  enveloped  with  tungstic  acid  that  some  of  the  former  is 
not  volatilized  as  fluoride.  In  such  cases,  as  Friedheim  *  has 
shown,  excellent  results  may  be  obtained  by  the 

Method  of  Perillon.-f 

The  mixture  of  the  ignited  oxides  is  introduced  into  a  plati- 
num boat  and  heated  to  redness  in  a  stream  of  dry  hydrogen 
chloride.  Thereby  the  tungsten  is  volatilized,  probably  as  an 
acid  chloride,  which  can  be  recovered  in  a  receiver  containing 
dilute  hydrochloric  acid;  the  silica  remains  behind  in  the  com- 
bustion tube. 

Frequently  the  tungsten  is  reduced  to  a  blue  lower  oxide,  which 
is  not  volatile  in  a  current  of  hydrochloric  acid  gas.  In  such  cases, 
after  the  apparatus  has  been  allowed  to  cool,  the  hydrogen 
chloride  is  replaced  by  air,  and  the  contents  of  the  boat  are  heated 
In  a  current  of  air.  The  tube  is  again  allowed  to  cool,  the  air 
replaced  by  hydrogen  chloride,  and  the  tube  once  more  heated 

*  Z.  anorg.  Chem.,  45,  398  (1905). 
f  Bull.  soc.  1'industrie  miner.,  1884. 


VANADIUM.  3°3 

to  redness.  The  process  is  repeated  if  necessary  until  finally  a 
residue  of  pure  white  8162  is  obtained.  The  tungsten  acid 
chloride  in  the  receiver  is  evaporated  to  dryness  with  nitric  acid, 
and  the  precipitated  WO3  is  filtered,  ignited  and  weighed.  Unless 
the  current  of  hydrogen  chloride  gas  is  perfectly  free  from  air,  the 
platinum  boat  will  be  strongly  attacked. 

The  bisulphate  method  of  effecting  the  separation  is  less 
accurate  (See  Vol.  I). 


VANADIUM,  V.    At.  Wt.  51.2. 

Vanadium  is  determined  as  the  pentoxide,  V2O5. 

The  most  convenient  method  for  determining  vanadium  is  a 
volumetric  process,  and  will  be  discussed  in  the  chapter  on  volu- 
metric analysis. 

If  vanadium  is  present  as  ammonium  or  mercurous  vanadate, 
it  can  be  easily  changed  to  the  pentoxide  by  ignition;  the  latter 
is  a  reddish-brown  fusible  substance  which  solidifies  as  a  radiating, 
crystalline  mass.  If  vanadic  sulphide  is  carefully  roasted  in  the 
air,  it  is  also  changed  quantitatively  to  the  pentoxide. 

In  the  analysis  of  most  minerals  containing  vanadium,  the 
vanadium  is  separated  from  the  other  metals  present  by  fusing  with 
a  mixture  of  six  parts  sodium  carbonate  and  one  part  potassium 
nitrate.  After  cooling,  the  melt  is  extracted  with  water,  whereby 
the  sodium  vanadate  goes  into  solution  while  most  of  the  metals 
are  left  behind  in  the  form  of  oxides  or  carbonates.  If  phosphorus, 
arsenic  (molybdenum,  tungsten),  and  chromium  are  present, 
these  elements  also  dissolve  on  treating  the  melt  with  water  in 
the  form  of  the  sodium  salts  of  the  corresponding  acids. 

In  practice,  therefore,  the  vanadium  is  usually  met  with  as 
the  sodium  salt  of  vanadic  acid,  and  it  is  a  matter  of  separating  it 
from  the  aqueous  solution  obtained  after  fusing  with  sodium 
carbonate  and  potassium  nitrate,  and  of  separating  it  from  the 
other  acids  which  are  likely  to  accompany  it  (phosphoric,  arsenic, 
and  chromic  acids). 


304  GRAVIMETRIC  ANALYSIS.  ' 

Precipitation  of  Vanadic  Acid  from  the  Solution  of 
Sodium  Vanadate. 

There  are  two  good  methods  for  the  separation  of  vanadic 
acid  from  a  solution  of  an  alkali  vanadate:  the  Rose  method, 
according  to  which  the  vanadium  is  precipitated  as  mercurous 
vanadate,  and  that  of  Roscoe,  by  which  it  is  precipitated  as  lead 
vanadate.  The  Berzelius-Hauer  method,*  in  which  the  vanadium 
is  precipitated  as  ammonium  metavanadate,  was  found  by  Holver- 
scheidt  f  to  give  always  too  low  results,  but  Gooch  and  Gilbert,  { 
as  well  as  E.  Champagne,  §  obtained  correct  results  by  working 
in  an  ammoniacal  solution,  which  was  saturated  with  ammonium 
chloride. 

1.   The  Mercurous  Nitrate  Method  of  Rose. 

The  alkaline  solution  is  nearly  neutralized  with  nitric  acid 
and  to  it  is  added,  drop  by  drop,  a  nearly  neutral  solution  of  mer- 
curous nitrate  II  until,  after  the  precipitate  has  settled,  a  further 
addition  of  the  reagent  causes  no  precipitation.  The  liquid  is  then 
heated  to  boiling,  the  gray-colored  precipitate  is  allowed  to  settle 
and  is  filtered  and  washed  with  water  to  which  a  few  drops  of 
mercurous  nitrate  solution  have  been  added.  The  precipitate  is 
dried,  ignited  under  a  good  hood,  and  the  residue  of  V2O5  is  weighed. 

Remark. — In  neutralizing  the  alkaline  solution  of  the  vanadate 
with  nitric  acid,  the  solution  must  on  no  account  be  made  acid, 
for  in  this  case  nitrous  acid  (from  the  nitrate  fusion)  will  be  set 
free  and  the  latter  reduces  some  of  the  vanadate  to  a  vanadyl 
salt  and  the  latter  is  not  precipitated  by  mercurous  nitrate.  In 
order  to  avoid  passing  over  the  neutral  point,  Hillebrand  recom- 
mends fusing  with  a  weighed  amount  of  sodium  carbonate  and 
adding  the  amount  of  nitric  acid  that  has  been  found  necessary 
by  a  blank  test  to  neutralize  this.  The  method  gives  good  results. 

*  Pogg.  Ann.,  22,  54  and  J.  prakt.  Chem.,  69,  388. 

t  Dissertation,   Berlin,   1890. 

J  Z.  anorg.  Chem.,  32,  175  (1902). 

§  Berichte,  1903,  3164. 

II  The  mercurous  nitrate  used  should  leave  no  residue  on  being  ignited. 


VAHADIUM.  3°5 

2.   The  Lead  Acetate  Method  of  Roscoe* 

Principle. — If  a  solution  weakly  acid  with  acetic  acid  is  treated 
with  lead  acetate,  orange-yellow  lead  vanadate  is  quantitatively 
precipitated.  The  lead  vanadate,  however,  does  not  possess  a 
constant  composition,  so  that  the  amount  of  vanadium  present 
cannot  be  determined  by  weighing  the  precipitate.  After  being 
washed,  it  is  dissolved  in  as  little  nitric  acid  as  possible,  the  lead 
precipitated  as  lead  sulphate,  and  the  vanadium  determined  in 
the  filtrate  by  evaporating  the  latter,  driving  off  the  excess  of  sul- 
phuric acid,  and  weighing  the  residual  V2O5. 

Procedure. — The  solution  from  the  sodium  carbonate  and 
potassium  nitrate  fusion  is  nearly  neutralized  as  before  with  nitric 
acid,  an  excess  of  lead  acetate  solution  is  stirred  into  it,  when  the 
voluminous  precipitate  will  collect  together,  rapidly  settle  to  the 
bottom  of  the  beaker,  and  the  supernatant  liquid  will  appear 
absolutely  clear.  The  precipitate  is  at  first  orange-colored,  but  OD 
standing  it  gradually  becomes  yellow  and  finally  perfectly  white 
It  is  filtered  off,  washed  with  water  containing  acetic  acid  until 
half  a  cubic  centimeter  of  the  filtrate  will  leave  no  residue  on 
evaporation.  The  precipitate  is  now  washed  into  a  porcelain 
dish,  the  part  remaining  on  the  filter  is  dissolved  in  as  little  as 
possible  of  hot  dilute  nitric  acid,  and  the  solution  added  to  the 
main  part  of  the  precipitate,  to  which  enough  nitric  acid  is  added 
to  completely  dissolve  it.  An  excess  of  sulphuric  acid  is  added 
to  the  solution,  and  it  is  evaporated  on  the  water-bath  as  far  as 
possible,  finally  heating  over  the  free  flame  until  dense  fumes  of 
sulphuric  acid  are  evolved.  After  cooling,  from  50  to  100  c.c.  of 
water  are  added,  the  lead  sulphate  is  filtered  off  and  washed  with 
dilute  sulphuric  acid  until  1  c.c.  of  the  filtrate  will  show  no  yellow 
color  with  hydrogen  peroxide.  The  lead  sulphate  should  be 
white  and  free  from  vanadium ;  it  will  be  so  provided  enough  sul- 
phuric acid  was  used  and  the  mass  was  not  heated  until  absolutely 
dry  before  diluting  with  water.  The  filtrate  containing  all  the 
vanadic  acid  is  evaporated  in  a  porcelain  dish  to  a  small  volume, 
transferred  to  a  weighed  platinum  crucible,  evaporated  further 
on  the  water-bath,  and  finally  in  an  air-bath  until  all  the  sulphuric 

*  Ann.  Chem.  Pharm..  Suppl.,  8.  102  (1872). 


306  GRAVIMETRIC   ANALYSIS. 

acid  is  removed.  The  open  crucible  is  then  ignited  for  some  time  * 
at  a  faint-red  heat  and  its  contents  finally  weighed  as  V205. 

Remark. — Instead  of  decomposing  the  lead  vanadate  by  means 
of  sulphuric  acid,  Holverscheidt  recommends  precipitating  the  lead 
as  sulphide-  by  means  of  hydrogen  sulphide  and  determining  the 
vanadium  in  the  filtrate.  For  this  purpose  the  blue-colored  filtrate 
from  the  lead  sulphide  precipitate  (which  contains  some  vanadyl 
salt)  is  boiled  to  expel  the  excess  of  hydrbgen  sulphide  and  the 
deposited  sulphur  is  filtered  off.  A  few  drops  of  nitric  acid  are 
added,  the  solution  evaporated  to  dryness,  and  the  reddish-yellow 
hydrate  of  vanadic  acid  is  changed  by  gentle  ignition  into  the 
pentoxide  of  vanadium. 

Lead  may  also  be  separated  from  the  vanadic  acid  as  lead 
chloride.  In  this  case  the  procedure  recommended  in  the  analysis 
of  vanadinite  (p.  308)  is  followed. 

The  separation  of  vanadium  as  the  sulphide  by  acidifying  a 
solution  of  an  alkali  vanadate  that  has  been  treated  with  an  excess 
of  ammonium  sulphide  is  not  admissible,  for  only  a  part  of  the 
vanadium  is  precipitated  as  the  brown  sulphide,  the  rest  remaining 
in  solution  in  the  form  of  vanadyl  salt.  H.  Rose  called  the  attention 
of  chemists  to  the  inaccuracy  of  this  method,  but  this  has  not  pre- 
vented its  being  recommended  in  some  of  the  most  recent  works 
on  analytical  chemistry.  The  author  has  carefully  tested  the 
method  and  found  it  useless. 

Separation  of  Vanadium  from  Arsenic  Acid. 

Most  minerals  containing  vanadium  also  contain  arsenic, 
and  after  extracting  the  melt,  obtained  by  fusion  with  sodium 
carbonate  and  nitre,  with  water,  both  elements  go  into  solution. 
For  their  separation,  the  solution  is  acidified  with  dilute  sulphuric 
acid  and  sulphur  dioxide  is  passed  into  the  hot  liquid,  whereby  the 
vanadic  acid  is  reduced  to  vanadyl  salt  and  the  arsenic  to  arsenious 
acid.  After  boiling  to  remove  the  excess  of  sulphur  dioxide,  the 
solution  is  saturated  with  hydrogen  sulphide  and  the  precipitate 
of  arsenic  trisulphide  is  filtered  off.  The  filtrate  is  freed  from 

*  On  expelling  the  sulphuric  acid,  there  is  finally  formed  some  green  and 
brown  crystals  of  a  compound  of  vanadic  acid  with  sulphuric  acid;  these  are 
decomposed  only  at  a  faint-red  heat. 


VANADIUM.  3°7 

hydrogen  sulphide  by  boiling,  evaporated  with  nitric  acid  in  order 
to  form  vanadic  acid  again,  the  solution  is  then  made  alkaline  with 
sodium  carbonate,  and  the  vanadium  determined  by  one  of  the 
above  methods. 

Separation  of  Vanadium  from  Phosphoric  Acid. 

If  the  solution  of  the  soda-nitre  fusion  contains  phosphoric  as 
well  as  vanadic  acid,  both  are  precipitated  by  mercurous  nitrate, 
the  precipitate  washed  with  dilute  mercurous  nitrate  solution  and 
weighed.  In  this  way  the  sum  of  the  V2O5+P2O5  is  obtained. 
When  P205  is  present  the  V2O5  does  not  melt,  but  only  sinters 
together.  The  ignited  oxides  are  fused  with  an  equal  weight  of 
sodium  carbonate,  the  melt  is  dissolved  in  water,  the  solution  made 
acid  with  sulphuric  acid  and  boiled  with  sulphurous  acid  in  order 
to  reduce  the  vanadic  acid  to  vanadyl  sulphate;  the  latter  will  be 
recognized  by  the  pure  blue  color  which  the  solution  assumes. 
Carbon  dioxide  is  passed  into  the  boiling  solution  until  the 
excess  of  sulphurous  acid  is  removed,  when  it  is  allowed  to 
cool.  To  the  cold  solution,  now  about  100  c.c.  '  i  vo'.ume,  200  c.c. 
of  a  75  per  cent,  solution  of  ammonium  nitrate  and  50  c.c.  of 
ammonium  molybdate  solution  are  added  (cf.  Remark,  below),  the 
solution  is  warmed  to  about  60°  C.,  set  aside  and  allowed  to  stand 
for  one  hour.  The  clear  liquid  is  then  decanted  through  a  filter, 
washed  three  times  by  decantation  with  50  c.c.  of  the  proper  wash 
liquid  (see  p.  437),  after  which  the  precipitate  is  dissolved  by 
passing  10  c.c.  of  8  per  cent,  ammonia  through  the  filter  into  the 
the  beaker  containing  the  bulk  of  the  precipitate  and  the  filter 
is  finally  washed  with  30  c.c.  of  water.  To  this  solution  20  c.c.  of 
a  34  per  cent,  ammonium  nitrate  solution  and  1  c.c.  more  of  ammo- 
nium molybdate  are  added,  the  solution  heated  until  it  begins  to 
boil,  and  the  phosphoric  acid  repreci  pita  ted  by  the  addition  of 
20  c.c.  of  hot  25  per  cent,  nitric  acid.  The  phosphoric  acid  is 
determined  by  the  method  of  Woy  (page  440).  The  amount  df 
phosphoric  acid  found  is  deducted  from  the  sum  of  the  oxides  and 
the  difference  gives  the  amount  of  V205. 

Remark. — A.  Gressly  tested  this  method  in  the  author's  labora- 
tory and  made  the  interesting  observation  that  if  about  0  15  gm. 
of  V2O5  was  present  with  0.1  gm.  P2O5,  no  trace  of  the  latter  could 


308  GRAVIMETRIC  ANA LYSIS. 

be  detected  according  to  the  procedure  of  Woy,  not  even  on  boiling 
the  solution.  On  the  other  hand,  an  immediate  precipitation  was 
produced  if  a  stronger  solution  of  ammonium  molybdate  were  used 
(75  gms.  of  ammonium  molybdate  dissolved  in  500  c.c.  of  water) 
and  this  solution  poured  into  500  c.c.  of  nitric  acid,  sp.  gr.  1:2. 

The  above-described  separation  gives  correct  results  only  when 
the  vanadium  is  present  as  vanadyl  sulphate;  if  vanadic  acid  is 
present  it  is  precipitated  with  the  phosphoric  acid.  If  the  solu- 
tion is  allowed  to  stand  after  the  addition  of  the  ammonium  molyb- 
date, the  vanadyl  sulphate  is  gradually  oxidized  to  vanadic  acid; 
the  precipitate  therefore  should  not  be  allowed  to  stand  long 
before  filtering. 

Separation  of  Vanadium  from  Molybdenum. 

The  solution  containing  the  alkali  salts  of  the  two  acids  is 
acidified  with  sulphuric  acid  and  the  molybdenum  precipitated 
in  a  pressure-flask  by  means  of  hydrogen  sulphide,  and  the  pre- 
cipitate filtered  off  through  a  Gooch  crucible,  as  described  on 
pp.  285  and  286) ,  and  weighed  as  Mo03.  After  removing  the  excess 
of  hydrogen  sulphide  from  the  filtrate,  the  vanadium  is  oxidized 
with  nitric  acid  and  determined  as  described  under  the  Separation  of 
Vanadium  from  Arsenic  Acid,  p.  306. 

Analysis  of  Vanadinite,  (Pb5(  VO4)3C1). 

Besides  lead,  vanadic  acid,  and  chlorine,  the  mineral  often  con- 
tains arsenic  and  phosphoric  acids. 

Determination  of  Chlorine. 

About  1  gm.  of  the  finely  powdered  mineral  is  dissolved  in 
dilute  nitric  acid  (in  order  to  avoid  loss  of  chlorine  the  solution  is 
kept  cold),  and  the  solution  is  diluted  with  considerable  water. 
The  chlorine  is  precipitated  with  silver  nitrate  and  the  weight  of 
the  silver  chloride  determined  as  described  on  p.  317. 

Determination  o]  Lead. 

The  filtrate  from  the  silver  chloride  is  treated  with  hydrochloric 
acid  in  order  to  precipitate  the  excess  of  silver,  filtered,  washed 
with  hot  water,  and  the  solution  thus  freed  from  silver  is  evaporated 


VANADIUM.  309 

to  dryness  to  remove  the  nitric  acid.  The  dry  mass  is  moistened 
with  hydrochloric  acid,  95  per  cent,  alcohol  is  added  in  order 
to  precipitate  completely  the  lead  chloride,  and  the  latter  is 
filtered  through  a  Gooch  crucible,  washed  with  alcohol,  dried 
at  110°  C.  and  weighed  as  PbCl2. 

Determination  of  Vanadium,  Phosphoric  Acid,  and  Arsenic. 

The  filtrate  from  the  lead  chloride  contains  the  vanadium 
as  vanadyl  salt.  The  alcohol  is  driven  off  by  careful  heating 
on  the  water-bath,  nitric  acid  is  added  to  the  solution,  and 
the  latter  is  repeatedly  evaporated  in  order  to  oxidize  the  blue 
vanadyl  salt  to  brown  vanadium  pentoxide.  The  dry  mass  is 
washed  by  means  of  as  little  water  as  possible  into  a  weighed  plati- 
num crucible,  the  residue  adhering  to  the  sides  of  the  dish  is  dis- 
solved in  a  little  ammonia  and  added  to  it.  The  crucible  is  then 
heated,  at  first  gradually  to  expel  the  ammonia,  and  afterward 
more  strongly  with  ready  access  of  air  (uncovered  crucible)  until 
the  reduced,  dark-colored  oxide  is  changed  over  to  the  brownish- 
red  pentoxide.  The  temperature  is  then  raised  until  the  vanadium 
oxide  begins  to  melt.  If  phosphoric  acid  is  present,  its  anhydride 
is  weighed  with  the  vanadium  and  the  amount  of  P2O5  is  deter- 
mined as  described  on  p.  307;  this  amount  is  deducted  from  the 
weight  of  the  two  oxides. 

The  determination  of  arsenic  is  best  carried  out  in  a  separate 
portion.  For  this  purpose  the  mineral  is  dissolved  in  hot  nitric 
acid,  the  greater  part  of  the  excess  of  the  acid  is  removed  by 
evaporation,  the  solution  is  diluted  with  water,  and  the  lead 
precipitated  by  the  addition  of  sulphuric  acid.  From  the  filtrate, 
the  last  portions  of  lead  and  arsenic  are  precipitated  by  hydro- 
gen sulphide,  after  previous  reduction  with  sulphurous  acid. 
The  filtered  precipitate  is  digested  with  sodium  sulphide  and  the 
arsenic  precipitated  from  the  solution  thus  obtained  by  the  addi- 
tion of  hydrochloric  acid.  The  arsenic  sulphide  is  then  changed  to 
arsenic  acid,  preferably  by  dissolving  in  ammoniacal  hydrogen 
peroxide,  and  is  precipitated  as  magnesium  ammonium  arsenate 
and  determined  according  to  p.  206. 


310  GRA yiMETRIC  ANALYSIS. 

Determination  of  Vanadium  and  Chromium  in  Iron  Ores 
and  Rocks. 

Method  of  W.  F.  Hillebrand* 

As  vanadium  often  occurs  in  many  ores  of  iron  and  in  rocks, 
although  in  very  small  amounts,  it  is  often  of  interest  and  of  im- 
portance to  be  able  to  determine  it  in  such  cases.  For  this  purpose 
it  is  best- to  proceed  as  follows: 

Five  gms.  of  the  finely  powdered  mineral  are  mixed  with  20  gms. 
sodium  carbonate  and  3  gms.  potassium  nitrate  and  fused  over  the 
blast-lamp.  The  green  fusion  (containing  manganese)  is  extracted 
with  water,  a  few  drops  of  alcohol  are  added  to  reduce  the  man- 
ganese, and  the  residue  is  filtered  off.f 

The  aqueous  solution  contains  sodium  vanadate  and  often  phos- 
phate, chromate,  molybdate,  aluminate,  and  considerable  silicate  as 
well.  First  of  all,  the  aluminium  and  the  greater  part  of  the  silicic 
acid  are  removed  by  nearly  neutralizing  the  alkaline  solution  with 
nitric  acid.J  It  is  very  important  that  the  solution  is  not  made 
acid  at  this  point  on  account  of  the  reducing  action  of  the  nitrous 
acid  set  free  from  the  nitrite  formed  during  the  fusion.  The 
almost  neutral  solution  is  evaporated  nearly  to  dryness,  taken  up 
in  water,  and  filtered. § 

The  cold  alkaline  solution  is  now  treated  with  an  almost  neutral 
solution  of  mercurous  nitrate  until  no  further  precipitation  takes 
place.  The  somewhat  voluminous  precipitate  contains,  besides 
mercurous  carbonate,  also  its  chromate,  vanadate,  molybdatc, 
arsenate,  and  phosphate,  if  the  corresponding  elements  are  present 
in  the  mineral.  If  the  precipitate  is  too  bulky,  a  little  nitric  acid 
is  cautiously  added,  and  then  a  drop  of  mercurous  nitrate  in 
order  to  see  if  the  precipitation  is  complete. 

*  See  U.  S.  Geol.  Survey  Bull.,  411. 

f  If  considerable  vanadium  is  present,  the  insoluble  residue  always  con- 
tains vanadium  and  must  be  fused  with  soda-nitre  again. 

J  The  amount  of  nitric  acid  necessary  to  neutralize  20  gms.  of  soda  is 
determined  by  a  blank  test. 

§  The  residue  of  aluminium  hydroxide  and  silicic  acid  almost  never  con- 
tains vanadium,  but  contains  chromium.  If  it  is  desired  to  determine  the 
latter,  the  residue  is  evaporated  to  dryness  with  hydrofluoric  and  sulphuric 
acids,  the  dry  mass  is  fused  with  soda  and  nitre  again,  and  the  aqueous  solu- 
tion of  the  melt  added  to  the  main  solution. 


VAKAD1UM.  311 

The  liquid  is  heated  to  boiling,  filtered,  the  precipitate  washed 
with  water  containing  ammonium  nitrate,  dried,  and  ignited  in  a 
platinum  crucible  at  as  low  a  temperature  as  possible.  The  ignited 
residue  is  fused  with  a  little  sodium  carbonate,  the  melt  extracted 
with  water  and,  if  yellow-colored,  it  is  filtered  into  a  25-c.c.  flask 
and  the  amount  of  chromium  determined  colorimetrically  by  com- 
paring its  color  with  a  carefully  prepared  solution  of  potassium 
chromate. 

The  solution  is  then  slightly  acidified  with  sulphuric  acid,  and 
the  molybdenum,  arsenic,  and  traces  of  platinum  precipitated  by 
hydrogen  sulphide  in  a  pressure-flask.  The  precipitated  sulphides 
are  filtered  off,  the  filter  together  with  the  precipitate  is  carefully 
ignited  in  a  porcelain  crucible,  a  few  drops  of  sulphuric  acid  are 
added  and  the  crucible  heated  again  until  the  acid  is  almost  com- 
pletely removed.  On  cooling  the  mass  is  colored  a  beautiful  blue  if 
molybdenum  is  present. 

The  filtrate  from  the  above  precipitate  is  freed  from  the  ex- 
cess of  the  hydrogen  sulphide  by  boiling  and  passing  a  stream  of 
carbon  dioxide  through  it,  and  the  hot  solution  is  then  titrated 

N 

to  a  pink  color  with  ~^r  potassium  permanganate  solution  (cf  .  Vol. 

Anal.).  In  order  to  obtain  absolutely  accurate  results,  the  solu- 
tion is  now  reduced  by  means  of  sulphur  dioxide  and  the  titration 
repeated.  The  mean  of  the  two  experiments  gives  the  vanadium 
value. 


This  method  gives  correct  results  only  when  the  amount  of 
chromium  present  is  very  small,  which  is  true  in  the  majority  of 
cases. 

In  case  more  than  5  mgm.  of  chromium  are  present  a  correction 
must  be  made,  for  a  measurable  amount  of  permanganate  is 
used  up  in  oxidizing  the  chromium.  This  is  determined  by  tak- 
ing a  solution  containing  the  same  amount  of  chromate  as  the 
analyzed  solution,  reducing  it  with  sulphurous  acid,  and  titrat- 
ing with  permanganate.  The  amount  of  permanganate  now  used 
must  be  subtracted  from  the  amount  used  in  the  analysis,  and  from 
the  difference  the  amount  of  vanadium  present  is  calculated. 


312  GRAVIMETRIC  ANALYSIS. 

Determination  of  Vanadium  and  Chromium  in  Pig  Iron. 

From  5  to  10  gms.  of  the  iron  are  dissolved  in  dilute  hydro- 
chloric acid  in  a  flask,  meanwhile  passing  a  current  of  carbon  diox- 
ide through  the  liquid.  For  each  gram  of  iron  taken,  5  c.c.  of 
hydrochloric  acid,  sp.  gr.  1.12  and  10  c.c.  of  water  are  used.  The 
solution  is  hastened  by  warming,  finally  boiling  it  until  there  is  no 
more  evolution  of  gas.  It  is  now  diluted  with  an  equal  volume  of 
water,  allowed  to  cool,  and,  without  filtering  off  the  slight  residue, 
an  excess  of  barium  carbonate  is  added  and  the  mixture  allowed  to 
stand  for  twenty-four  hours  with  frequent  shaking.  The  residue 
is  filtered  off,  rapidly  washed  with  cold  water,  dried  and  ignited  in 
a  platinum  crucible  in  order  to  burn  off  the  carbon.  Five  parts  of 
sodium  carbonate  and  one  part  of  nitre  are  then  added  to  the  con- 
tents of  the  crucible  and  the  mixture  is  heated  to  quiet  fusion. 

The  fusion  is  leached  with  water  and  the  solution  thus  obtained 
contains  all  the  chromium  as  chromate,  and  the  vanadium  as  vana- 
date  in  the  presence  of  alkali  silicate  and  phosphate.  The  aque- 
ous solution  is  now  nearly  neutralized  with  nitric  acid,  being  care- 
ful not  to  make  the  solution  acid  as  the  nitrous  acid  set  free  will 
reduce  some  of  the  vanadium  and  chromium.  The  barely  alkaline 
solution  is  then  treated  with  an  almost  neutral  solution  of  mercu- 
rous  nitrate  until  no  further  precipitation  takes  place,  the  liquid  is 
heated  to  boiling,  filtered  and  washed  with  water  containing  a  little 
mercurous  nitrate.  After  drying,  as  much  of  the  precipitate  as 
possible  is  transferred  to  a  platinum  crucible,  the  filter  burned  by 
itself  and  its  ash  added  to  the  main  portion  of  the  precipitate,  which 
is  ignited  to  remove  the  mercury.  The  residue  is  fused  with  a  little 
sodium  carbonate,  the  melt  extracted  with  water,  and  the  solution 
filtered.  In  case  the  filtrate  is  colored  yellow,  the  amount  of 
chromium  present  is  determined  colorimetrically  *  by  placing  the 
solution  in  a  graduated  cylinder  and  comparing  its  color  with  a  potas- 

*  The  colorimetric  determination  is  only  suitable  when  small  amounts  are 
present.  When  considerable  chromium  is  present  (chrome  steel)  the  results 
obtained  by  the  colorimetric  determination  may  be  as  much  as  2  per  cent,  too 
high.  In  such  cases  the  chromic  acid  is  titrated  with  ferrous  sulphate  (see 
Vol.  Anal.). 


VANADIUM.  313 

sium-chromate  solution  containing  a  known  amount  of  chromium. 
The  solution  is  then  acidified  with  sulphuric  acid  and  hydrogen 
sulphide  is  conducted  into  the  boiling  solution  to  precipitate  ar- 
senic and  platinum.  After  filtering,  the  hydrogen  sulphide  is  re- 

\ 

moved  and  the  hot  solution  titrated  with  ^--  potassium  perman- 
ganate solution  (cf.  Volumetric  Analysis). 


Determination    of    Vanadium,    Molybdenum,    Chromium,    and 
Nickel  in  Steel.     Method  of  A.  A.  Blair.* 

The  method  is  given  in  this  edition  of  the  book  as  illustrating 
the  removal  of  the  greater  part  of  the  iron  from  a  ferric  chloride 
solution  by  shaking  with  ether.  Although  it  is  difficult  to 
effect  a  perfect  separation  in  this  way,  still  when  the  conditions 
are  right,  almost  all  of  the  iron  can  be  removed  so  that  a  large 
sample  of  steel  can  be  taken  for  analysis.  This  particular 
method  has  not  been  tested  in  the  author's  laboratory  and 
is  not  given  in  the  German  edition.  The  ether  separation, 
however,  is  discussed  in  many  other  text-books  and  deserves 
mention. — (TRANSLATOR.) 

Molybdenum,  in  this  analysis,  follows  the  iron  so  that  when  a 
small  amount  of  the  former  is  present,  as  is  the  case  in  steels, 
the  ether  solution  may  be  regarded  as  containing  all  of  the 
molybdenum,  as  well  as  the  greater  part  of  the  iron. 

Procedure. — Two  grams  of  the  steel  are  dissolved  in  nitric  acid 
with  the  addition  of  hydrochloric  acid  if  necessary.  The  resulting 
solution  is  evaporated  to  dryness  and  the  residue  dissolved  by 
treatment  with  hot  concentrated  hydrochloric  acid.  If  silica  is 
present,  the  solution  is  diluted  and  filtered.  The  hydrochloric 
acid  solution  is  evaporated  to  a  sirup,  the  latter  dissolved  in  a  little 
hydrochloric  acid,  sp.  gr.  1.1, f  and  transferred  with  the  aid  of  a 
little  more  of  the  same  acid  to  a  separatory  funnel  of  about  250 
c.c.  capacity  which  is  provided  with  tightly  fitting  stop-cock 
and  glass-stopper.  About  80  c.c.  of  ether  are  added  to  the  cold 

*  J.  Am.  Chem.  Soc.,  30,  1228. 

t  The  separation  works  best  with  acid  of  this  concentration. 


314  GRAVIMETRIC  ANALYSIS. 

solution*  and  the  mixture  is  shaken  vigorously  for  half  a  minute. 
When  the  two  liquids  are  in  equilibrium,  the  lower  layer  is 
transferred  to  a  second  separatory  funnel.  The  stopper  of  the 
first  funnel  is  carefully  rinsed  with  hydrochloric  acid,  sp.  gr. 
1.1,  the  contents  of  the  funnel  once  more  shaken,  and  the 
lower  layer  added  to  the  contents  of  the  second  funnel.  The 
solution  in  the  latter  is  shaken  with  50  c.c.  more  of  ether,  and  the 
acid  solution  containing  all  the  vanadium,  chromium,  nickel, 
manganese  and  copper,  is  run  into  a  beaker  and  freed  from 
dissolved  ether  by  evaporating  on  the  water  bath  nearly  to  dry- 
ness.  An  excess  of  nitric  acid  is  added  and  the  solution  again 
evaporated  to  remove  all  the  hydrochloric  acid.  When  the  solu- 
tion is  almost  sirupy,  20  c.c.  of  hot  water  are  added  and  the 
solution  is  heated  with  the  addition  of  a  little  sulphurous  acid  to 
reduce  any  chromic  acid  that  may  have  been  formed.  The  hot 
solution  is  slowly  poured,  while  stirring  vigorously,  into  a  boiling 
10  per  cent,  solution  of  sodium  hydroxide.  After  boiling  a  few 
minutes,  the  precipitate  is  allowed  to  settle,  is  washed  twice  by 
decantation,  and  finally  on  the  filter  until  the  volume  of  the 
filtrate  is  about  300  c.c.  The  precipitate  contains  the  hydrated 
oxides  of  chromium,  nickel  and  iron  with  the  greater  part  of  the 
manganese  and  any  copper  that  may  have  been  in  the  sample. 
The  filtrate  contains  the  vanadium,  some  silicate  and  aluminate 
(from  the  reagents)  and  sometimes  a  little  chromium.  It  is 
made  barely  acid  with  nitric  acid,  once  more  made  alkaline  with 
a  few  drops  of  sodium  hydroxide,  boiled  and  filtered. 

The  vanadium  is  determined  in  this  last  filtrate.  It  is  pre- 
cipitated as  lead  vanadate  by  the  addition  of  10  c.c.  of  a  10  per 
cent,  solution  of  lead  nitrate,  eventually  adding  enough  acetic 
acid  to  make  the  sojution  decidedly  acid  and  boiling  for  several 
minutes.  The  lead  vanadate  is  filtered  and  washed  with  hot 
water.  The  precipitate  is  dissolved  in  hot,  dilute  hydrochloric 
acid,  the  solution  evaporated  nearly  to  dryness,  treated  with  50 
c.c.  of  hydrochloric  acid,  evaporated  again,  cooled,  treated  with 
10  c.c.  concentrated  sulphuric  acid,  and  evaporated  until  fumes 

*  The  warm  solution  would  result  in  the  reduction  of  some  of  the  iron  by 
the  ether. 


DETERMINATION  OF  YANADIUM,  MOLYBDENUM,  ETC.        315 

of  sulphuric  anhydride  are  evolved.  When  cold  150  c.c.  of  water 
are  added,  the  solution  heated  to  between  60  and  70°  and  titrated 
with  permanganate.  The  evaporation  of  the  vanadate  solution 
with  hvdrochloric  acid  and  subsequent  treatment  with  sulphuric 
acid  results  in  the  reduction  of  the  vanadium  from  the  pentavalent 
to  the  tetravalent  condition,*  and  the  titration  with  permanganate 
makes  the  vanadium  pentavalent  again.  Consequently  2  atoms 
of  V  are  equivalent  to  1  atom  of  oxygen  in  the  titration.  The 
presence  of  a  little  iron  does  not  interfere  when  the  vanadium  is 
reduced  in  the  above  manner. 

The  two  precipitates  obtained  by  the  sodium  hydroxide  treat- 
ment contain  chromium,  nickel  and  copper  besides  iron  and 
manganese.  The  two  niters  are  ignited  and  the  precipitates 
fused  with  about  2  gms.  of  sodium  carbonate  and  half  a  gram  of 
potassium  nitrate.  The  fused  mass  is  treated  with  water  and 
the  solution  filtered.  The  residue  contains  the  nickel,  copper, 
iron  and  part  of  the  manganese;  the  filtrate  contains  the  chromium 
and  the  rest  of  the  manganese.  To  the  filtrate,  enough  ammonium 
nitrate  is  added  to  convert  all  the  sodium  salts  to  nitrates,  and  the 
solution  evaporated  to  small  volume  with  the  addition  of  a  few 
drops  of  ammonia  from  time  to  time.  The  evaporated  solution 
is  diluted  to  50  c.c.  and  filtered.  The  precipitate  consists  of 
hydrated  manganese  dioxide  (alumina  and  silica  from  the 
reagents).  Tne  filtrate  is  boiled  to  drive  off  the  ammonia,  sul- 
phurous acid  added  to  reduce  any  chromic  acid,  the  excess  of 
reducing  agent  boiled  off,  and  the  chromium  precipitated  by  the 
careful  addition  of  ammonia  to  the  boiling  solution.  The  precip- 
itate is  filtered  off,  washed  and  weighed  as  Cr2O3. 

The  filter  containing  the  insoluble  residue  from  the  above 
fusion  is  returned  to  the  same  crucible  in  which  the  fusion  was 
made  and  ignited.  The  ignited  oxides  are  dissolved  in  hydro- 
chloric acid,  the  solution  diluted  and  the  copper  precipitated  by 
hydrogen  sulphide.  The  filtrate  is  evaporated  with  sulphuric 
acid  until  the  hydrochloric  acid  is  all  expelled,  whejeupon  it  is 

*Campagne,  Compt,  rend.,  13",  570  (1903). 


316  GRAVIMETRIC  A NA LYSIS. 

diluted  and  treated  with  a  large  excess  of  ammonia  and  the 
nickel  determined  by  electrolysis  (see  p.  136). 

The  ether  solution  from  the  two  separatory  funnels  is 
shaken  with  water,  which  causes  the  separation  of  an  ether 
layer  from  the  solution  containing  the  iron  and  molybdenum. 
The  lower  layer  is,  therefore,  drawn  off;  the  solution  of  ferric 
chloride  containing  all  the  molybdenum  is  evaporated  nearly  to 
drynees,  the  cold  solution  treated  with  10  c.c.  of  concentrated 
sulphuric  acid  and  evaporated  until  the  sulphuric  acid  fumes 
freely.  The  cold  sulphuric  acid  solution  is  diluted  with  100  c.c. 
of  water,  reduced  by  the  careful  addition  of  ammonium  acid 
sulphite,  the  excess  of  sulphurous  acid  boiled  off,  and  the  cold 
solution  saturated  with  hydrogen  sulphide  in  a  200  c.c.  pressure 
bottle.  The  bottle  is  stoppered  and  heated  on  the  water  bath  for 
several  hours.  After  slowly  cooling,  the  precipitate  is  filtered  into 
a  Gooch  crucible  and  washed  with  water  containing  a  little  sul- 
phuric acid  and  finally  with  alcohol.  The  Gooch  crucible  is 
placed  within  a  larger  porcelain  crucible  so  that  the  bottom  of  the 
former  does  not  touch  that  of  the  latter,  covered  with  a  watch- 
glass  and  heated  gradually  until  there  is  no  more  odor  of  sulphur 
dioxide.  The  watch-glass  is  then  replaced  by  a  porcelain  crucible 
cover  and  the  heating  is  continued  until  the  ignited  precipitate 
becomes  bluish  white  in  color. 

The  Gooch  crucible  is  then  heated  to  faint  redness,  cooled  and 
weighed.  The  heating  and  weighing  is  repeated  until  the  pre- 
cipitate ceases  to  lose  in  weight.  The  crucible  is  then  placed  in 
the  suction  bottle  and  washed  with  ammonia  until  the  washings 
are  free  from  molybdenum.  The  crucible  is  again  heated  and 
weighed.  The  difference  in  weight  corresponds  to  the  amount 
of  molybdenum  trioxide.  A  small  amount  of  ferric  oxide  always 
remains  on  the  felt. 


SILVER.  3X7 

METALS  OF  GROUP  I. 

SILVER,  LEAD,  MERCUROUS  MERCURY   (AND   THALLIUM). 
The   determination   of   lead   and   mercury   has   already   been 
considered;  it  remains  for  us  to  discuss  the  determination  of  silver. 

SILVER,  Ag.    At.  Wt.  107.88. 

Forms :  AgCl  and  Ag. 
Determination  as  Silver  Chloride,  AgCl. 

The  solution,  slightly  acid  with  nitric  acid,  is  heated  to  boiling 
and  the  silver  precipitated  by  the  addition  of  hydrochloric  acid, 
drop  by  drop,  until  no  more  precipitate  is  formed.  The  precipitate 
is  allowed  to  settle  in  a  dark  place,  filtered  through  a  Gooch  cruci- 
ble and  washed,  first  with  water  containing  a  little  nitric  acid  until 
the  chloride  test  can  no  longer  be  obtained,  then  twice  with  alcohol 
or  water  in  order  to  remove  the  nitric  acid.  The  precipitate  is 
dried  first  at  100°  C.  and  finally  at  130°  C.  till  a  constant  weight  is 
obtained.  If  it  is  not  desired  to  use  a  Gooch  crucible  for  this 
determination,  the  silver  chloride  can  be  filtered  upon  an  ordinary 
washed  filter,  washed  as  before  and  dried  at  100°  C.  As  much 
of  the  precipitate  as  possible  is  transferred  to  a  weighed  porcelain 
crucible,  the  filter  burned  (as  described  on  page  21)  in  a  platinum 
spiral  whereby  some  of  the  silver  chloride  adhering  to  it  will  be 
reduced  to  metal.  The  ash  of  the  filter  is  added  to  the  main  por- 
tion of  the  precipitate.  It  is  moistened  with  a  little  nitric  acid  and  a 
drop  or  two  of  concentrated  hydrochloric  acid,  dried  on  the  water- 
fyath  and  then  heated  over  a  free  flame  until  the  silver  chloride 
begins  to  melt.  After  cooling  in  a  desiccator  it  is  weighed. 

Solubility  of  Silver  Chloride*  One  liter  of  water  dissolves 
0.00154  g.  AgCl  at  20°  and  0.0217  g.  at  100°.  In  water  con- 
taining a  little  hydrochloric  acid,  the  AgCl  is  less  soluble  than  in 
pure  water  but  as  the  quantity  of  hydrochloric  acid  is  increased, 
the  solubility  of  AgCl  rises  rapidly.  Thus  one  liter  of  1  per  cent. 
HC1  dissolves  only  0.0002  g.  AgCl  at  21°,  but  1 1.  of  5  per  cent.  HC1 
dissolves  0.0003  g.  and  1  1.  of  10  per  cent.  HC1  dissolves  0.0555  g. 
AgCl. 

*  G.  S.  Whitby,  Z.  anorg.  Chem.,  67,  108  (1910). 


3i8  GRAVIMETRIC  ANALYSIS 

Determination  as  Metal,  Ag. 

Metallic  silver  is  obtained  by  the  ignition  of  silver  oxide,  car- 
bonate, cyanide  or  the  salt  of  an  organic  acid.  In  the  latter  case, 
the  substance  must  be  heated  very  cautiously  at  first  in  a  covered 
crucible.  When  the  organic  substance  is  completely  charred, 
the  cover  is  removed  from  the  crucible  and  the  contents  are  ignited 
until  the  carbon  is  completely  burned,  and  the  crucible  then  weighed. 

From  the  chloride,  bromide  (but  not  the  iodide)  and  sulphide, 
the  metal  may  be  obtained  by  igniting  in  a  current  of  hydrogen. 
The  reduction  of  the  chloride,  bromide,  and  iodide  may  be  effect- 
ed very  conveniently  by  passing  the  electric  current  through 
the  substance  after  it  has  been  melted  together.  The  porcelain 
crucible  containing  the  silver  halide  is  placed  in  a  crystallizing 
dish  and  near  it  is  placed  a  second  crucible  containing  a  little 
mercury  and  a  small  piece  of  zinc.  Upon  the  silver  salt  is  placed 
a  small  disk  of  platinum  foil,  which  is  fastened  to  a  platinum  wire; 
the  other  end  of  the  wire  dips  in  the  mercury  in  the  other  crucible. 
The  crystallizing  dish  is  filled  with  dilute  sulphuric  acid  (1:20) 
so  that  the  crucible  is  entirely  covered  with  the  acid  and  it  is  then 
allowed  to  stand  over  night.  Next  morning  all  of  the  silver  salt 
will  be  found  to  be  reduced.  The  crucible  is  removed  from  the 
acid,  washed  with  water,  dried,  ignited,  and  weighed.  By  this 
simple  method,  E.  Lagutt  obtained  excellent  results.  If  the 
silver  halide  has  not  been  fused  to  a  compact  mass  small  particles 
of  the  silver  precipitate  are  likely  to  float  around  during  the  opera- 
tion, and  escape  reduction. 

Silver  can  also  be  deposited  electrolytically,  but  this  method 
will  not  be  described  in  this  book,  for  it  offers  no  particular  advan- 
tages over  the  determination  as  silver  chloride  with  a  Gooch  cru- 
cible. 

Separation  of  Silver  from  Other  Metals. 
As  almost  all  metal  chlorides  *  are  soluble  in  dilute  hydro- 

*  Thallium  chloride  is  difficultly  soluble  in  water.  If  thallium  is  present 
the  silver  is  precipitated  from  a  nitrate  solution  by  means  of  H2S,  ignited 
in  a  stream  of  hydrogen,  and  weighed  as  metal.  To  determine  the  thallium, 
the  nitrate  is  evaporated  to  dryness,  the  residue  dissolved  in  a  little  water 


SILVER.  319 

chloric  acid,  suver  is  separated  from  the  other  metals  by  the  addi- 
tion of  hydrochloric  acid  to  the  solution.  If  the  solution  contains 
mercurous  salts  these  are  oxidized  before  the  addition  of  the 
hydrochloric  acid  by  boiling  with  nitric  acid. 

For  the  separation  of  silver  from  gold  and  platinum  in  alloys 
consult  pages  259  and  270. 

and  the  thallium  precipitated  by  the  addition  of  potassium  iodide.  The 
thallium  iodide  precipitate  is  washed  with  dilute  potassium  iodide  solution, 
then  with  alcohol,  dried  at  150°  and  weighed  as  Til. 


GRAVIMETRIC   DETERMINATION  OF  THE 
METALLOIDS   (ANIONS). 

GROUP  I. 

HYDROCHLORIC,  HYDROBROMIC,  HYDRIODIC,  HYDROCYANIC, 
HYDROFERROCYANIC,  HYDROFERRICYANIC,  SULPHOCY- 
ANIC,  AND  HYPOCHLOROUS  ACIDS. 

HYDROCHLORIC  ACID,  HC1.    Mol.  Wt.  36.47. 

Form :  Silver  Chloride,  AgCl. 
We  can  distinguish  between  two  cases: 

A.  The  chloride  solution  is  present  either  as  free  hydrochloric 
acid  or  as  a  chloride  soluble  in  water. 

B.  It  is  present  in  the  form  of  an  insoluble  chloride. 

A.  The  Chloride  is  Present  in  Aqueous  Solution. 

If  only  metals  of  the  alkali  or  alkaline  earth  groups  are  present, 
the  cold  solution  is  made  slightly  acid  with  nitric  acid,  and  silver 
nitrate  is  slowly  added  with  constant  stirring  until  the  precipitate 
collects  together  and  further  addition  of  the  reagent  produces  no 
more  precipitation.  The  liquid  is  now  heated  to  boiling,  the  pre- 
cipitate allowed  to  settle  in  the  dark,  filtered  through  a  Gooch 
crucible  and  then  treated  exactly  as  described  in  the  determina- 
tion of  silver,  page  317. 

If  the  aqueous  solution  contains  a  chloride  of  a  heavy  metal, 
it  is  not  always  possible  to  follow  the  above  procedure.  If,  for 
exampb,  substances  are  present  which  on  boiling  are  changed  to 
insoluble  basic  salts,  it  is  evident  that  the  precipitate  of  silver  chlo- 
ride would  b3  contaminated  with  these  substances  and  too  high 
results  would  be  obtained.  This  is  particularly  true  of  stannic 
and  ferric  salts.  Ferrous  salts,  on  the  other  hand,  in  case  only  little 

320 


HYDROCHLORIC  ACID.  321 

nitric  acid  is  present,  reduce  silver  nitrate  to  metallic  silver  on 
heating  the  solution;  if  enough  nitric  acid  is  present  to  prevent 
the  reduction  to  silver,  the  danger  of  forming  basic  salts  still 
remains  to  be  feared.  In  such  cases  the  precipitation  is  effected 
as  before  from  a  cold  solution  and  the  subsequent  heating  is 
omitted. 

In  all  cases,  however,  it  is  better  to  first  remove  the  heavy 
metal  by  precipitation  with  ammonia,  caustic  soda  or  sodium 
carbonate. 

Example  :  Analysis  of  Commercial  Tin  Chloride. 

Tin  chloride  is  obtained  either  as  a  solid  salt  corresponding 
to  the  formula  SnQ4  +  5H2O,  or  as  a  concentrated  aqueous  solu- 
tion. 

As  both  the  solid  salt  and  its  concentrated  solution  are  very 
hygroscopic,  it  is  necessary  to  weigh  out  the  portion  for  analysis 
from  a  stoppered  vessel.  It  is  best  to  proceed  as  follows: 

A  large  sample  of  the  substance  (about  10  gm.)  is  placed  in  a 
tared  weighing  beaker,  closed  and  weighed.  About  10  c.c.  of  water 
are  added,  the  salt  is  completely  dissolved  to  a  homogeneous 
syrup  by  shaking,  and  the  beaker  is  again  weighed.  Four  more 
weighing  beakers  are  now  tared  and  into  each  is  placed  about 
2  c.c.  of  the  syrup.  Each  beaker  is  quickly  stoppered  and  then 
weighed. 

Determination  of  Tin. — The  contents  of  one  of  the  weighing 
beakers  is  washed  into  a  400-500-c.c.  beaker,  diluted  to  about 
300  c.c.  and  a  few  drops  of  methyl  orange  are  added,  whereby  the 
liquid  is  colored  red.  Ammonia  solution  (free  from  chloride)  is 
now  added  until  .the  color  of  the  solution  is  changed  to  yellow  (an 
excess  of  ammonia  must  be  carefully  avoided  for  tin  hydrox- 
ide is  somewhat  soluble  in  ammonia).  The  solution  is  then  treated 
with  ammonium  nitrate  (5  c.c.  of  concentrated  ammonia  exactly 
neutralized  with  nitric  acid,  sp.  gr.  1.2),  boiled  for  one  or  two  min- 
utes, filtered,  and  washed  with  water  containing  ammonium  nitrate, 
and  weighed  as  SnO2. 

Determination  of  Chlorine. — The  filtrate  from  the  tin  hydroxide 
precipitate  is  acidified  with  nitric  acid,  and  precipitated  in  the  cold 
with  silver  nitrate.  The  solution  is  then  heated  to  boiling  and, 


322      GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

after   the  precipitate  has  settled,  it  is  filtered  through  a  Gooch 
crucible,  washed  with   cold  water  containing  a  little  nitric  acid, 
then  with  cold  water  or  alcohol,  dried  at  130°  C.,  and  weighed. 
The  amount  of  tin  and  chlorine  present  is  computed  as  follows  : 

Weight  of  Solid  Salt  =4. 

Weight  of  the  Solid  Salt  +  Water  =B. 

Weight  of  the  Solution  taken  for  Analysis  =  a. 

Weight  of  the  SnO2  found=p. 

Weight  of  the  AgCl  found  =  p'. 

Since  B  gm.  of  the  solution  contain  A  gm.  of  the  solid  salt, 
then  the  amount  a  of  the  solution  taken  for  analysis  will  contain  : 


A-a 

x=  -5-  =  wt.  of  substance  taken. 
n 

This  amount  of  substance  yielded  p  gm.  SnO2;  corresponding 
to: 

SnO2:Sn  =p:x? 


,  =      . 
Sn02 


and  in  percentage : 


In  the  same  way  the  amount  of  chlorine  present  is  found  to  be: 
100C1  p'-B_^ 

3Sa"?Z  %l 

This  analysis  may  be  accomplished  much  more  rapidly  by  a 
volumetric  process.  (Consult  Vol.  Anal.) 

If  antimony  or  stannous  compounds  are  present,  the  above 
procedure  cannot  be  used.  It  has  been  proposed  to  add  tartaric 
acid  to  the  solution,  then  dilute  with  water  and  precipitate  the 
chlorine  with  silver  nitrate.  It  is  better,  however,  to  proceed  as 
follows:  The  antimony  is  precipitated  by  hydrogen  sulphide  as  its 


HYDROCHLORIC  ACID.  323 

sulphide,  the  excess  of  the  latter  is  removed  by  passing  carbon 
dioxide  through  the  solution,  after  which  the  precipitate  is  filtered 
and  washed.  The  filtrate  containing  all  the  chlorine  is  made 
slightly  ammoniacal,  a  little  hydrogen  peroxide  or  potassium  per- 
carbonate  is  added  (both  reagents  must  be  free  from  chloride)  and 
the  solution  boiled  until  the  excess  of  the  peroxide  is  destroyed. 
By  this  treatment  traces  of  hydrogen  sulphide  remaining  in  the 
solution  are  oxidized  to  sulphuric  acid.  After  cooling,  the  solu- 
tion is  acidified  with  nitric  acid,  and  the  chlorine  determined  as 
silver  chloride  as  described  above. 

According  to  this  method,  chlorine  may  be  determined  in  the 
presence  of  large  amounts  of  hydrogen  sulphide  without  difficulty. 

It  is  less  practical  to  proceed  as  follows :  The  solution  is  satu- 
rated with  ammonia  and  the  hydrogen  sulphide  is  precipitated  by 
the  addition  of  ammoniacal  silver  nitrate  solution,  the  deposited 
silver  sulphide  is  filtered  off,  washed  with  ammonia,  and  the  silver 
chloride  precipitated  from  the  filtrate  by  acidifying  with  nitric 
acid. 

B.  Analysis  of  an  Insoluble  Chloride. 

The  substance  is  boiled  with  sodium  carbonate  solution  *  (free 
from  chloride) ,  and  the  chlorine  determined  in  the  filtrate  as  before. 

Many  chlorides,  e.g.  silver  chloride,  many  minerals  such  as 
apatite,f  sodalite,  and  rocks  'containing  the  latter,  are  not  decom- 
posed by  boiling  them  with  sodium  carbonate.  In  such  cases,  the 
substance  must  be  fused  with  sodium  carbonate. 

Silver  chloride  should  be  mixed  with  three  times  as  much 
sodium  carbonate  and  heated  in  a  porcelain  crucible  until  the 
mass  has  sintered  together.  The  mass  is  treated  with  water,  the 
insoluble  silver  filtered  off,  and  the  chlorine  determined  in  the  fil- 
trate as  under  (a). 

For  the  determination  of  chlorine  in  rocks,  1  gm.  of  the  finely- 

*  Mercurous  chloride  is  decomposed  only  slowly  by  sodium  carbonate, 
solution,  but  readily  acted  upon  by  potassium  or  sodium  hydroxide. 

f  According  to  Jannasch,  chlorine  in  apatite  may  be  determined  by  treat- 
ing the  finely  powdered  mineral  with  nitric  acid  and  silver  nitrate  on  the 
water-bath.  Everything  goes  into  solution  with  the  exception  of  silver 
chloride,  which  is  filtered  off  and  weighed.  (This  does  not  apply  to  a  sample- 
of  apatite  contaminated  with  silica  or  silicates.) 


324      GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

powdered  material  is  fused  with  four  or  five  times  as  much  sodium 
carbonate  (or  with  a  mixture  of  equal  parts  sodium  and  potassium 
carbonates)  at  first  over  a  Bunsen  burner,  afterward  over  a  Teclu 
burner  or  the  blast-lamp.  The  melt  is  extracted  with  hot  water. 
After  cooling,  methyl  orange  is  added,  the  solution  is  acidified 
with  nitric  acid  and  allowed  to  stand  overnight.  If  silicic  acid 
has  .precipitated  out  by  the  next  morning,  a  little  ammonia  is 
added,  the  solution  is  boiled,  filtered,  and  washed  with  hot  water. 
The  cold  filtrate  is  acidified  with  nitric  acid  and  the.  chlorine 
determined  as  above. 

If  there  is  no  separation  of  silicic  acid  on  acidifying  the  water 
extraction  of  the  fusion  with  nitric  acid,*  the  chlorine  is  precipi- 
tated at  once  from  the  cold  solution. 

Free  Chlorine. 

If  it  is  desired  to  determine  gravimetrically  the  amount  of 
chlorine  in  a  sample  of  chlorine  water,  it  is  not  feasible  to  simplify 
add  silver  nitrate,  for  all  of  the  chlorine  is  not  precipitated  as  silver 
chloride;  a  part  of  it  remains  in  solution  as  soluble  silver  chlorate: 

6C1+  3H2O+  6AgNO3  =  5AgCl+  AgClO3+  6HNO3. 

The  chlorine,  therefore,  must  be  changed  to  hydrochloric'  acid 
or  to  one  of  its  salts  before  attempting  to  precipitate  with  silver 
nitrate.  This  may  be  accomplished  in  several  ways : 

1 .  A  definite  amount  of  the  chlorine  water  is  transferred  by  means 
of  a  pipette  to  a  flask  containing  ammonia  and  after  mixing  the  solu- 
tion is  heated  to  boiling.  After  cooling  the  liquid  is  acidified  with 
nitric  acid  and  precipitated  by  silver  nitrate.  The  ammonia  con- 
verts the  chlorine  partly  to  ammonium  chloride  and  partly  to  am- 
monium hypochlorite.  The  latter  is  decomposed  partly  in  the  cold 
and  quantitatively  on  warming  into  ammonium  chloride  and 
nitrogen: 

(a)  2NH4OH  +  2C1  =  NH4C1  +  NH4OC1  +  H20  ; 

(6)   3NH4OC1  +  2NH3  =  3H2O  +  N2  +  3NH4C1. 

*  According  to  W.  F.  Hillebrand,  there  is  no  separation  of  silicic  acid  to 
be  feared  from  1  gm.  of  the  substance. 


HYDROCHLORIC  ACID.  325 

2.  The  chlorine  water  is  treated  with  an  excess  of  sulphurous 
acid,  the  solution  is  made  ammoniacal,  hydrogen  peroxide  is  added, 
and  the  liquid  boiled  until  the  excess  of  hydrogen  peroxide  is  re- 
moved.    After  cooling  the  solution  is  acidified  with  nitric  acid, 
diluted  with  water,  and  the  chlorine  precipitated  by  means  of  silver 
nitrate. 

3.  The  chlorine  water  is  treated  with  dilute  caustic  soda  solu- 
tion,  an  aqueous  solution  of  sodium  arsenite  is  added   (arsenic 
trioxide  dissolved  in  sodium  carbonate)  until  a  drop  of  the  liquid 
will  not  turn  a  piece  of  iodo-starch  paper  blue.     It  is  then  acidi- 
fied with  nitric  acid  and  the  chlorine  precipitated  by  a  soluble 
silver  salt. 

If  the  solution  contains  both  free  chlorine  and  hydrochloric 
acid,  the  total  chlorine  is  determined  by  one  of  the  above  methods, 
while  the  free  chlorine  is  determined  in  a  separate  sample  by  a 
volumetric  process  (see  lodimetry). 

Determination   of   Chlorine   in  Non-electrolytes  (Organic 
Compounds). 

A.  Method  of  Carius* 

Principle. — The  method  is  based  upon  the  fact  that  all  organic 
compounds  are  decomposed  by  heating  with  concentrated  nitric 
acid  at  a  high  temperature  under  pressure.  If  the  substance  con- 
tains halogen,  sulphur,  phosphorus,  or  arsenic,  it  is  first  set 
free  as  such,  but  on  account  of  the  reducing  action  of  the  nitrous 
acid  formed  it  is  then  changed  over  into  its  hydrogen  compound. 
The  latter,  however,  is  partly  oxidized  by  the  nitric  acid.  The 
reaction  is  therefore  a  reversible  one.  If,  on  the  other  hand,  the 
substance  is  heated  under  the  same  conditions  with  nitric  acid  in 
the  presence  of  silver  nitrate,  the  halogen  hydride  is  converted  into 
silver  halide  as  fast  as  it  is  formed  and  the  halogen  is  in  this  case 
quantitatively  changed  into  its  silver  salt.  Sulphur,  phosphorus, 
and  arsenic  are  oxidized  in  the  same  way  to  sulphuric,  phosphoric, 
and  arsenic  acids  and  any  metals  present  form  nitrates. 

*  Ann.  d.  Chem.  u.  Pharm.  (1865),  136,  p.  129,  and  Zeit.  f.  anal  Chem. 
4,  p.  451. 


326     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 


Procedure  for  the  Halogen  Determination. 

A  glass  tube  made  of  difficultly  fusible  glass  (about  50  cm. 
long,  2  cm.  in  diameter,  with  walls  about  2  mm.  thick)  is  sealed  at 
one  end,  thoroughly  cleaned  and  dried  by  sucking  air  through  it. 

About  0.5  gm.  of  powdered  silver  nitrate  (or  in  the  case  of 
substances  rich  in  halogen  as  much  as  1  gm.  may  be  used)  is  trans- 
ferred to  the  tube  by  pouring  the  powder 
through  a  cylinder  made  by  rolling  up  a 
piece  of  glazed  paper  and  shoving  the  paper 
into  the  tube  until  it  reaches  about  the 
middle  of  it.  About  40  c.c.  of  pure  nitric 
acid  (sp.  gr.  1.5)  free  from  chlorine  are 
poured  into  the  tube  through  a  funnel 
whose  stem  is  about  40  cm.  long.  In  this 
way  only  the  lower  half  of  the  tube  is  wet 
with  the  acid.  The  tube  is  then  inclined 
to  one  side  and  from  0.15-0.2  gm.  of  the 
substance  contained  in  a  small  glass  tube 
closed  at  one  end  is  introduced  into  it  (this 
smaller  tube  should  be  about  5  cm.  long 
and  5  mm.  wide).  As  soon  as  the  tube 
plQ  53  containing  the  substance  has  reached  the 

acid,  it  remains  suspended  (Fig.  53,  a).     It 

is  very  important  that  the  substance  should  not  come  in  contact 
with  the  acid  before  the  tube  is  closed  at  the  upper  end,  as  other- 
wise there  is  likelihood  of  some  halogen  escaping. 

The  upper  end  of  the  tube  is  now  heated  very  cautiously  in 
the  flame  of  the  blast-lamp  until  the  tube  begins  to  soften  and 
thicken  (Fig.  53,  6).  It  is  then  drawn  out  into  a  3-5  cm.  long, 
thick-walled  capillary  and  the  end  fused  together  (Fig.  53,  c). 

After  the  tube  has  become  cold,  it  is  enveloped  in  asbestos 
paper,  carefully  shoved  into  the  iron  mantle  of  a  "bomb  furnace," 
and  gradually  heated.  Aliphatic  substances  are  usually  decom- 
posed by  heating  four  hours  at  150-200°  C;  substances  of  the 
aromatic  series  usually  require  from  eight  to  ten  hours'  heating  at 
250-300°  C.,  while  in  some  cases  an  even  longer  heating  at  a  higher 
temperature  is  necessary.  The  time  and  temperature  must  be 


a 


HYDROCHLORIC  ACID.  327 

found  out  for  each  substance  by  experiment.  The  decomposition 
is  complete  when  on  cooling  the  contents  of  the  tube  neither 
crystals  nor  drops  of  oil  are  to  be  recognized.*  The  heating  is  so 
regulated  that  after  three  hours  the  temperature  of  about  200°  C. 
is  reached,  after  three  hours  more  250-270°  C.,  and  finally  after 
another  three  hours  a  temperature  of  about  300°  C.  is  attained.! 
After  the  heating  is  finished,  the  tube  is  allowed  to  cool  completely 
in  the  furnace,  the  iron  mantle  together  with  the  tube  is  then 
removed,  and  by  slightly  inclining  the  mantle  the  capillary  of  the 
tube  is  brought  out  into  the  open  air.  In  most  cases  a  drop  of 
liquid  will  be  found  in  the  point  of  the  latter.  In  order  not  to  lose 
this,  the  outer  point  of  the  capillary  is  carefully  heated  with  a 
free  flame,  and  by  this  means  the  liquid  is  driven  back  into  the 
other  part  of  the  tube.  The  point  of  the  capillary  is  now  more 
strongly  heated  %  until  the  glass  softens,  when  it  will  be  blown  out 
in  consequence  of  the  pressure  within  the  tube.  The  gas  escapes 
with  a  hissing  sound.  When  the  contents  of  the  tube  are  at  the 
atmospheric  pressure,  a  scratch  is  made  upon  it  with  a  file  jus,t 
below  the  capillary,  and  this  is  touched  with  a  hot  glass  rod,  whereby 
the  tube  usually  breaks  and  the  upper  part  can  be  removed.  The 
contents  of  the  tube  are  then  carefully  poured  into  a  fairly  large 
beaker  without  breaking  the  little  tube  in  which  the  substance  was 
weighed  out,  and  the  inner  part  of  the  tube  as  well  as  its  capillary 
is  washed  out  with  water.  The  liquid  in  the  beaker  is  diluted  to 
about  300  c.c.  and  heated  to  boiling.  After  cooling,  the  insoluble 
silver  halide  is  filtered  off  through  a  Gooch  crucible,  and  after 
washing  and  drying  at  130°  C.  its  weight  is  determined. 

If  it  is  thought  that  the  precipitate  is  contaminated  by  frag- 
ments of  broken  glass,  as  is  often  the  case  even  with  careful  work, 
the  clear  liquid  is  decanted  through  a  filter,  the  residue  washed  by 

*  Sometimes,  with  substances  rich  in  sulphur,  crystals  of  nitrosyl  sulphuric 
acid  are  formed  and  adhere  to  the  sides  of  the  tube.  They  are  easily  distin- 
guished from  crystals  of  the  undecomposed  substance. 

f  Such  a  high  pressure  is  often  attained  that  the  tube  bursts  as  soon  as  it 
is  heated  very  hot.  In  such  cases  it  should  be  heated  to  only  200°  C.,  allowed 
to  cool,  the  capillary  opened  and  the  gas  set  free.  It  is  then  fused  together 
again  and  heated  to  the  desired  temperature. 

I  Before  heating,  the  tube  and  the  hand  should  be  wrapped  in  a  towel  to 
avoid  accidents. 


328     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

decantation  with  very  dilute  nitric  acid  to  the  disappearance  of 
the  silver  reaction,  and  the  residue  (except  when  it  is  silver  iodide) 
is  dissolved  in  warm  ammonia  water.  The  solution  is  filtered 
through  the  same  filter,  but  the  filtrate  is  this  time  collected  in  a 
fresh  beaker.  After  washing  the  filter  with  dilute  ammonia,  the 
filtrate  is  acidified  with  nitric  acid,  heated  to  boiling,  and  after 
allowing  the  silver  chloride  or  bromide  to  settle  in  the  dark,  it  is 
filtered  through  a  Gooch  crucible,  dried  at  130°  C.,  and  weighed. 

In  the  case  of  silver  iodide,  it  cannot  be  dissolved  in  ammonia 
and  in  this  way  separated  from  splinters  of  glass.  In  this  case  the 
substance,  together  with  the  glass,  is  filtered  through  an  ordinary 
washed  filter  (not  a  Gooch  crucible),  completely  washed  with  dilute 
nitric  acid,  then  once  with  alcohol  in  order  to  remove  the  nitric 
acid,  and  dried  at  100°  C.  As  much  of  the  precipitate  as  possible 
is  transferred  to  a  watch-glass,  the  filter  burned,  and  its  ash  placed 
in  a  weighed  porcelain  crucible.  A  little  dilute  nitric  acid  is  added 
(in  order  to  change  any  reduced  silver  into  the  nitrate),  the  liquid  is 
evaporated  on  the  water-bath,  a  few  drops  of  water  and  a  drop  of 
pure  hydriodic  acid  are  added,  and  the  contents  of  the  crucible  are 
again  evaporated  to  dryness,  when  the  main  part  of  the  precipitate  is 
added,  heated  until  it  begins  to  fuse,  and  then  weighed.  The  mass 
in  the  crucible  is  then  covered  with  pure  dilute  sulphuric  acid,  a 
piece  of  chemically  pure  zinc  is  added,  and  the  crucible  allowed  to 
stand  overnight.  After  this  time  the  silver  iodide  will  be  com- 
pletely reduced  to  metallic  silver.  The  zinc  is  removed,  and  the 
residue  washed  by  decanting  several  times  with  water  until  the 
iodine  reaction  can  no  longer  be  detected.  The  residue  is  then 
warmed  with  dilute  nitric  acid  upon  the  water-bath,  in  order  to 
dissolve  the  silver,  the  solution  is  filtered  through  a  small  filter; 
and  the  latter  is  washed  with  water  and  dried.  This  filter  is  ig- 
nited in  a  crucible  and  the  residue  (the  glass)  is  weighed.  This 
second  weight  deducted  from  the  former  gives  the  amount  of  silver 
iodide  present. 

This  method  is  also  suitable  for  obtaining  lead  and  mercury 
from  organic  compounds  in  a  form  which  can  be  precipitated  by 
hydrogen  sulphide. 

The  method  of  Carius  is  by  far  the  best  for  the  determination  of 
halogens  in  organic  substances  when  only  one  of  the  halogens  is 


HYDROBROMIC  ACID.  329 

present.     If  two  or  three  of  them  are  present  at  the  same  time, 
the  "  lime  method  "  is  to  be  preferred. 

The  Lime  Method. 

Into  a  glass  tube  made  of  difficultly  fusible  glass  (about  40  cm. 
long,  1  cm.  wide  and  closed  at  one  end),  a  layer  of  lime  (free 
from  chloride)  from  5  to  6  cm.  long  is  introduced,  then  about  0.5 
gm.  of  substance,  and  finally  5  cm.  more  of  lime.  The  substance  is 
then  mixed  thoroughly  with  the  lime  by  means  of  a  copper  wire 
whose  end  is  wound  into  a  spiral.  The  tube  is  nearly  filled 
with  lime,  placed  on  its  side,  and  gently  tapped  so  that  a  small 
canal  is  formed  above  the  lime.  The  tube  is  then  placed  in  a  small 
combustion  furnace  (cf .  Carbon)  and  heated.  First  of  all  the  front 
end  of  the  tube,  free  from  substance,  is  heated  to  a  dull  redness, 
then  the  back  end,  and  afterward  the  other  burners  are  lighted  one 
after  another  until  finally  the  whole  tube  is  at  a  dull-red  heat.  After 
cooling,  the  contents  of  the  tube  are  transferred  to  a  large  beaker 
and  the  lime  dissolved  in  dilute  nitric  acid  free  from  chlorine.  The 
carbon  is  filtered  off,  and  the  halogen  precipitated  with  silver  nitrate. 

If  the  lime  contains  calcium  sulphate,  this  is  reduced  to  sul- 
phide, so  that  some  silver  sulphide  is  likely  to  be  precipitated  with 
the  silver  halide.  In  this  case  the  solution  is  treated  with  hydrogen 
peroxide  (free  from  halogen)  before  enough  nitric  acid  has  been 
added  to  make  the  solution  acid,  the  liquid  is  boiled  to  remove 
the  excess  of  the  reagent,  then  acidified,  filtered,  and  precipitated 
with  silver  nitrate.*  In  the  analysis  of  substances  rich  in  nitrogen, 
it  is  possible  that  some  soluble  calcium  cyanide  will  be  forme:!. 
In  this  case  care  must  be  taken  that  the  silver  precipitate  con- 
tains no  silver  cyanide  (cf.  Separation  of  Cyanogen  from  Chlorine- 
Bromine,  and  Iodine,  p.  339). 

HYDROBROMIC  ACID,  HBr.    Mol.  Wt.  80.93. 

Form:  Silver  Bromide,  AgBr. 

Hydrobromic  acid  is  determined  exactly  the  same  as  hydro- 
chloric acid.  This  is  also  true  of  the  determination  of  free  bro- 
mine, and  bromine  in  non-electrolytes. 

*  W.  Biltz  (Chem.  Ztg.,  1903,  Rep.  142),  separates  the  halides  from  sulphide 
by  treating  the  precipitated  silver  salts  with  an  ammoniacal  sodium  thiosul- 
phate  solution,  whereby  the  silver  halide  goes  into  solution,  from  which  the 
silver  is  precipitated  as  silver  sulphide,  by  adding  ammonium  sulphide,  and 
determined  as  silver. 


330     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

HYDRIODIC  ACID,  HI.    Mol.  Wt.  127.93. 

Forms;  Silver  Iodide,  Agl,  and  Palladous 
d  Iodide,  PdI2. 

(a)  Determination  as  Silver  Iodide. 

The  determination  of  hydriodic  acid  is  carried  out  in  exactly 
the  same  way  as  the  analysis  of  hydrochloric  acid.  If  it  is  de- 
sired to  filter  the  silver  iodide  through  an  ordinary  washed  filter 
instead  of  through  a  Gooch  crucible,  the  procedure  described  on 
p.  328  is  used,  converting  the  reduced  metal  to  iodide  by  dissolv- 
ing in  nitric  acid  and  adding  hydriodic  acid.  In  case  there  is  no 
hydriodic  acid  at  one's  disposal,  the  main  portion  of  the  precipitate 
is  placed  in  a  weighed  porcelain  crucible  and  heated  until  it  begins 
to  melt  and  then  weighed.  The  filter  ash  is  placed  in  another 
crucible,  and  treated  with  nitric  and  hydrochloric  acids,  whereby 
the  silver  and  any  unreduced  iodide  are  changed  to  silver  chloride. 
The  silver  chloride  is  weighed  and  the  equivalent  amount  of  silver 
iodide  is  added  to  the  weight  of  the  main  part  of  the  precipitate. 

Example.  —  Suppose  a  grams  substance  gave  p  grams  silver 
iodide  and  p'  grams  silver  chloride,  then 


We  have,  therefore,  in  a  grams  substance  p+  A   ™p'  grams  silver 


iodide,  and  the  amount  of  iodine  present  may  be  calculated  in  the 
usual  manner. 

(6)  Determination  as  Palladous  Iodide. 

This  important  method  for  the  separation  of  iodine  from 
bromine  and  chlorine  is  carried  out  as  follows: 

The  solution  is  acidified  with  hydrochloric  acid,  and  palladous 
chloride  solution  is  added  until  no  more  precipitate  is  formed. 
After  standing  one  or  two  days  in  a  warm  place,  the  brownish- 
black  precipitate  of  palladous  iodide  is  filtered  through  a  Gooch 


SEPARATION  OF  IODINE  FROM  CHLORINE.  331 

crucible,  or   through  a  tared  filter  that  has  been  dried  at  100°  C., 
washed  with  warm  water,  dried  at  100°  C.,  and  weighed  as  PdI2. 

According  to  Rose,  the  PdI2  may  be  changed  to  palladium  by 
igniting  in  a  current  of  hydrogen,  and  from  the  weight  of  the  palla- 
dium the  amount  of  iodine  calculated. 


SEPARATION  OF  THE  HALOGENS  FROM  ONE  ANOTHER. 

i.  Separation  of  Iodine  from  Chlorine. 

(a)   The  Palladous  Iodide  Method. 

The  iodine  is  determined  as  above  as  palladous  iodide,  and  in  a 
second  sample  the  sum  of  the  chlorine  and  iodine  is  determined 
from  the  weight  of  their  insoluble  silver  salts. 

(6)  Method  of  Gooch. 

This  method  depends  upon  the  fact  that  in  a  dilute  solution  of 
the  three  halogens,  nitrous  acid  sets  free  iodine  alone: 

2KI+  2KNO2+  4H2SO4  =  4KHSO4+  2NO+  211,0+ 12, 

which  escapes  from  the  solution  on  boiling.  In  one  sample,  there- 
fore, the  halogens  are  precipitated  together  in  the  form  of  their 
silver  salts,  in  a  second  sample  the  amount  of  the  chlorine  is  deter- 
mined after  setting  free  the  iodine  by  means  of  nitrous  acid,  and 
the  amount  of  iodine  determined  by  difference.  In  order  to  obtain 
correct  results  by  this  method,  the  solution  must  be  very  dilute 
when  it  is  boiled  to  expel  the  iodine;  otherwise  some  chlorine 
escapes. 

Procedure. — The  mixture  of  the  halogen  salts  (about  0.5  gin. 
of  the  substance  should  be  dissolved  in  600-700  c.c.  water  in  a 
liter  flask)  is  treated  with  2-3  c.c.  of  dilute  sulphuric  acid, 
0.5-1  gm.  of  solid  potassium  nitrite  (free  from  halogen)  is  added, 
and  the  solution  is  boiled  until  entirely  colorless ;  in  most  cases  this 
is  accomplished  in  about  three-quarters  of  an  hour.  The  contents 
of  the  flask  are  now  treated  with  silver  nitrate  solution,  and  the 
resulting  precipitate  is  allowed  to  settle.  It  is  filtered  through  a 
Gooch  crucible,  and  weighed. 


332      GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

(c)  Method  of  Jannasch* 

Jannasch  proceeds  in  exactly  the  same  way  as  Gooch,  but  in- 
stead of  letting  the  iodine  escape,  he  collects  in  it  a  mixture  of 
caustic  soda  and  hydrogen  peroxide,  whereby  it  is  transformed  to 
sodium  iodide  and  is  subsequently  determined  as  silver  iodide. 
In  the  other  solution  the  chlorine  is  determined  in  the  usual  way. 

Procedure. — The  solution  containing  the  two  halogens  is  placed 
in  a  1^-liter  round-bottomed  flask  and  diluted  to  a  volume  of  600-700 
c.c.  Like  a  wash-bottle,  this  flask  is  provided  with  one  glass  tube 
reaching  to  the  bottom,  through  which  vapor  can  be  conducted 
into  the  flask,  and  with  another  shorter  tube  for  the  escape  of  gas. 
This  second  tube  is  connected  with  an  Erlenmeyer  flask  for  a  re- 
ceiver, and  this  is  in  turn  connected  with  a  Peligot  tube.  About 
50  c.c.  of  pure  5  per  cent,  caustic  soda  solution  are  placed  in  the 
Erlenmeyer  flask,  an  equal  volume  of  hydrogen  peroxide  free  from 
chlorine  is  added,  and  the  mixture  cooled  by  surrounding  the  flask 
with  ice  or  snow.  The  Peligot  tube  is  likewise  filled  with  a  suitable 
amount  of  caustic  soda  and  hydrogen  peroxide.  From  5-10  c.c. 
of  dilute  sulphuric  acid  (1:5)  and  10  c.c.  of  10  per  cent,  sodium 
nitrite  solution  are  now  added  to  the  solution  containing  the  halo- 
gens, the  flask  is  immediately  closed,  and  the  contents  of  the  flask 
are  heated  over  a  free  flame  while  water  vapor  is  at  the  same  time 
conducted  into  it.  As  soon  as  the  liquid  begins  to  boil,  the  space 
above  is  filled  with  violet  vapors  of  iodine,  which  are  gradually 
driven  over  into  the  Erlenmeyer  flask,  where,  with  evolution  of 
oxygen,  they  are  completely  absorbed  by  the  hydrogen  peroxide 
solution.  The  iodine  is  changed  into  sodium  iodide  and  sodium 
hypoiodite  by  means  of  the  dilute  alkali : 

21+  2NaOH  =  NaI+  NaOI+  H2O. 

The  sodium  hypoiodite.  however,  is  reduced  by  the  hydrogen 
peroxide  to  sodium  iodide: 

NaOI  +  H2O2  =  H2O  +  O2  +  Nal. 

When  all  the  iodine  is  driven  over  into  the  receiver  (which  is 
always  the  case  after  the  solution  in  the  flask  has  become  colorless 

*  Zeit.  fiir  anorg.  Chem.  1,  p.  144,  and  Prakt.  Leit.  der  Gewichts-analyse, 
p.  182  et  seq. 


SEPARATION  OF  IODINE  FROM  CHLORINE.  333 

and  has  been  boiled  for  twenty  minutes  longer),  the  delivery-tube 
between  the  distilling-flask  and  the  Erlenmeyer  flask  is  removed, 
the  liquid  within  it  is  washed  with  hot  water  into  the  Erlenmeyer 
and  the  current  of  steam  is  stopped.  The  contents  of  the  Peligot 
tube  are  added  to  the  Erlenmeyer  flask  and  the  solution  heated  to 
boiling  in  order  to  remove  the  excess  of  hydrogen  peroxide.  After 
cooling,  the  liquid  is  acidified  with  a  little  sulphuric  acid;  this  always 
causes  a  yellow  coloration  due  to  free  iodine.*  The  solution,  there- 
fore, is  treated  with  a  few  drops  of  sulphurous  acid,  whereby  it  is 
completely  decolorized.  An  excess  of  silver  nitrate  and  a  little 
nitric  acid  are  then  added,  the  liquid  is  boiled,  and  the  silver 
iodide  filtered  through  a  Gooch  crucible  and  weighed. 

For  the  chlorine  determination,  the  contents  of  the  distilling- 
flask  are  transferred  to  a  beaker  and  the  chlorine  determined  as 
silver  chloride. 

Remark. — This  method  has  been  carefully  tested  in  the  author's 
laboratory  by  O.  Brunner,  and  in  the  above  form  has  been  found 
to  give  very  exact  results. 

Jannasch  recommends  a  slightly  different  procedure.  He 
adds  silver  nitrate  directly  to  the  alkaline  solution  in  the  Erlen- 
meyer flask.  In  this  way  accurate  results  are  obtained  provided 
there  is  no  iodate  formed  by  the  absorption  of  the  iodine.  In 
the  latter  case,  due  to  insufficient  cooling  of  the  contents  of  the 
receiver,  the  addition  of  silver  nitrate  results  in  the  formation  of 
some  silver  iodate,  and  this  amount  of  iodine  escapes  determination, 
for  silver  iodate  is  soluble  (though  difficultly  so).  In  such  cases 
the  results  obtained  are  too  low."  If  the  solution  is  acidified,  how- 
ever, the  presence  of  the  iodate  is  shown  by  the  separation  of  a 
little  iodine,  and  this  can  be  changed  by  sulphurous  acid  to  iodide, 
and  accurate  results  will  be  obtained. 


*  If  the  above  directions  are  closely  followed,  there  should  not  be  much 
separation  of  iodine.  It  may  be  caused  by  the  presence  of  a  small  amount 
of  nitrous  acid  which  is  not  oxidized  to  nitric  acid  by  hydrogen  peroxide  or 
if  the  contents  of  the  Erlenmeyer  flask  are  not  kept  cool,  appreciable  amounts: 
of  sodium  iodate  (NaIO3)  are  formed,  and  the  latter  is  not  reduced  by  hydro- 
gen peroxide.  In  this  case  there  is  a  separation  of  a  considerable  amount  of 
iodine  on  acidifying  the  solution,  but  the  addition  of  sulphurous  acid  changes 
it  to  iodide  without  loss. 


334     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

Determination  of  the  Halogens  by  Indirect  Analysis. 

(a)  Determination  of  Bromine  together  with  Chlorine. 

Principle.  —  In  this  method  the  sum  of  the  weights  of  the  silver 
salts  of  the  two  halogens  is  first  determined  and  afterwards  the 
silver  bromide  converted  to  silver  chloride  by  heating  in  a  current 
of  chlorine. 

Procedure.  —  The  solution  containing  about  0.5  gm.  of  the 
halogen  salt  is  acidified  with  a  little  nitric  acid  (free  from  chlorine) 
and  precipitated  in  the  cold  by  the  addition  of  a  slight  excess  of 
silver  nitrate.  The  liquid  is  heated  to  boiling,  with  frequent  stir- 
ring, and  after  cooling  again,  the  precipitate  is  filtered  through 
a  15  cm.  long,  asbestos  filter-tube  made  of  difficultly  fusible  glass. 
The  precipitate  is  dried  at  150°  C.  and  weighed  after  cooling. 

For  the  transformation  of  the  bromide  into  chloride,  the  asbes- 
tos is  shoved  forward  a  little  in  the  tube  by  means  of  a  glass  rod 
(in  order  that  the  gas  may  pass  through  it  more  readily),  the  tube 
is  fastened  in  a  slightly  inclined  position,  and  a  current  of  dry 
chlorine  gas  is  passed  through  it.  At  the  same  time  the  tube  is 
heated  cautiously  by  moving  a  small  flame  back  and  forth.  During 
the  first  half  hour  the  precipitate  should  not  be  heated  hot 
enough  to  melt  it;  finally,  however,  the  temperature  is  raised 
until  it  begins  to  melt,  after  which  the  chlorine  is  replaced  by  air, 
and  after  cooling  the  residue  is  again  weighed. 

If  p  represents  the  combined  weight  of  the  two  silver  salts, 
and  q  the  weight  after  the  silver  has  been  completely  changed 
to  chloride,  then 

AgCl       AgBr 

1.  x    +    y    =  p 

2.x    +  my   =  q  (AgCl) 

and  from  this  it  follows:  . 

3-    --- 


In  this  equation  m=-£       =  0.7633. 

If  this  value  is  substituted  in  equation  (3),  we  obtain 
(AgBr)  y-  4.224  (p-q) 


DETERMINATION  OF  IODINE   TOGETHER  WITH  CHLORINE-   335 

and 

(AgCl)  x  =  p-y 

from  which  the  amount  of  bromine  and  chlorine  may  be  calculated. 

(6)  Determination  of  Iodine  together  with  Chlorine. 

The  same  procedure  is  used  as  above  described 
If  p  represents  the  weight  of  silver  iodide  +  silver  chloride  and 
q  the  weight  after  the  silver  has  been  converted  to  chloride,  then 

AgCl     Agl 

1.  x  +    y   =  p 

2.  x  +  my  =  q  (AgCl) 

and  from  this  it  follows: 


m=  0.6105. 

Agl 

If  this  value  is  substituted  in  equation  (3),  we  obtain 

(Agl)     y=2M7(p-q) 
and 

(AgCl)  x=p-y. 

(c)  Determination  of  Bromine  in  the  Presence  of  Iodine. 

In  this  case  p  represents  the  weight  of  the  silver  bromide  and 
silver  iodide,  and  q  as  before  the  corresponding  weight  of  silver 
chloride  : 

Agl    AgBr 

1.  x   +    y   =  p 

2.  mx  +  ny  =   g(AgCl) 
and 


o 

3.  x= p 

n—m^    n—m 


in  which 


0.6105     and    n-          =0.7633. 
AgBr 


336     GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS, 

If  these  values  for  m  and  n  are  substituted  in  equation  (3), 
we  obtain 

(Agl)  z=4.996-;p-6.545.2 
and 

(AgBr)  y  =  p-x. 

(d)  Determination  of  Iodine,  Bromine,  and  Chlorine  in  the  Presence 

of  One  Another. 

In  one  portion  of  the  substance  the  total  weight  (P)  of  the 
halogen  salts  is  determined,  and  this  is  changed  over  into  chloride 
whose  weight  (Q)  is  obtained.  In  a  second  portion  of  the  substance, 
the  iodine  is  determined  as  palladous  iodide,  whose  weight  is  (t). 

If  (t)  is  multiplied  by  1.303,  the  corresponding  weight  of  silver 
iodide  is  obtained  (p). 

If  (p)  is  subtracted  from  (P),  the  sum  of  the  weights  of  the 
silver  bromide  and  silver  chloride  is  obtained  (P  —  p). 

Again,  if  (t)  is  multiplied  by  0.7951,  the  corresponding  weight 
of  silver  chloride  is  obtained  (q),  and  if  this  is  subtracted  from  (Q), 
the  amount  of  silver  chloride  (Q  —  q)  will  be  obtained  which  corre- 
sponds to  the  amount  that  would  be  obtained  from  the  weight 

(p-p). 

If,  then,  the  amount  of  silver  chloride  is  designated  by  x  and 
the  amount  of  silver  bromide  by  y,  we  have  : 

AgCl    AgBr 

1.  x  +      y  =  (P-p) 

2.  x  +  my  =  (Q-q) 
from  which  follows  from  p.  334,  (a)  : 

3-   -P--- 


(AgBr)  y  =  4. 
and 

(AgCl)  t-(P-p)-y. 

Instead  of  determining  the  iodine  as  palladous  iodide  it  may 
be  removed  'as  on  page  331,  6,  by  treatment  with  nitrous  acid 
and  the  weight  of  the  silver  bromide  +  silver  chloride  obtained. 


HYDROCYANIC  ACID.  337 

The  amount  of  chlorine,  bromine,  and  iodine  follows  from  the 
above  calculation. 

For  the  determination  of  bromine  and  iodine  volumetrically 
consult  Part  II,  lodimetry. 

HYDROCYANIC  ACID,  HCN.    Mol.  Wt.  27.02. 

Forms:  Silver  Cyanide,  AgCN,  and  Metallic 
Silver,  Ag. 

Free  hydrocyanic  acid  as  well  as  the  cyanides  of  the  alkalies 
and  alkaline  earths  are  decomposed  quantitatively  by  silver  nitrate 
with  the  formation  of  insoluble  silver  cyanide. 

If,  therefore,  it  is  desired  to  determine  gravimetrically  the 
amount  of  cyanide  present  in  an  aqueous  solution  of  hydrocyanic 
acid  or  of  an  alkaline  cyanide,  the  cold  solution  is  treated  with  an 
excess  of  silver  nitrate,  stirred,  a  little  dilute  nitric  acid  is  added, 
the  precipitate  allowed  to  settle  and  it  is  then  filtered  through  a 
weighed  filter,  dried  at  100°  C.  and  weighed.  To  confirm  the 
result,  the  silver  cyanide  is  placed  in  a  porcelain  crucible,  the  filter 
burned  in  a  platinum  spiral,  its  ash  added  to  the  main  portion 
of  the  precipitate,  and  the  contents  of  the  crucible  ignited,  at 
first  gently  and  finally  until  the  silver  begins  to  melt;  it  is  then 
weighed. 

By  the  decomposition  of  the  silver  cyanide,  difficultly  volatile 
paracyanide  is  formed,  but  this  is  gradually  burned  away  by 
igniting  the  contents  of  the  open  crucible. 

Example :  Determination  of  Hydrocyanic  acid  in  Bitter-almond 
Water. — Bitter-almond  water  contains  cyanogen  as  free  hydrocyanic 
acid  and  as  ammonium  cyanide,  but  the  greater  part  is  present  as 
mandelic  acid  nitrile,  C6H5CH(OH)CN.  The  latter  compound  is  not 
decomposed  in  aqueous  solution  by  means  of  silver  nitrate,  but  is 
readily  acted  upon  by  the  latter  if  the  solution  is  made  ammoniacal 
after  the  addition  of  the  silver  nitrate  and  then  made  acid. 

The  gravimetric  determination  of  the  cyanogen  present  is  per- 
formed according  to  the  method  of  Feldhaus  *  as  follows : 

100  gms.  of  bitter-almond  water  are  treated  with  10  c.c.  of  a  10 
per  cent,  silver  nitrate  solution,  2-3  c.c.  of  concentrated  ammonia 
*  Z.  anal.  Chem.  Ill  (1864),  p.  34. 


338     GRAVIMETRIC  DETERMIANTION  OF   THE  METALLOIDS. 

are  added,  the  solution  is  immediately  acidifi3d  with  nitric  acid, 
the  precipitate  allowed  to  settle,  and  the  HCX  determined  as 
described  above. 

Liebig's  volumetric  method  is  much  more  satisfactory  for  this 
determination  (see  Part  II,  Precipitation  Analyses). 

If  it  is  desired  to  determine  the  amount  of  cyanogen  in  a  solid 
alkali  cyanide,  a  weighed  amount  of  the  salt  is  dissolved  in  water 
containing  silver  nitrate,  and  the  solution  then  acidified  with 
nitric  acid  and  the  precipitate  treated  as  above. 

If  the  cyanide  is  dissolved  in  water  before  the  addition  of  the 
silver  nitrate,  there  is  always  a  slight  loss  of  hydrocyanic  acid. 

Some  complex  cyanides  are  quantitatively  decomposed  by 
silver  nitrate,  e.g.  those  of  nickel,  zinc,  and  copper  (the  latter 
only  slowly) ;  while  others  such  as  the  f erro-  and  f erricyanides  of 
the  alkalies  (and  mercuric  cyanide)  are  not. 

Determination  of  Cyanogen  in  Mercuric  Cyanide,  Method  of  Rose. 

Mercuric  cyanide  is  a  non-electrolyte  and  is  consequently  not 
precipitated  by  silver  nitrate,  but  it  is  acted  upon  by  hydrogen 
sulphide  with  the  formation  of  insoluble  mercuric  sulphide  and 
hydrocyanic  acid: 

Hg(CN)2+H2S=HgS  +  2HCN. 

This  reaction,  however,  cannot  take  place  in  neutral  or  acid 
solutions  on  account  of  the  volatility  of  the  hydrocyanic  acid;  it 
must  be  performed  in  an  alkaline  solution.  In  order  to  avoid 
the  introduction  of  an  excess  of  hydrogen  sulphide  into  the  solu- 
tion, the  following  procedure  is  necessary: 

The  solution  of  the  mercuric  cyanide  is  treated  with  about 
twice  as  much  zinc  sulphate  dissolved  in  ammonia.  If  this  should 
cause  a  turbidity,  enough  ammonia  is  added  to  clear  it  up  and 
hydrogen  sulphide  water  is  then  slowly  poured  in.  This  causes  at 
first  a  brown  precipitate  which  becomes  black  on  stirring.  The 
hydrogen  sulphide  water  is  added  until  the  upper  liquid  shows 
a  pure  white  precipitate  of  zinc  sulphide.  The  zinc  sulphate, 
therefore,  serves,  as  it  were,  as  an  indicator,  inasmuch  as  the 
pure  white  precipitate  will  not  be  formed  until  the  mercury  is 
completely  precipitated.  The  precipitated  sulphides  are  now 


SULPHOCYANIC  ACID.  339 

filtered  off  and  washed  with  dilute  ammonia.  The  filtrate  con- 
tains all  of  the  hydrocyanic  acid  and  is  treated  with  an  excess  of 
silver  nitrate,  acidified  with  nitric  acid  filtered  and  the  weight  of 
the  silver  cyanide  determined  as  described  on  page  337. 

Determination  of  Hydrocyanic  Acid  and  Halogen  Hydride  in  the 
Presence  of  One  Another,  according  to  Neubauer  and 
Kerner.* 

The  solution  is  treated  with  silver  nitrate  in  the  cold,  the 
precipitate  filtered,  dried  at  100°  C.  and  in  this  way  the  total 
weight  of  the  silver  salts  is  determined.  A  definite  amount  of 
the  precipitate  is  placed  in  a  porcelain  crucible,  heated  until  it 
is  completely  melted,  and  then  reduced  with  zinc  and  sulphuric 
acid  as  described  on  page  328.  The  metallic  silver  and  para- 
cyanogen  are  filtered  off  and  the  halogen  determined  in  the  fil- 
trate according  to  page  320  et  seq. 

The  above  separation  can  be  more  satisfactorily  effected  by 
means  of  a  volumetric  process  (See  Precipitation  Analyses). 


SULPHOCYANIC  ACID,  HCNS.    Mol.  Wt.  59.09. 

Forms:  Cu2(CNS)2,  Ag(CNS),  BaSO4. 
i.  Determination  as  Cuprous  Sulphocyanate,  Cu2(CNS)2. 

The  solution  of  the  alkali  sulphocyanate,  which  is  neutral  or 
slightly  acid  with  hydrochloric  or  sulphuric  acid,  is  treated  with 
20  to  50  c.c.  of  a  saturated  solution  of  sulphurous  acid,  and  copper 
sulphate  solution  is  introduced  with  constant  stirring  until  a 
slightly  greenish  tint  is  imparted  to  the  liquid.  After  standing 
a  few  hours,  the  precipitate  is  filtered  into  a  Munroe  crucible, 
washed  with  cold  water  containing  sulphurous  acid,  then  once 
with  alcohol,  and  dried  at  130°  to  140°  to  constant  weight.  The 
results  are  good. 

*  Ann.  d.  Chem.  u.  Pharm.  (1857),  101,  p.  344. 


340      GRAVIMETRIC  DETERMINATION   OF  THE  METALLOIDS. 

2.  Determination  as  Silver  Sulphocyanate,  AgfCNS). 

This  excellent  method  for  estimating  sulphocyanic  acid  is  only 
applicable  in  the  absence  of  the  halogen  acids,  or  hydrocyanic 
acid. 

The  dilute  solution  of  the  alkali  sulphocyanate  is  treated  in 
the  cold  with  a  slight  excess  of  silver  nitrate  solution,  which  has 
been  slightly  acidified  with  nitric  acid.  After  stirring  well,  the 
precipitate  is  filtered  into  a  Munroe  crucible,  washed  with  water, 
then  with  a  little  alcohol,  dried  at  130°  to  150°  and  weighed. 

R.  Philipp  obtained  good  results  by  this  method. 

3.  Determination  as  Barium  Sulphate. 

In  the  absence  of  all  other  compounds  containing  sulphur, 
thiocyanic  acid  may  be  determined  with  accuracy  by  oxidizing 
it  and  precipitating  the  sulphuric  acid  formed  as  barium  sul- 
phate. Bromine  water  is  the  most  suitable  oxidizing  agent  for 
this  purpose.  The  alkali  sulphocyanate  solution  is  treated  with 
an  excess  of  bromine  water,  heated  for  from  thirty  minutes  to  an 
hour  on  the  water-bath,  the  solution  acidified  with  hydrochloric 
acid,  and  the  sulphuric  acid  precipitated  (according  to  the  direc- 
tions on  p)  464  et  seq.)  by  means  of  barium  chloride,  and 
weighed  as  barium  sulphate. 

Instead  of  bromine,  nitric  acid  may  be  employed  as  the 
oxidizing  agent. 

It  will  not  do  at  all,  however,  to  treat  a  solid  alkali  sulpho- 
cyanate with  strong  nitric  acid  in  an  open  vessel,  for  on  account  of 
the  violent  action  some  of  the  hydrocyanic  acid  is  volatilized  and 
escapes  oxidation.  It  is  better,  as  E.  Heberlein  found  in  the 
author's  laboratory,  to  dissolve  the  alkali  sulphocyanate  in  water 
(Heberlein  used  20  c.c.  of  a  one-tenth  normal  potassium  sulpho- 
cyanide  solution)  and  add  10  c.c.  of  fuming  nitric  acid,  keeping  the 
beaker  surrounded  with  ice.  The  solution  is  at  first  colored  yellow, 
then  deep  red,  reddish  brown,  and  finally  becomes  colorless.  The 
sulphur  is  then  by  no  means  entirely  oxidized  to  sulphuric  acid; 
to  accomplish  this  the  solution  must  be  kept  boiling  gently 


SULPHOCYANIC  ACID.  341 

for  two  hours.  It  is  then  evaporated  almost  to  dryness,  taken 
up  in  200  c.c.  of  water,  precipitated  hot  with  barium  chloride 
solution  and  the  barium  sulphate  filtered  off  and  weighed.  Heber- 
lein  found  99.79 — 99.94  per  cent,  of  the  potassium  sulphocyanate 
taken.  The  oxidation  is  more  certain,  if  the  solution  of  the  alkali 
sulphocyanate  is  placed  in  a  flask  connected  with  a  return-flow 
condenser,  treated  with  an  excess  of  fuming  nitric  acid,  boiled 
two  hours  and  then  treated  as  above.  In  this  way  Heberlein 
found  100.1  and  100.2  per  cent,  of  the  theoretical  amount  of  sul- 
phocyanic  acid.  The  oxidation  of  the  sulphpcyanic  acid  is  still 
better  effected  by  first  precipitating  the  acid  in  the  form  of  its 
silver  salt  *  and  filtering  it  off  (it  is  only  necessary  to  wash  the 
precipitate  when  a  sulphate  is  also  present).  The  funnel  contain- 
ing the  precipitate  is  then  placed  over  a  small  flask,  the  apex  of 
the  filter  is  broken  with  a  glass  rod  and  the  precipitate  washed 
into  the  flask  by  means  of  a  stream  of  nitric  acid  (sp.  gr.  1.37-1.40). 
In  this  way  there  is  no  violent  reaction  and  no  loss  of  sulphocyanic 
acid  to  be  feared.  The  contents  of  the  flask  are  heated  to  boil- 
ing for  three-quarters  of  an  hour.  If  at  the  end  of  this  time,  red 
vapors  are  still  evolved  from  the  flask  (usually  due  to  small  par- 
ticles of  filter  paper)  it  makes  no  difference;  the  oxidation  of  the 
sulphocyanic  acid  is  sure  to  have  been  complete.  The  contents  of 
the  flask  are  evaporated  to  a  small  volume  in  order  to  remove  the 
excess  of  nitric  acid,  taken  up  with  water  and  the  silver  precipitated 
as  chloride  and  filtered  off.  The  sulphuric  acid  is  precipitated  in 
the  filtrate  as  barium  sulphate  and  the  latter  is  weighed. f 

Hydrogen  peroxide  in  ammoniacal  solution  also  oxidizes  sulpho- 
cyanic acid  completely  to  sulphuric  acid  but  the  oxidation  requires 
more  time  than  in  the  case  of  nitric  acid.  By  this  method,  accord- 
ing to  Heberlein,  the  alkali  sulphocyanate  is  treated  with  a  large 
excess  of  3  to  4  per  cent,  hydrogen  peroxide  (for  20  c.c.  of  one-tenth 
normal  sulphocyanate  solution,  120  c.c.  of  3  to  4  per  cent,  hydrogen 
peroxide  are  used),  the  solution  made  ammoniacal,  allowed  to  stand 
twenty-four  hours  at  the  ordinary  temperature,  then  heated  two 
hours  on  the  water-bath,  and  finally  boiled.  After  acidifying  with 

*  W.  Borchers,  Repertorium  der  anal.  Ghemie,  1881,  p.  130. 
f  Borchers  precipitates  the  sulphuric  acid    without  removing  the  silver 
by  means  of  barium  nitrate.     The  procedure  given  here  is  better. 


342      GRAVIMETRIC  DETERMINATION  OF   THE   METALLOIDS. 

hydrochloric  acid  the  sulphuric  acid  is  precipitated  with  barium 
chloride  and  the  barium  sulphate  formed  is  weighed. 

The  oxidation  is  effected  even  more  slowly  by  potassium  per- 
carbonate. 

Determination  of  Sulphocyanic  and  Hydrocyanic  Acids  in  the 
Presence  of  One  Another  (Borchers).* 

The  amount  of  silver  nitrate  necessary  to  precipitate  both  of 
the  acids  is  determined  volumetrically  in  one  sample  of  the  substance 
(see  Precipitation  Analysis)  and  in  a  second  portion  the  weight  of 
the  barium  sulphate  formed  after  the  oxidation  of  the  sulpho- 
cyanic  acid  is  determined.  From  the  latter  weight  the  amount 
of  sulphocyanic  acid  present  can  be  computed  and  also  the  amount 
of  silver  nitrate  that  would  be  required  to  precipitate  it.  If  this 
amount  is  subtracted  from  the  amount  of  silver  nitrate  required 
to  precipitate  both  of  the  acids,  the  amount  of  silver  nitrate 
equivalent  to  the  hydrocyanic  acid  present  is  obtained. 

Determination    of    Sulphocyanic    Acid    together    with    Halogen 
Hydrides  (Volhard). 

In  one  portion  the  amount  of  sulphocyanic  acid  present  is  deter- 
mined as  barium  sulphate  after  oxidation  with  nitric  acid.  A 
second  portion  is  heated  in  a  closed  tube  with  concentrated  nitric 
acid  and  silver  nitrate  (Carius  Method, f  page  325)  after  which  the 
halogen  silver  salts  are  filtered  off,  weighed,  and  subsequently 
changed  to  silver  chloride  as  described  on  page  334.  A  third  por- 
tion is  fused  with  sodium  carbonate  and  potassium  nitrate  and 
the  iodine  determined  from  the  melt  as  palladous  iodide  (see 
page  330).  From  the  data  thus  obtained,  the  three  halogens  are 
computed  (see  page  336). 

HYDROFERROCYANIC  ACID,  H4Fe(CN)6.    Mol.  Wt.  215.9. 

Form:  Silver  Cyanide,  AgCN. 

The  most  accurate  procedure  for  the  analysis  of  cyanides  is  to  de- 
termine the  carbon  and  nitrogen  by  elementary  analysis  (which  see) . 

*  Loc.  cit. 

t  Instead  of  using  the  Carius  method,  the  halogens  and  sulphocyanide  may 
be  precipitated  by  silver  nitrate,  filtered  through  a  Gooch  crucible,  dried  at 
160°  and  weighed. 


HYDROFERROCYAN1C  ACID.  343 

Determination  as  Silver  Cyanide  (Rose-Finkener). 

This  method  depends  upon  the  fact  that  all  salts  of  hydroferro- 
cyanic  acid  on  being  heated  with  yellow  mercuric  oxide  give  up 
their  cyanogen  to  the  mercury,  forming  soluble  mercuric  cyanide, 
while  the  iron  is  changed  to  insoluble  ferric  hydroxide.  Thus 
Prussian  blue  is  decomposed  as  follows: 

Fe/"[Fe"(CN)6]3+ 9HgO  +  9H2O= 

=  9Hg(CN)2+4Fe(OH)3+3Fe(OH)2. 

A  weighed  amount  of  the  substance  is  treated  with  an  excess 
of  mercuric  oxide  and  the  liquid  is  boiled  until  the  blue  color  has 
completely  disappeared,  when  the  precipitate  is  filtered  off. 

On  filtering  off  the  insoluble  oxides,  at  first  a  clear  filtrate  is 
obtained,  but  on  washing  some  of  the  precipitate  usually  passes 
through  the  filter.  By  washing  with  a  solution  containing  a  dis- 
solved salt,  preferably  mercuric  nitrate,  it  is  possible  to  obtain,  how- 
ever, a  clear  filtrate.  Even  then  the  operation  is  tedious,  so  that 
the  attempt  has  been  made  to  avoid  the  washing  of  the  precipitate 
by  diluting  the  liquid  containing  the  precipitate  suspended  in  it 
to  a  definite  volume,  filtering  through  a  dry  filter,  measuring  off 
a  definite  volume  of  the  filtrate,  and  subsequently  determining 
the  cyanogen  as  silver  cyanide  after  first  precipitating  out  the 
mercury  as  sulphide  (see  p.  338) .  The  amount  of  cyanide  found 
is  then  -calculated  over  into  the  amount  that  would  have  been 
obtained  in  case  the  whole  of  the  solution  had  been  used  for  the 
analysis.  In  this  way  an  error  is  introduced  which  in  some 
cases  is  considerable.  Let  us  assume  that  the  Prussian  blue  was 
decomposed  in  a  100-c.c.  flask  and  after  the  decomposition  was 
complete,  the  liquid  was  diluted  up  to  the  mark;  and  that  in 
50  c.c.  of  the  filtrate  p  gms.  of  cyanide  were  found. 

The  amount  of  cyanide  in  the  portion  weighed  out  is  not  2  p  gms., 
for  the  volume  of  the  liquid  before  filtering  was  not  100  c.c.,  but 
100— v  c.c.,  where  v  is  the  volume  of  the  suspended  oxides. 
This  volume  v  can  be  determined  only  approximately,  so  that  the 
cyanogen  determination  by  this  method  will  never  be  abso- 
lutely certain.  In  order  to  obtain  exact  results,  the  first-men- 
tioned procedure  should  be  followed;  or,  better  still,  the  amount 


344         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

of   carbon   and   nitrogen   should   be    determined   by   elementary 
analysis. 

Soluble  ferrocyanides  may  be  satisfactorily  determined  by 
titration  with  potassium  permanganate  (cf.  Part  II,  Oxidation 
and  Reduction  Methods).  For  the  determination  of  the  iron 
and  other  metals,  the  substance  is  heated  with  concentrated  sul- 
phuric acid,  the  residue  after  evaporation  is  dissolved  in  water, 
and  the  solution  analyzed  as  usual. 

HYDROFERRICYANIC  ACID,  H3Fe(CN)6.    Mol.  Wt.  214.9. 

The  ferricyanides  are  analyzed  in  the  same  way  as  the  ferro- 
cyanides. 

HYPOCHLOROUS  ACID,  HC10.    Mol.  Wt.  52.47. 

Hypochlorous  acid  is  always  determined  volumetrically  and 
will  be  discussed  in  Part  II  of  this  book,  under  Oxidation  Methods. 

GROUP  II. 

NITROUS,  HYDROSULPHURIC,  ACETIC,  CYANIC,  AND  HYPO- 
PHOSPHOROUS  ACIDS. 

NITROUS  ACID,  HNO2.     Mol.  Wt.  47.02. 
Nitrous  acid  is  either  determined  volumetrically,  gasometric- 
ally,  or  colorimetrically.      The  two  former  methods  will  be  dis- 
cussed in  Parts  II  and  III  of  the  book. 

Colorimetric  Determination,  of  Peter  Griess. 

This  method  serves  only  for  the  determination  of  extremely 
small  amounts  of  nitrous  acid  (e.g.,  in  drinking-waters),  and 
depends  upon  the  formation  of  intensively  colored  azo-dyes. 

Inasmuch  as  azo-compounds  are  formed  only  when  nitrous 
acid  is  present,  they  can  all  be  used  in  testing  for  this  acid,  but 
the  different  substances  do  not  prove  equally  sensitive  as  reagents. 
Thus  in  the  production  of  tri-amido-azo-benzene  (Bismarck  brown) 
not  less  than  Tfg-  mgm.  of  nitrous  acid  in  a  liter  can  be  detected, 
while  according  to  the  following  procedure  y^^  mgm.  in  a  liter 
can  be  detected  with  certainty.  To  carry  out  the  determination 
two  solutions  are  necessary,  one  of  sulphanilic  acid  and  one  of 


NITROUS  ACID.  345 

a-naphthylamine.     Both  substances  are  dissolved  in  acetic  acid  * 
and  prepared  according  to  the  directions  of  Ilosvay  f  as  follows : 

1.  0.5  gm.  of  sulphanilic  acid  is  dissolved  in  150  c.c.  of  dilute 
acetic  acid. 

2.  0.1  gm.  of  solid  a-naphthylamine  is  boiled  with  20  c.c.  of 
water,  the  colorless  solution  is  poured  off  from  the  bluish-violet 
residue,  and  150  c.c.  of  dilute  acetic  acid  are  added. 

These  two  solutions  are  now  mixed. {  It  is  not  necessary  to 
protect  the  reagent  from  the  action  of  light,  but  it  is  desirable  to 
keep  impure  air  away  from  it.  As  long  as  the  solution  remains 
colorless  it  can  be  used.  If  it  comes  in  contact  with  nitrous  acid, 
which  is  often  present  in  the  air,  the  reagent  becomes  red,  and  in 
this  case  it  must  be  decolorized  by  shaking  with  zinc-dust  before 
using. 

Besides  the  above  reagent,  it  is  necessary  to  prepare  a  solution 
of  sodium  nitrite  of  known  strength.  For  this  purpose  a  concen- 
trated solution  of  commercial  potassium  nitrite  is  treated  with 
silver  nitrate  solution,  the  precipitated  silver  nitrite  is  filtered  off 
and  washed  a  few  times  with  cold  water.  In  order  to  obtain  abso- 
lutely pure  silver  nitrite  the  precipitate  is  dissolved  in  as  little  hot 
water  as  possible  and  quickly  cooled .  The  mass  of  crystals  is  placed  in 
a  funnel  provided  with  a  platinum  cone,  and  after  being  sucked  free 
from  mother-liquor,  it  is  washed  with  a  small  amount  of  distilled 
water.  The  silver  nitrite  is  placed  in  a  calcium  chloride  desiccator 
and  allowed  to  dry  in  the  dark.  As  soon  as  it  has  become  dry 
(shown  by  its  having  assumed  a  constant  weight)  exactly  0.4047  gm. 
of  it  is  weighed  out  into  a  liter  flask  and  dissolved  in  hot  distilled 
water.  About  0.2  to  0.3  gm.  of  pure  sodium  chloride  is  added  (i.e., 
a  little  more  than  the  theoretical  amount)  in  order  to  convert  the 
silver  nitrite  into  silver  chloride  and  sodium  nitrite.  After  becom- 
ing cold,  the  solution  is  diluted  to  exactly  one  liter  with  pure 
water,  then  thoroughly  shaken,  and  the  precipitate  allowed  to  settle. 
After  this,  100  c.c.  of  the  clear  liquid  are  pipetted  into  a  second 

*  P.  Griess  used  dilute  sulphuric  acid  to  set  free  the  nitrous  acid.     Ilosvay 
showed  that  if  acetic  acid  were  used  the  reaction  was  much  more  sensitive, 
t  Bull.  chim.  [2]  2,  p.  317. 
%  Lunge,  Zeitschr.  f.  angew.  Chem.  1899,  Heft  23. 


346         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

liter  flask  and  diluted  up  to  the  mark  with  water  free  from  nitrous 
acid.     1  c.c.  of  this  solution  contains  0.01  mgm;  N2O3. 

Procedure  for  the  Determination. 

50  c.c.  of  the  water  to  be  examined  are  placed  in  a  cylinder, 
such  as  is  shown  on  p.  61,  treated  with  5  c.c.  of  the  reagent, 
and  the  contents  of  the  cylinder  mixed  with  the  aid  of  the 
stirrer  shown  in  Fig.  25;  the  cylinder  is  placed  in  water  at 
about  70-80°  C.  If  as  much  as  y^-  mgm.  of  nitrous  acid  is  pres- 
ent in  a  liter  of  the  water  tested,  the  red  coloration  will  appear 
within  one  minute;  with  relatively  larger  amounts  (e.g.,  as  much 
as  1  mgm.  per  liter)  the  solution  is  simply  colored  yellow,  unless 
a  concentrated  solution  of  naphthylamine  is  used.  Meanwhile 
in  three  other  cylinders  are  placed  respectively  0.1  c.c.,  0.5  c.c.,  and 
1  c.c.  of  the  solution  containing  a  known  amount  of  sodium  nitrite; 
each  is  diluted  with  water  up  to  the  mark  and  treated  with  the 
reagent  in  the  same  way.  As  soon  as  a  distinct  red  coloration  is 
apparent,  the  colors  are  compared  with  that  produced  by  the  water 
to  be  analyzed.  If  the  color  of  the  unknown  water  lies  between 
two  of  the  standards — e.g.,  between  that  produced  with  0.1  and  0.5 
c.c.  of  the  standard — then  three  more  standards  are  prepared  con- 
taining, say,  0.2,  0.3,  and  0.4  c.c.  of  the  known  solution.  When 
the  color  of  the  unknown  solution  is. matched,  then  the  water  con- 
tains the  same  amount  of  nitrous  acid  as  the  standard. 

If  the  water  contains  considerable  nitrous  acid  (e.g.,  over  0.3 
mgm.  per  liter),  then  the  red  coloration  will  be  so  dark  that  the 
colorimetric  determination  cannot  be  performed  with  certainty. 
In  this  case  a  definite  volume  of  the  water  is  diluted  with  distilled 
water  and  the  nitrous  acid  present  in  this  diluted  water  is  deter- 
mined as  before. 

TromsdorfT  recommends  for  the  determination  of  nitrous  acid 
in  drinking-water  the  use  of  zinc  iodide  of  starch,  and  comparing 
the  blue  color  produced  by  the  nitrous  acid  (cf.  Vol.  I,  p.  287).  If 
TV  mgm.  of  nitrous  acid  is  present  in  a  liter,  the  blue  color  produced 
can  be  distinctly  seen;  with  T%  mgm.  per  liter,  however,  the  color 
is  so  intense  that  it  is  unsuited  for  a  colorimetric  determination. 
This  method  is  not  to  be  recommended  because  in  the  first  place 


HYDROSULPHURIC  ACID.  347 

it  is  far  lass  sensitive  than  the  Griess  method,  and  second  because 
it  can  easily  lead  to  error  inasmuch  as  a  blue  color  will  be  often 
produced  when  there  is  no  nitrous  acid  present.  Traces  of  hydrogen 
peroxide  or  ferric  salts,  which  are  likely  to  be  present  in  a  drinking- 
water,  will  also  cause  the  solution  of  zinc  iodide  of  starch  to  turn 
blue. 

HYDROSULPHURIC  ACID,  H2S.    Mol.  Wt.  34.09. 

Forms:  Barium  Sulphate,  BaSO4,  Hydrogen  Sulphide,  H2S, 
and  colorimetrically. 

There  are  four  cases  to  be  considered: 

I.  The  determination  of  free  hydrogen  sulphide. 
II.  The  determination  of  sulphur  in  sulphides  soluble  in  water. 

III.  The  determination  of  sulphur    in   sulphides   insoluble   in 
water  but  decomposable  by  dilute  acids  with  evolution  of  hydro- 
gen sulphide. 

IV.  The  determination  of  sulphur  in  insoluble  sulphides. 

I.  Determination  of  Free  Hydrogen  Sulphide. 
(a)  Determination  of  Hydrogen  Sulphide  in  Gas  Mixtures. 

In  case  it  is  desired  to  know  the  per  cent,  of  hydrogen  sulphide 
present  in  a  mixture  of  gases,  the  analysis  is  best  made  volumetri- 
cally  (s°e  Part  II,  lodhietry),  but  it  is  possible  to  accomplish  the 
same  end  by  a  gravimetric  process. 

The  source  of  the  gas  is  connected  by  means  of  rubber  tubing 
with  the  ten-bulb  absorption-tube  shown  in  Fig.  55,  page  359,  *  which 
contains  a  solution  of  ammoniacal  hydrogen  peroxide  free  from  sul- 
phuric acid.  The  other  end  of  the  absorption- tube  is  connected  with 
an  aspirator,  i.e.  a  large  bottle  of  about  4-5  liters  capacity  filled 
with  water  and  closed  by  means  of  a  double-bored  stopper.  Through 
one  hole  of  the  stopper  is  passed  a  right-angled  glass  tube  which 
reaches  just  below  the  bottom  of  the  stopper  in  the  bottle,  and  its 
other  end  is  connected  with  the  absorption-tube.  Through  the  other 
hole  in  the  stopper  is  placed  a  glass  tube  reaching  to  the  bottom 

*  Usually  two  of  these  tubes  are  used  in  order  to  make  sure  that  none  of 
the  gas  escapes  absorption. 


348        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

of  the  bottle.  The  upper  end  of  this  tube  is  likewise  bent,  and  is 
connected  with  a  rubber  tube  to  serve  as  a  siphon  ;  on  the  lower 
end  of  the  rubber  tube  is  a  screw-cock. 

Before  beginning  the  experiment,  the  air  in  the  rubber  tubing 
between  the  source  of  gas  and  the  absorption-tube  is  removed 
by  conducting  the  gas  to  be  analyzed  through  it.  When  this  is 
accomplished  the  tubing  is  connected  with  the  absorption-tube. 
Water  is  now  allowed  to  run  slowly  from  the  aspirator  into  a 
vessel  graduated  in  liters;  after  from  2-5  liters  of  the  water  have 
run  out,  the  aspirator  is  closed  by  screwing  up  the  cock  on  the 
siphon  arm.  The  contents  of  the  absorption-tube  are  poured  into 
a  beaker,  slowly  heated  to  boiling,  and  kept  at  this  temperature 
for  from  five  to  ten  minutes.  The  solution  is  then  evaporated  on 
the  water-bath  to  a  small  volume,  a  little  hydrochloric  acid  is 
added,  the  solution  filtered  if  necessary,  and  the  sulphuric  acid  pre- 
cipitated at  a  boiling  temperature  with  a  boiling  solution  of 
barium  chloride.  After  the  precipitate  has  settled,  it  is  filtered 
off,  ignited  wet  in  a  platinum  crucible,  and  weighed  as  barium 
sulphate. 

Both  at  the  beginning  and  end  of  the  experiment  it  is  necessary 
to  note  the  temperature  of  the  room  and  the  barometer  reading. 
The  mean  of  these  readings  is  used  for  the  calculation.  The 
amount  of  hydrogen  sulphide  present  in  the  gas  is  computed  as 
follows  : 

The  volume  of  water  which  has  flowed  out  of  the  aspirator 
represents  the  volume  of  the  gas  that  has  been  sucked  through 
the  apparatus  less  the  amount  absorbed  by  the  ammoniacal  hy- 
drogen peroxide  solution.  Let  V  represent  the  volume  of  water  in 
liters  which  has  flown  from  the  aspirator  and  p  the  weight  of 
barium  sulphate  found. 

Since  one  gram  molecule  of  barium  sulphate  corresponds  to 
one  gram  molecule  of  hydrogen  sulphide  and  the  latter  assumes  at 
0°  C.  and  760  mm.  pressure  a  volume  of  22.159  liters,*  we  have: 

BaSO4:22.159:p:Fi; 
*   y  =  —  '  d'P  —  the  volume  of  the  hydrogen  sulphide  absorbed. 


*  According  to  Leduc,  Comptes  rendus,  125,  571  (1897)  the  density  of  H2S 
(referred  to  air  =1)  is  1.1895,  from  which  the  molecular  volume  is  computed 
as  22.159  liters. 


DETERMINATION  OF  SULPHUR   IN  SULPHIDES.  349 

Now  the  volume  (V)  of  the  gas  that  passed  through  the  apparatus 
was  at  t°  and  B  mm.  pressure,  while  FI  is  measured  at  0°  C.  and 
760  mm.  pressure.     It  is  necessary,  therefore,  to  reduce  V  to  0°  C 
and  760  mm.  pressure. 

_7-(B-uQ273 
°~   760(273  +  0  ' 
The  volume  of  the  gas  drawn  through  the  apparatus  is  then; 

and  we  have: 


V  •  100 

£=  TT37  \f  =  the  Per  cen^.  by  volume  of  hydrogen  sulphide  present. 
Ko+  M 

(6)  Determination  of  the  Amount  of  Hydrogen  Sulphide  Present 

in  Solution. 

By  means  of  a  pipette  a  definite  volume  of  the  solution  is 
measured  out  and  allowed  to  run  'into  ammoniacal  hydrogen  per- 
oxide with  constant  stirring  of  the  latter  by  means  of  the  pipette 
itself.  After  heating  to  boiling  and  acidifying  with  hydrochloric 
acid,  the  amount  of  sulphuric  acid  formed  is  determined  as  barium 
sulphate. 

II.  Determination  of  Sulphur  in  Sulphides  Soluble  in  Water. 

(a)  The  solution  is  treated  with  an  excess  of  ammoniacal 
hydrogen  peroxide  water,  slowly  heated  to  boiling  and  kept  at 
that  temperature  until  the  excess  of  the  reagent  is  destroyed,  when 
the  sulphuric  acid  is  precipitated  with  barium  chloride  and  weighed 
as  barium  sulphate. 

(3)  The  solution  is  treated  with  bromine  water  until  a  perma- 
nent brown  color  is  obtained,  when  it  is  warmed,  acidified  with 
hydrochloric  acid,  and  the  sulphuric  acid  determined  as  barium 
sulphate. 

If  the  solution  contains  thiosulphate,  sulphide,  and  sulphate, 
as  is  likely  to  be  the  case  after  standing  in  the  air  for  some  time, 
the  sulphide  sulphur  is  precipitated  by  means  of  cadmium  acetate 
and  the  sulphur  in  the  precipitate  is  determined  as  under  III,  or 
the  cadmium  sulphide  is  oxidized  with  either  bromine  water  or 


350       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

fuming  nitric  acid,  and  the  sulphuric  acid  formed   determined   as 
barium  sulphate. 

The  determination  of  thiosulphate,  sulphide,  and  sulphite 
sulphur  will  be  discussed  in  Part  II  of  this  book  under  lodimetry. 

III.  The  Determination  of  Sulphur  in  Sulphides  Soluble  in 
Dilute  Acids. 

Principle. — The  hydrogen  sulphide  is  evolved  by  treatment 
of  the  sulphide  with  dilute  acids,  and  absorbed  in  ammoniacal  hydro- 
gen peroxide  solution  as  under  I;  or  the  hydrogen  sulphide  is 
absorbed  in  caustic  soda  solutioa  and  the  sodium  sulphide  formed 
analyzed  according  to  II ;  or  finally  the  gas  may  be  absorbed  in  a 
weighed  tube  containing  pumice  soaked  with  copper  sulphate 
solution,  in  which  case  the  gain  in  weight  represents  the  amount  of 
gas  absorbed. 

Evolution  and  Absorption  of  the  Hydrogen  Sulphide. 

In  the  case  of  sulphides  rich  in  sulphur  0.25-0.50  gm.  of  the 
substance  should  betaken  for  the  analysis,  whereas  of  those  contain- 
ing less  sulphur  a  correspondingly  larger  amount  should  be  taken. 
The  substance  is  placed  in  an  Erlenmeyer  flask  (Fig.  54,  a)  the  con- 
nection between  the  flask  and  the  receiver  is  broken  and  the  air  is 
expelled  from  K  by  conducting  hydrogen  gas  through  the  delivery 
tube  and  out  through  the  open  stop-cock  of  T.  After  a  rapid 
current  of  hydrogen  has  passed  through  the  apparatus  for  about 
five  minutes,  the  receivers  V  and  P  are  partly  filled  with  an 
ammoniacal  solution  of  hydrogen  peroxide  *  (about  3-4  per  cent. 
H2O2);  placing  about  100  c.c.  of  the  solution  in  V  and  about 
10-20  c.c.  in  P. 

The  receiver,  V,  is  now  connected  with  the  delivery-tube  from 
the  evolution-flask  K,  and  hydrogen  is  conducted  from  T  throughout 
the  whole  apparatus  for  five  minutes  more  in  order  to  remove  as 

*  In  case  hydrogen  peroxide  is  not  at  hand,  the  receivers  should  contain 
100  c.c.  of  dilute  sodium  hydroxide  solution  (250  gm.  to  1  liter).  After  the 
decomposition  is  complete  the  contents  of  the  receiver  are  transferred  to  a 
beaker,  30-50  c.c.  of  bromine  water  are  added,  the  solution  acidified  with 
hydrochloric  acid  (sp.  gr.  1.19)  and  boiled  while  carbon  dioxide  is  passed 
through  it  until  the  excess  of  bromine  is  completely  expelled.  The  sulphuric 
acid  formed  is  then  precipitated  with  a  hot  solution  of  barium  chloride. 
Instead  of  oxidizing  the  sodium  sulphide  to  sodium  sulphate  it  can  be 
titrated  with  iodine  (cf.  lodimetry). 


ANALYSIS   OF  SULPHIDES. 


351 


much  as  possible  of  the  air  from  the  receivers*  After  this,  about 
20  c.c.  of  boiled  water  are  introduced  into  K  through  T  so  that  the 
substance  is  entirely  covered,  then  dilute  hydrochloric  acid  (1  vol. 
concentrated  acid  -f  1  vol.  of  boiled  water)  is  slowly  added  to  the 
contents  of  the  flask  and  the  decomposition  is  promoted  by  warm- 


- 


%M  FIG.  54. 

ing  somewhat.  When  the  evolution  of  the  gas  has  ceased,  the 
contents  of  K  are  heated  to  a  gentle  boiling  and  a  slow  current 
of  hydrogen  *  is  conducted  through  the  apparatus  from  T  for 
twenty  minutes,  when  the  flame  is  removed  and  the  current  of 
hydrogen  is  continued  for  fifteen  minutes  longer.  At  the  end  of 
this  time,  the  hydrogen  sulphide  will  surely  completely  be  driven 
over  into  F.f 

*  The  hydrogen  is  evolved  from  zinc  and  sulphuric  acid  in  a  Kipp  gener- 
ator. The  gas  is  washed  first  with  an  alkaline  lead  solution  in  order  to  remove 
traces  of  hydrogen  sulphide  and  then  with  water. 

t  By  the  absorption  of  the  hydrogen  sulphide  hi  the  ammoniacal  solution 


352        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

The  contents  of  the  two  receivers  are  washed  into  a  beaker 
and  slowly  heated  to  boiling  in  order  to  effect  the  complete  oxi- 
dation of  the  thiosulphuric  and  sulphurous  acids  and  to  expel  the 
excess  of  the  hydrogen  peroxide.  The  solution  is  finally  acidi- 
fied with  hydrochloric  acid  and  the  sulphuric  acid  determined  as 
barium  sulphate. 

This  method  yields  excellent  results  and  can  be  applied 
to  the 

Determination  of  Sulphur  in  Iron  and  Steel. 

As  Phillips  and  Blair  have  shown,*  the  sulphur  present  in 
different  kinds  of  iron  and  steel,  especially  cast  iron,  may  be 
present  in  four  different  conditions: 

1.  By  far  the  greater  part  is  evolved  as  hydrogen  sulphide 
when  the  metal  is  treated  with  hydrochloric  acid. 

2.  Another    part    is   evolved    probably   as   methyl   sulphide 
(CH3)2S,  an  extremely  stable   sulphur   compound  which  is  not 
very  much  affected  by  ammoniacal  hydrogen  peroxide,  bromine 
in  hydrochloric  acid,  or  aqua  regia.     The  sulphur  in  this  compound 
is  changed   completely  into  hydrogen  sulphide  on  being  passed 
through  a  tube  heated  to  redness,   in  which  hydrogen  is  also 
present. 

3.  Another  part  of  the  sulphur  present  is  not  volatilized  by 
the  action  of  hot  dilute  hydrochloric  acid,  but  can  be  oxidized  to 
sulphuric  acid  by  treating  the  contents  of  the  evolution  flask  with 
nitric  acid  or  aqua  regia. 

4.  Another  very  small  part  of  the  sulphur  may  be  present  in 
the  form  of  an  insoluble  sulphide  which  is  not  oxidized  by  nitric 
acid  or  aqua  regia  and  can  only  be  obtained  in  solution  after 
fusion  with  sodium  carbonate  and  potassium  nitrate. 

of  hydrogen  peroxide  the  latter  is  always  colored  somewhat  yellow  owing  to 
the  formation  of  a  little  ammonium  disulphide.  This,  yellow  color  can  be 
distinctly  seen  in  the  delivery -tube,  where  it  dips  into  the  solution  in  the 
receiver  and  later  disappears  owing  to  further  oxidation: 

(NH4)2S2->(NH4)2S203-^(NH4)2S03->(NH4)2S04. 

When  the  color  can  no  longer  be  detected,  it  is  a  sign  that  the  greater  part 
of  the  hydrogen  sulphide  has  been  driven  over. 
*  Cf.  J.  Am.  Chem.  Soc  ,  19,  114  (1897). 


DETERMINATION  OF  SULPHUR  IN  IRON  AND  STEEL.         353 

Inasmuch  as  the  amount  of  sulphur  present  is  so  small,  a  large 
amount  of  the  substance  must  be  taken  for  the  analysis.  For 
pig  iron  2-5  gms.  are  sufficient,  while  with  steel  5  gms.,  and  with 
wrought  iron  as  much  as  10  gms.  should  be  used. 

The  determination  is  carried  out  in  the  same  way  as  before, 
except  in  this  case  a  stronger  acid  should  be  used  (HC1  sp.  gr.  1.12) ; 
this  is  allowed  to  act  upon  the  iron  at  once  without  first  cover- 
ing it  with  water,  and  the  boiling  is  continued  for  at  least  twenty 
minutes  after  the  gas  evolution  has  ceased. 

Remark. — The  sulphur  present  in  steel  or  cast  iron  made  by 
the  Thomas-Gilchrist,  or  basic  Bessemer,  process  can  as  a  rule  be 
determined  accurately  by  this  method.  In  the  case  of  certain 
other  steels  and  cast  irons,  however,  the  results  are  likely  to  be 
low.  In  order  to  carry  out  an  accurate  determination  in  such 
cases,  a  tube  made  of  difficultly  fusible  glass  (about  30  cm.  long  and 
1  cm.  wide)  is  inserted  between  the  evolution  flask  K  and  the 
absorption  flask  V  (Fig.  54).  After  the  air  has  been  replaced  by 
hydrogen,  this  tube  is  heated  to  dark  redness  by  means  of  a  small 
furnace  of  from  four  to  six  burners,  whereby  the  sulphur  in  the 
methyl  sulphide  passing  through  the  tube  is  converted  completely 
into  hydrogen  sulphide. 

When  the  use  of  this  tube  is  adopted,  care  must  be  taken  that 
no  drops  of  water  enter  the  red-hot  tube.  To  this  end,  the  liquid 
in  the  flask  K  should  only  be  boiled  very  gently,  or  what  is  better, 
the  flask  should  be  connected  with  a  return  flow  condenser.  (Cf. 
p.  381). 

The  insoluble  residue  which  is  obtained  especially  in  the  case 
of  irons  containing  considerable  silicon,  often  contains  considerable 
amounts  of  sulphur.  The  residue  is,  therefore,  filtered  off,  washed, 
dried,  fused  with  sodium  carbonate  and  potassium  nitrate  (cf. 
p  357),  the  melt  extracted  with  water,  the  resulting  solution  evap- 
orated with  hydrochloric  acid,  any  deposited  silicic  acid  filtered 
off,  and  the  sulphuric  acid  in  the  final  filtrate  determined  as  barium 
sulphate  in  the  usual  way. 


354        GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS, 


Bamber  Method  for   Determining  Sulphur   in   Iron   and   Steel. 

On  account  of  the  uncertainty  in  obtaining  all  the  sulphur 
present  in  iron  or  steel  by  the  above  evolution  method,  the  Com- 
mittee on  Standard  Methods  for  the  Analysis  of  Iron — American 
Foundrymen's  Association,  have  recommended  the  following 
method,  which  is  that  proposed  by  Bamber. 

A  3-gm.  sample  of  drillings  is  dissolved  in  concentrated  nitric 
acid.  After  the  iron  is  completely  dissolved,  2  gms.  of  potassium 
nitrate  are  added,  the  solution  is  evaporated  to  dryness  in  a 
platinum  dish  and  the  dry  residue  is  ignited  over  an  alcohol 
lamp  at  a  red  heat.  After  the  ignition,  50  c.c.  of  a  1  per  cent, 
solution  of  sodium  carbonate  are  added,  the  liquid  boiled  for  a 
few  minutes,  and  then  filtered,  washing  the  precipitate  with  hot 
1  per  cent,  sodium  carbonate  solution.  The  filtrate  containing 
all  the  sulphur  is  evaporated  to  dryness  with  hydrochloric  acid, 
the  residue  thus  obtained  is  taken  up  in  50  c.c.  of  water  and  2  c.c. 
of  concentrated  hydrochloric  acid,  and  the  resulting  solution  is 
filtered.  The  filtrate  is  diluted  to  a  volume  of  about  100  c.c.  and 
precipitated  hot  with  barium  chloride  solution. 

During  the  determination  great  care  should  be  taken  to  prevent 
the  absorption  of  fumes  containing  sulphur.  For  this  reason  a 
gas  flame  should  not  be  used  at  any  stage  in  the  process. 

Colorimetric  Determination  of  Sulphur  in  Iron  and  Steel.* 

Principle. — The  hydrogen  sulphide  evolved  from  a  weighed 
amount  of  iron  is  passed  into  a  flask  containing  cadmium  acetate 
whereby  the  latter  is  colored  yellow  owing  to  the  formation  of 
cadmium  sulphide;  the  intensity  of  the  color  is  proportional  to 
the  amount  of  hydrogen  sulphide. 

If  a  grams  of  substance  produce  a  certain  shade  then  it  would 
take  2a  grams  of  a  substance  containing  half  as  much  sulphur  to 
duplicate  it,  or,  in  other  words,  the  relations  holds,  as  =  aV,  where 
a  and  a'  represent  the  amount  of  substance  taken  for  the  analysis 

*  J.  Wiborgh;  Stahl  und  Eisen,  6  (1866;,  p.  240. 


DETERMINATION  OF  SULPHUR  IN  IRON  AND  STEEL       355 


and  s  and  s'  the  percentage  of  sulphur  present.  In  the  first  place, 
then,  a  scale  must  be  prepared  of  different  shades  representing 
different  percentages  of  sulphur.  For  this  purpose,  Wiborgh 
uses  the  apparatus  shown  in  Fig.  55.  It  consists  of  a  250-300-c.c. 

Erlenmeyer  flask  A  with  a  side- 
arm  funnel  T  and  with  a  ground- 
glass  connection  between  the  cylin- 
der B.  The  latter  is  about  20  cm. 
long,  and  is  from  5.5-6.0  cm.  wide 
at  the  top  and  about  8  mm.  at  the 
bottom.  The  upper  edge  of  the 
cylinder  is  rounded  over  and  ground 
perfectly  smooth.  Upon  this  upper 
edge  are  placed  two  rubber  rings 
of  the  same  inner  diameter  as  the 
glass  cylinder.  Between  these  two 
rings  is  laid  a  circular  piece  of  cloth 
C  that  has  been  dipped  in  a  solution 
of  cadmium  acetate,  and  upon  the 
upper  rubber  ring  is  placed  a  wooden 
ring  H  which  is  held  firmly  against 
the  edge  of  the  cylinder  by  means 
of  three  clamps  K  (only  two  are 
shown  in  the  illustration). 
The  flask  A  is  filled  not  quite  half  full  with  distilled  water,  the 
contents  boiled  a  few  minutes  to  remove  the  air,  the  flame  is  re- 
moved, and  a  weighing-tube  containing  a  definite  amount  of  a 
substance  whose  sulphur  content  is  known  is  thrown  into  the 
flask.  The  cylinder,  with  the  cadmium  acetate  cloth  in  position, 
is  placed  upon  the  flask,  and  the  gentle  boiling  is  continued  until 
the  cloth  is  uniformly  moistened  with  the  aqueous  vapor  which 
is  seen  to  pass  through  it.  The  water  must  not  be  boiled  too 
strongly  and  the  cloth  must  not  be  allowed  to  puff  up,  for  in  that 
case  it  will  become  distorted  and  afterward  an  unevenly  colored 
surface  will  be  obtained.  After  boiling  for  three  or  four  minutes 
sulphuric  acid  (1:5)  is  cautiously  added,  drop  by  drop,  to  the 
contents  of  the  flask  (3  c.c.  for  each  0.1  gm.  of  iron)  through  the 


FIG.  55. 


356        GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

funnel  T.  The  evolution  of  hydrogen  sulphide  begins  at  once 
and  is  recognized  by  the  cadmium  acetate  cloth  becoming  yellow. 
After  the  acid  has  all  been  added,  the  boiling  is  continued  until 
there  is  no  more  gas  evolved  from  the  substance,  and  then  for 
ten  minutes  more  in  order  to  completely  expel  it  from  the  solu- 
tion. 

The  piece  of  cloth  is  now  removed  and  placed  upon  a  piece  of 
white  filter-paper,  so  that  the  side  which  was  toward  the  flask  is  on 
top.  In  the  same  way  a  scale  of  six  different  shades  is  prepared 
corresponding  to  the  following  table : 

Tint  No.  1.  Tint  No.  4. 

Amount  Per  Cent.  Amount                         Per  Cent. 

Weighed  Sulphur  Weighed                         Sulphur 

Out.  Present.  Out.                             Present. 

0-8  0-0025  0-8  0-015 

0-4  0-005  0-4  0-030 

0-2  0-010  0-2  0-060 

0-1  0-020  0-1  0-120 

0-08  0-025  0-08  0-150 

0-04  0-050  0-04  0-300 

0-02  0-100  0-02  0-600 

Tint  No.  2.  Tint  No.  5. 

0-8  0-005  0-8  0-025 

0-4  0-010  0-4  0-050 

0-2  0-020  0-2  0-100 

0-1  0-040  0-1  0-200 

0-08  0-050  0-08  0-250 

0-04  0-100  0-04  0-500 

0-02  0-200  0-02  1-000 

Tint  No.  3.  Tint  No.  6. 

0-8  0-01  0-8  0-035 

0-4  0-02  0-4  0-070 

0-2  0-04  0-2  0-140 

0-1  0-08  0-1  0-280 

0-08  0-10  0-08  0-350 

0-04  0-20  0-04  0-700 

0-02  0-40  0-02  1-400 

To  illustrate  the  use  of  this  table,  suppose  we  wish  to  prepare 
the  scale  from  a  sample  of  steel  containing  exactly  0.17  per  cent, 
of  sulphur.  How  much  of  it  should  be  weighed  out  in  order  to 
prepare  Tint  No.  1  ? 

From  the  table  we  know  that  this  shade  can  be  prepared  by 


DETERMINATION  OF  SULPHUR  IN  INSOLUBLE  SULPHIDES.     357 

weighing  out  0.8  gm.~of  an  iron  containing  0.0025  per  cent,  sul- 
phur,  and  it  follows  from  what  has  been  said  : 

0.8X  0.0025  =  xX  0.017 


We  must,  therefore,  weigh  out  0.0118  gm.  of  the  steel  in  order 
to  prepare  Tint  No.  1. 

In  the  same  way  the  amount  necessary  to  produce  Tint  No.  2 
will  be  found  to  be  0.0235  gm.,  etc.  For  the  determination  proper, 
from  0.1-0.8  gm.  of  the  substance  (according  to  its  supposed  sul- 
phur content)  is  weighed  out  and  treated  in  the  same  way.  If 
with  a  sample  of  0.2  gm.  a  shade  corresponding  to  Tint  No.  5  is 
obtained,  the  table  shows  us  that  0.1  per  cent,  of  sulphur  is  present. 

Remark.  —  The  above  process  is  very  simple  and  to  be  recom- 
mended in  case  a  large  number  of  sulphur  determinations  are  to  be 
made,  as  is  the  case  in  iron  and  steel  laboratories.  It  is  to  be 
noted,  however,  that  an  accurate  value  is  obtained  only  when  all 
the  sulphur  is  present  in  a  form  such  that  it  is  evolved  as  hydrogen 
sulphide  on  treatment  with  acid. 

IV.  Determination  of  Sulphur  in  Insoluble  Sulphides. 

For  this  analysis  the  sulphur  is  either  oxidized  to  sulphuric 
acid  and  determined  as  barium  sulphate,  or  the  sulphide  is  acted 
upon  in  a  suitable  apparatus  with  nascent  hydrogen,  whereby  the 
sulphur  is  evolved  as  hydrogen  sulphide  and  is  absorbed  by  one  of 
the  above-described  methods. 

The  oxidation  of  the  sulphide  can  take  place: 

(a)    In  the  Dry  Way. 
(6)    In  the  Wet  Way, 

(A)    OXIDATION    IN   THE    DRY   WAY. 

1.  Fresenius'  Method:  Fusion  with  Sodium  Carbonate  and 
Potassium  Nitrate. 

About  0.5  gm.  of  the  extremely  finely  powdered  sulphide  is 
intimately  mixed  in  a  spacious  nickel  crucible  with  twelve  times 


358       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

as  much  of  a  mixture  of  four  parts  sodium  carbonate  and  one  part 
potassium  nitrate,*  covered  with  a  thin  layer  of  the  mixture  and 
heated  at  first  gently,  then  gradually  increasing  the  temperature 
until  the  contents  of  the  crucible  are  melted ;  it  is  then  kept  at  this 
temperature  for  fifteen  minutes.  After  cooling,  the  melt  is 
extracted  with  water,  filtered,  the  residue  boiled  with  pure  dilute 
sodium  carbonate  solution  and  finally  washed  with  water  to  the 
disappearance  of  the  alkaline  reaction.  The  filtrat  e  is  treated  in  a 
covered  beaker  with  an  excess  of  hydrochloric  acid  boiled  to 
expel  the  carbon  dioxide,  and  evaporated  to  dryness.  In  order 
to  remove  all  of  the  nitric  acid,  the  dry  mass  is  treated  with  10  c.c. 
concentrated  hydrochloric  acid  and  again  evaporated  to  dryness. 
This  last  residue  is  moistened  with  1  c.c.  concentrated  hydro- 
chloric acid,  treated  with  100  c.c.  water,  and  filtered  if  necessary. 
The  filtrate  is  diluted  to  450  c.c.,  heated  to  boiling  and  precipitated 
with  24  c.c.  of  normal  barium  chloride  solution!  which  is  diluted 
to  100  c.c.  and  added  as  quickly  as  possible  while  stirring  vigor- 
ously (cf.  sulphuric  acid). 

Remark. — This  is  the  most  reliable  method  for  determining 
the  total  amount  of  sulphur  hi  insoluble  sulphides  and  serves  for 
testing  values  obtained  by  other  methods. 

It  is  important,  however,  to  conduct  the  fusion  in  such  a  man- 
ner that  none  of  the  combustion  products  of  the  sulphur  in  the 
illuminating-gas  comes  in  contact  with  the  contents  of  the  crucible. 
This  is  accomplished,  as  suggested  by  Lowe,!  by  placing  the  cruci- 
ble in  an  inclined  position  within  a  hole  in  a  piece  of  asbestos  board. 

2.  Method  of  Bockmann. 

In  order  to  avoid  the  tedious  operation  of  destroying  the  nitrate 
which  is  necessary  in  the  method  of  Fresenius,  Bockmann  fuses  0.5 
gm.  of  the  substance  with  25  gms.  of  a  mixture  of  six  parts  sodium 
carbonate  and  one  part  potassium  chlorate.  The  contents  of  the 
crucible  are  heated  gently  at  first  and  finally  until  there  is  no  more 

*  Glaser  recommends  sodium  peroxide.     See  Chem.  Ztg.,  18,  1448,  and 
Z.  anal.  Chem.,  04,  594  (1895).     In  this  case  a  nickel  crucible  should  be  used, 
t  122  gm.  of  the  solid  BaCl2-2H2O  dissolved  in  a  liter  of  water. 
%  Z.  anal.  Chem.,  XX  (1881),  p.  224. 


DETERMINATION  OF  SULPHUR  IN  INSOLUBLE  SULPHIDES-      359 

evolution  of  oxygen.  After  cooling  the  melt  is  extracted  with 
water,  the  filtrate  acidified  with  hydrochloric  acid  and  precipitated 
at  a  boiling  temperature  with  barium  chloride. 

This  method  is  held  to  be  less  accurate  than  that  of  Fresenius, 
but  according  to  the  author's  experience  it  is  equally  good. 

3.  Oxidation  by  Chlorine  (Rose). 

This  very  important  method  is  used  less  to  determine  the  amount 
of  sulphur  present  in  insoluble  sulphides  than  it  is  to  effect  the 
solution  of  the  sulphide  for  the  separation  and  determination 
of  the  metals.  As  an  example  of  this  sort  of  an  analysis  we  will 
consider  the 

Analysis  of  Tetrahedrite  (Fahlerz). 

Tetrahedrite  is  a  sulpho-salt  corresponding  to  the  general  for- 
mula "IMS-RjSj  in  which  M  is  Cu2,  Ag2,  Fe,  Zn,  or  Hg2,  and  R  is 
As,  Sb,  or  Bi. 

From  0.5-1  gm.  of  the  finely-powdered  mineral  is  introduced 
by  means  of  a  long  weighing-tube  into  the  bulb  of  the  tube  R,  Fig.  56, 
which  is  30  cm.  long  and  1^  cm.  wide  and  made  of  difficultly  fusible 
glass. 


FIG.  56. 

In  the  receivers  V  and  Z  are  placed  about  100  c.c.  of  hydrochloric 
acid  (1:4)  to  which  3.5  gms.  of  tartaric  acid  have  been  added,  and  a 
slow  but  steady  stream  of  chlorine  *  is  conducted  through  the  appa- 
ratus. 

*  The  chlorine  is  generated  in  a  Kipp  apparatus  from  chloride  of  lime  and 
hydrochloric  acid.  In  order  to  purify  the  gas  it  is  passed  through  the  wash- 
bottles  A,  B,  and  C.  The  first  contains  water  and  the  other  two  contain  con- 


360     GRAVIMETRIC  DETERMINATION   OF   THE  METALLOIDS. 

As  soon  as  the  chlorine  reaches  the  substance  in  R,  the  decom- 
position begins.  The  contents  of  R  become  heated  and  the  volatile 
chlorides  collect  in  the  front  part  of  the  tube.  When  the  action 
begins  to  diminish,  the  decomposition  is  assisted  by  heating  R 
with  a  small  flame  kept  in  constant  motion.  The  heating  is  con- 
tinued until  only  brown  vapors  of  ferric  chloride  are  given  off;  as 
little  as  possible  of  these  should  pass  into  the  receiver.  The  easily 
volatile  chlorides,  however,  are  driven  over  into  V  as  much  as 
possible  by  carefully  heating  with  the  flame.  After  allowing  to 
cool  in  an  atmosphere  of  chlorine,  the  tube  R  is  broken  by  first 
scratching  with  a  file  near  the  drawn-out  part  and  then  touch- 
ing it  with  a  hot  glass  rod.  Over  each  of  the  open  ends  of  the 
tube  a  clean,  moist  test-tube  is  placed  and  allowed  to  stand  this 
way  overnight;  in  this  way  the  sublimate  absorbs  water  and  can 
be  easily  washed  off  in  the  morning.  The  contents  of  V  and  Z 
are  poured  into  a  beaker  and  the  drawn-out  part  of  R  is  washed  out 
with  hydrochloric  acid  containing  tartaric  acid. 

The  Residue  A 

consists  of  silver,  lead,  and  copper  chlorides,  almost  all  of  the  zinc, 
lead,  considerable  amounts  of  iron,  and  the  gangue. 

The  Solution  B 

contains  all  of  the  sulphur  as  sulphuric  acid,  the  bismuth  as  chloride, 
the  arsenic  and  antimony  as  their  pentoxide  compounds,  a  part 
of  the  iron  and  zinc  and  often  small  amounts  of  lead. 

Treatment  of  the  Residue  A. 

This  is  warmed  for  a  long  time  with  dilute  hydrochloric  acid, 
diluted  with  water,  allowed  to  settle,  and  the  residue  consisting  of 
silver  chloride  and  the  gangue  is  filtered  off,  washed  thoroughly 
with  hot  water  in  order  to  make  sure  that  all  lead  chloride  is  re- 
moved, treated  with  ammonia  on  the  filter  and  the  silver  precipi- 
tated from  the  ammoniacal  filtrate  by  acidifying  with  hydrochloric 
acid,  and  determined  as  the  chloride.  The  residue,  insoluble  in 
ammonia,  is  ignited  wet  in  a  platinum  crucible  and  weighed. 

centrated  sulphuric  acid.  It  is  also  well  to  introduce  a  calcium  chloride  tube 
filled  with  pieces  of  calcite  between  C  aud  R  in  order  to  remove  traces  of  acid. 


DETERMINATION  OF  SULPHUR  IN  INSOLUBLE  SULPHIDES.      361 

Into  the  nitrate  from  the  silver  chloride,  hydrogen  sulphide  is 
passed  until  the  solution  is  saturated  with  the  gas,  the  precipitate 
consisting  of  copper  and  lead  sulphides  is  filtered  off,  and  the  lead 
separated  from  the  copper  as  sulphate  according  to  p.  200.  The 
filtrate  from  the  hydrogen  sulphide  precipitate  is  combined  with 
that  obtained  from  Solution  B  after  hydrogen  sulphide  has  been 
passed  into  it. 

Treatment  of  Solution  B. 

A  stream  of  carbon  dioxide  is  passed  through  the  solution  for 
some  time  in  order  to  remove  the  greater  part  of  the  excess  of  chlo- 
rine, and  hydrogen  sulphide  is  then  passed  into  it  at  the  tempera- 
ture of  the  water-bath.  The  precipitate,  consisting  of  sulphides 
of  arsenic,  antimony,  mercury,  and  possibly  bismuth,  is  filtered 
oft  after  standing  twelve  hours,  and  the  arsenic  and  antimony 
separated  from  the  mercury  and  bismuth  by  means  of  ammonium 
sulphide  as  described  on  p.  235.  From  the  ammonium  sulphide 
solution  the  arsenic  and  antimony  are  precipitated  by  acidifying 
with  dilute  hydrochloric  or  sulphuric  acid,  the  precipitated  sul- 
phides filtered  off  and  the  arsenic  separated  from  the  antimony  as 
described  on  p.  241  et  seq. 

The  precipitate  insoluble  in  ammonium  sulphide  usually  consists 
almost  entirely  of  mercuric  sulphide  and  sulphur,  in  which  case 
it  is  washed  first  writh  alcohol,  then  a  few  times  with  carbon 
bisulphide,  then  with  alcohol  again,  dried  at  100°  C.  (preferably 
in  a  Paul's  drying-oven)  and  weighed.  If  bismuth  is  present,  how- 
ever, the  mixture  of  the  two  sulphides  is  treated  with  nitric 
acid  of  sp.  gr.  1.2-1.3,  boiled,  an  equal  volume  of  water  added, 
the  residue  filtered  and  the  bismuth  determined  in  the  filtrate 
according  to  p.  157,  while  the  mercury  is  determined  as  above 
described. 

The  filtrate  from  the  hydrogen  sulphide  precipitate  contains 
iron  and  zinc  and  is  combined  with  the  corresponding  filtrate 
from  the  Residue  A,  which  likewise  contains  these  metals.  These 
are  precipitated  by  the  addition  of  ammonia  and  ammonium 
sulphide,  filtered  off,  dissolved  in  hydrochloric  acid,  the  solution 
oxidized  with  nitric  acid,  and  the  iron  separated  from  the  zinc, 
preferably  by  the  barium  carbonate  method  (see  p.  150). 


362        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

It  is  best  to  determine  the  sulphur  in  a  separate  portion  by 
fusion  with  sodium  carbonate  and  potassium  nitrate  as  described 
on  p.  358. 

The  determination  of  the  sulphur  in  an  aliquot  part  of  the 
Solution  B  is  not  to  be  recommended  on  account  of  the  fact  that 
the  metals  present  are  likely  to  contaminate  the  precipitate  of 
barium  sulphate. 

(B)    OXIDATION   OF  SULPHUR  IN   THE   WET  WAY. 

For  this  purpose  aqua  regia,  fuming  nitric  acid,  bromine, 
hydrochloric  acid  and  potassium  chlorate,  and,  in  some  cases, 
ammoniacal  hydrogen  peroxide  have  been  proposed. 

Aqua  regia  is  most  frequently  used  in  practice  and  in  the  pro- 
portion first  recommended  by  J.  Lefort,*  viz.,  3  volumes  of  nitric 
acid  of  sp.  gr.  1.4  and  1  volume  of  hydrochloric  acid  of  sp.  gr.  1.2. 
As  an  example  we  will  cite  the 

Determination  of  Sulphur  in  Pyrite,  G.  Lunge's  Method,  f 

First  of  all,  the  sample  must  be  ground  to  an  impalpable 
powder,  or  sulphur  will  separate  out  during  the  solution  of  the 
sample.  Of  the  fine  powder,  0.5  gm.  is  treated  with  10  c.c.  of  a 
mixture  consisting  of  3  parts  nitric  acid,  sp.  gr.  1.42,  and  1  part 
hydrochloric  acid,  sp.  gr.  1.2,  in  a  300  c.c.  beaker  which  is  covered 
with  a  watch-glass.  At  first  the  acid  is  allowed  to  act  upon  the 
pyrite  in  the  cold,  but  at  the  last  the  reaction  is  completed  by 
heating  upon  the  water-bath.  Then  the  solution  is  transferred 
to  a  porcelain  evaporating  dish  and  evaporated  to  dryness  on  the 
water-bath.  The  residue  is  treated  with  5  c.c.  of  concentrated 
hydrochloric  acid  and  again  evaporated  'to  dryness.  The  dry 
mass  is  now  treated  with  1  c.c.  of  concentrated  hydrochloric 
acid  and  100  c.c.  of  hot  water,  the  solution  filtered  through  a 
small  filter  and  the  residue  washed  first  with  cold  water  and  then 
with  hot  water.  The  hot  filtrate,  of  not  more  than  150  c.c.  at 

*  J.  de  Phann.  et.  de  Chimie  [IV],  Vol.  9,  p.  99,  and  Zeit.  fur  anal.  Chem., 
IX,  p.  81. 

t  The  procedure  is  given  here  as  recommended  by  the  Report  of  the  Sixth 
International  Congress  of  Applied  Chemists,  Rome,  1906,  Vol.  VI,  p.  15. 


DETERMINATION  OF  SULPHUR  IN  INSOLUBLE  SULPHIDES.     363 

the  most,  is  treated  with  20  c.c.  of  10  per  cent,  ammonia  and 
kept  at  about  70°  for  fifteen  minutes.  The  ferric  hydroxide 
precipitate  is  filtered  and  washed  with  hot  water,  whereby  the 
precipitate  is  well  "  churned/'  until  a  volume  of  about  450  c.c. 
is  reached.  The  filtrate  is  neutralized  with  hydrochloric  acid, 
using  methyl  orange  as  indicator,  and  1  c.c.  of  concentrated  hydro- 
chloric acid  added  in  excess.  Thereupon  the  solution  is  heated 
until  it  begins  to  boil,  when  it  is  treated  with  a  boiling-hot  solu- 
tion made  by  taking  24  c.c.  of  10  per  cent,  barium  chloride 
solution  and  diluting  to  100  c.c.  The  reagent  is  added  as  quickly 
as  possible  at  one  time  while  stirring  the  solution  vigorously. 

The  barium  sulphate  precipitate  is  washed  three  times  by 
decantation  with  boiling  water,  then  transferred  to  a  filter  and 
washed  free  from  chlorides,  dried,  ignited  and  weighed. 

To  test  the  ammonia  precipitate  for  sulphur,  transfer  it  from 
the  filter  into  a  beaker  by  means  of  a  stream  of  water  from  the 
wash  bottle  and  dissolve  it  by  the  addition  of  as  little  hydrochloric 
acid  as  possible.  The  resulting  solution  is  precipitated  with 
ammonia,  filtered,  and  the  filtrate  and  washings  treated  as  in  the 
case  of  the  main  analysis.  Should  any  barium  sulphate  be 
obtained  in  this  way,  it  should  be  filtered  off  and  weighed  with  the 
main  part  of  the  barium  sulphate  precipitate. 

Remark. — It  is  still  better  to  filter  the  precipitate  through  a 
Munroe  crucible.  After  washing,  the  precipitate  is  dried  as  much 
as  possible  by  suction,  the  crucible  placed  within  a  larger  porcelain 
or  platinum  crucible,  heated  gently  and  weighed. 

The  above  method  gives  excellent  results,  which  as  a  rule  agree 
closely  with  those  obtained  by  the  Fresenius  method.  If  the 
pyrite,  however,  contained  barium  or  any  considerable  amount  of 
lead,  some  sulphate  will  always  remain  undissolved  with  the 
gangue.  In  such  cases  the  Lunge  method  will  give  lower  results 
but  on  the  other  hand  it  represents  more  nearly  the  quantity  of 
sulphur  in  the  pyrite  which  is  available  for  the  manufacture  of 
sulphuric  acid.  In  spite  of  the  strong  oxidizing  power  of  the  above 
mixture  of  nitric  and  hydrochloric  acids,  it  is  not  sufficient  to 
permit  the  determination  of  sulphur  in  roasted  pyrite,  on  account 
of  the  danger  of  losing  some  sulphur  as  hydrogen  sulphide.  Such 


364        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

products  are  fused  with  a  mixture  of  two  parts  sodium  carbonate 
and  one  part  potassium  nitrate  and  the  analysis  carried  out  as 
described  under  the  Fresenius  method. 

Determination  of  Sulphur  in  Cast  Iron  and  Steel.     (Noyes  and 

Helmer  *) . 

About  5  gms.  of  iron  or  steel,  in  the  form  of  fine  borings,  are 
introduced  gradually  into  an  Erlenmeyer  flask  containing  a  cooled 
mixture  of  200  c.c.  water  and  8  c.c.  bromine,  free  from  sulphur. 
As  soon  as  all  the  metal  has  dissolved,  the  contents  of  the  flask  are 
heated  to  boiling,  in  order  to  expel  the  slight  excess  of  bromine, 
and  the  solution  is  filtered  from  any  residue.  Inasmuch  as  the 
latter  frequently  contains  an  appreciable  amount  of  sulphur,  it  is 
dried,  transferred  to  a  platinum  crucible,  the  ash  of  the  filter  paper 
added,  and  a  fusion  is  made  with  2  gms.  sodium  carbonate. 
The  crucible  should  be  inclined  within  an  asbestos  shield  to  pro- 
tect its  contents  from  being  contaminated  with  any  sulphur 
from  the  gas  flame.  After  the  sodium  carbonate  has  melted,  the 
crucible  is  allowed  to  cool  somewhat,  a  crystal  of  potassium  nitrate 
is  added,  and  the  heating  is  continued.  After  cooling,  the  melt 
is  dissolved  in  water,  the  resulting  solution  filtered,  the  filtrate 
acidified  with  hydrochloric  acid,  heated  to  boiling,  and  the 
sulphuric  acid  precipitated  by  the  addition  of  5  c.c.  barium 
chloride  solution.  In  the  following  calculation,  the  weight  of  this 
precipitate  is  called  p. 

The  original  solution  of  the  iron,  containing  the  greater  part  of 
the  sulphur,  is  poured,  while  constantly  stirring,  into  130  c.c.  of 
10  per  cent,  ammonia  water  which  is  contained  in  a  500-c.c. 
calibrated  flask.  The  contents  of  the  flask  are  well  shaken, 
diluted  with  water  up  to  the  mark,  mixed  by  pouring  back  and  forth 
several  times  into  a  dry  beaker,  and  then  filtered  through  a  dry 
filter,  rejecting  the  first  few  c.c.  of  the  filtrate.  From  the  strongly 
ammoniacal  filtrate,  300  c.c.  are  transferred  by  a  pipette  into  a 
new  beaker,  evaporated  to  100  c.c.,  while  avoiding  any  contamina- 
tion from  a  gas  flame,  treated  with  five  or  six  drops  of  dilute 

*  J.  Am.  Chem.  Soc.,  23,  675  (1901). 


DETERMINATION  OF  SULPHUR  IN  INSOLUBLE  SULPHIDES.    365 

hydrochloric  acid  and  the  sulphuric  acid  precipitated  by  the 
addition  of  5  c.c.  of  hot,  normal  barium  chloride  solution.  The 
weight  of  this  precipitate  is  taken  as  p\  in  the  following  computa- 
tion. 

The  sulphur  content  of  the  sample  of  iron  or  steel,  weighing  a 
gms.,  is  then  found  to  be 

(*ft+&X?Xl(M-13.73xte±g-per  cent,  sulphur. 
BaSO4Xa  a 

Remark. — This  method  is  accurate  and  easily  carried  out.  All 
the  sulphur  is  obtained  with  the  exception  of  that  small  amount 
which  is  combined  with  an  organic  radical.  A  great  advantage 
is  gained  by  not  having  to  wash  the  ferric  hydroxide  precipitate. 
A  very  slight  error  is  introduced  by  neglecting  the  volume  of  the 
ferric  hydroxide  precipitate,  but  this  is  negligible  in  the  deter- 
mination of  such  small  amounts  of  sulphur.  The  iron  must  be 
introduced  into  the  bromine  in  very  small  portions  in  order  to 
prevent  overheating  which  would  result  in  the  formation  of  a 
basic  salt  that  is  hard  to  get  back  into  solution. 


Determination  of  Sulphur  in  Iron  and  Steel.     Method  of  Krug* 

Procedure. — 5  gms.  of  borings  are  treated  in  a  500-c.c.  round- 
bottomed  flask  "with  50  c.c.  concentrated  nitric  acid,  sp.  gr.  1.4 
and  the  contents  of  the  flask  are  gently  heated.  After  the  reddish- 
brown  vapors  cease  to  form,  the  acid  is  gradually  heated  up  to  the 
boiling  point.  When  at  the  end  of  an  hour  or  two  the  solution  of 
the  iron  is  complete,  0.25  gm.  of  potassium  nitrate,  dissolved  in  a 
little  water,  is  added,  the  liquid  evaporated  to  dryness,  and  the 
residue  ignited  until  no  more  brown  fumes  are  evolved.  After 
cooling,  the  ferric  oxide  is  dissolved  by  heating  with  50  c.c. 
concentrated  hydrochloric  acid,  the  solution  evaporated  nearly 
to  dryness,  and  the  treatment  with  hydrochloric  acid  and  evapora- 
tion repeated  until  no  more  chlorine  is  evolved.  The  hydro- 

*  Stahl  und  Eisen,  25,  887  (1905). 


366       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

chloric  acid  solution  is  then  rinsed  into  a  beaker,  and  any  residue 
of  silica,  carbon,  etc.,  is  filtered  off  into  a  porcelain  evaporating 
dish.  The  nitrate  is  evaporated  until  a  film  of  ferric  chloride 
forms  on  the  solution,  which  is  redissolved  by  the  addition  of  a 
few  drops  of  hydrochloric  acid.  After  cooling  the  ferric  chloride 
solution  is  introduced  into  a  double  separatory  funnel,  washing 
out  the  dish  with  hydrochloric  acid,  sp.  gr.  1.1,  but  keeping  the 
volume  below  60  c.c.  30  c.c.  of  fuming  hydrochloric  acid  and 
ether  mixture  (prepared  by  gradually  pouring  ether  into  cold  con- 
centrated hydrochloric  acid,  sp.  gr.  1.2,  solution  until  a  little  layer 
of  ether  is  formed  on  top)  and  100  c.c.  of  ether.  The  mixture  is  well 
cooled  under  the  water  tap  and  thoroughly  shaken.  The  upper 
olive-green  ether  layer  contains  nearly  all  of  the  iron,  the  lower 
light  yellow  solution  contains  all  the  sulphuric  acid.  The  lower  layer 
is  carefully  withdrawn  into  the  other  separatory  funnel  and  the 
ether  solution  is  washed  once  with  a  few  c.c.  of  dilute  hydrochloric 
acid,  sp.  gr.  1.1  which  has  been  saturated  with  ether.  The  ether 
solution  is  shaken  with  this  last  mixture  and  after  standing  until 
two  layers  again  separate,  the  lower  one  is  added  to  the  contents 
of  the  other  separatory  funnel.  75  c.c.  of  pure  ether  are  now 
introduced  into  the  second  separatory  funnel  and  the  contents 
well  shaken,  this  time  the  cooling  is  unnecessary.  The  upper 
layer  will  contain  an  ether  solution  of  practically  all  the  remaining 
iron,  whereas  the  lower  hydrochloric  acid  layer  will  contain  all  the 
sulphuric  acid  and  some  dissolved  ether.  The  lower  layer  is 
withdrawn  to  a  porcelain  evaporating  dish,  and  the  ether  contained 
in  it  is  removed  by  evaporating  on  the  water  bath  to  dry  ness.  To 
the  residue,  a  few  drops  of  hydrochloric  acid  and  a  little  water 
are  added.  The  solution  is  filtered  and  the  hot  filtrate  treated 
with  hot  barium  chloride  solution. 

Remark. — In  testing  this  method,  Dr.  Krug  established  the 
fact  that  a  mixture  of  pure  ferric  chloride,  corresponding  to  5  gms. 
iron,  could  be  treated  with  10  c.c.  of  tenth-normal  sulphuric  acid, 
and  all  of  the  latter  recovered  after  the  ether  separation.  Further- 
more, the  results  were  compared  with  the  bromine  method  of 
Johnston  and  found  to  be  more  accurate. 


DETERMINATION  OF  SULPHUR  IN  INSOLUBLE  SULPHIDES.     367 


(C)    EXPULSION    OF   HYDROGEN   SULPHIDE   FROM   INSOLUBLE 
SULPHIDES. 

(a)  The  Iron  Method* 

In  1881,  M.  Groger  showed  that  by  heating  pyrite  with  iron 
out  of  contact  with  the  air  the  former  is  quantitatively  changed 
into  ferrous  sulphide, 

FeS2  +  Fe=2FeS, 

and  from  the  latter  all  of  the  sulphur  will  be  given  off  as  hydrogen 
sulphide  on  treatment  with  hydrochloric  acid.  In  1891  the  author 
independently  came  to  the  same  conclusion  and  worked  out  a 
method  which  permits  of  the  determination  of  sulphur  not  only 
in  pyrite  but  in  all  other  insoluble  sulphides. 

Procedure. — First  of  all  the  finely  powdered  sulphide  is  heated 
out  of  contact  with  the  ah-  with  iron  powder.  In  this  way 
part  of  the  sulphur  is  usually  given  up  to  the  iron,  and  the  com- 
pound itself  is  reduced  to  compounds  which  are  acted  upon  by 
hydrochloric  acid  with  evolution  of  hydrogen  sulphide;  the  latter 
is  absorbed  in  ammoniacal  hydrogen  peroxide  solution,  as  de- 
scribed on  p.  347.  The  heating  with  iron  is  accomplished  in  a 
small  glass  crucible  about  30  mm.  long  and  10  mm.  in  diameter 
(Fig.  54,  6),  which  can  be  easily  made  from  an  ordinary  piece  of 
combustion  tubing.  About  3  gms.  of  iron  powder  that  has  been 
previously  ignited  in  hydrogen  is  placed  in  the  crucible,  from  0.3- 
0.5  gm.  of  the  sulphide  is  thoroughly  mixed  with  it,  and  the  mix- 
ture is  finally  covered  with  a  thin  layer  of  iron  powder.  The  cru- 
cible is  now  placed  in  the  opening  of  the  piece  of  asbestos  board  A 
(Fig.  54,  b)  and  upon  it  is  placed  the  gas-delivery  tube  B  which  has 
been  prepared  from  difficultly  fusible  glass.  A  stream  of  dry 
carbon  dioxide  f  is  passed  through  the  apparatus  for  a  few  min- 

*  Berichte,  XXIV,  p.  1937. 

f  The  carbon  dioxide  is  prepared  from  marble  and  hydrochloric  acid  in  a 
Kipp  generator.  To  purify  the  gas  it  is  passed  through  a  wash-bottle  con- 
taining water,  then  through  one  containing  potassium  permanganate,  then 
through  a  tube  filled  with  pumice  soaked  in  copper  sulphate  solution,  and 
finally  through  a  calcium  chloride  tube.  Potassium  permanganate  and  cop- 
per sulphate  serve  to  remove  traces  of  hydrogen  sulphide  that  the  carbon 
dioxide  might  contain. 


368         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

utes  and  the  crucible  is  gently  heated  with  a  small  flame.  Usu- 
ally there  is  a  distinct  glowing  visible,  but  no  trace  of  the  sulphur 
is  lost  by  volatilization.  As  soon  as  the  contents  of  the  crucible 
have  ceased  to  glow,  the  temperature  is  raised  until  a  dull-red  heat 
is  obtained,  and  the  crucible  is  kept  at  this  temperature  for  ten 
minutes. 

After  cooling  in  the  carbon  dioxide,  the  crucible  together  with 
its  contents  is  placed  in  the  400-c.c.  flask  K  and  is  connected  with 
the  absorption  vessels  V  and  P  as  shown  in  the  figure.  The  rest 
of  the  procedure  is  carried  out  as  described  on  p.  350. 

Remark. — Commercial  iron  powder  always  contains  a  small 
amount  of  sulphur,  so  that  a  blank  experiment  is  first  made  with 
a  weighed  amount  of  the  same,  and  for  the  experiment  proper  the 
same  amount  of  iron  is  used.  The  amount  of  sulphur  found  to 
be  present  in  the  iron  is  subtracted  from  the  amount  found  in 
the  analysis. 

The  author  was  disappointed  in  not  being  able  by  this  method 
to  distinguish  between  the  sulphur  present  in  insoluble  sulphides 
as  sulphide  and  that  present  as  sulphate  (barium  sulphate).  If 
the  amount  of  sulphate  present  is  small,  it  is  completely  reduced 
to  sulphide  by  this  method,  while  if  a  large  amount  of  sulphate 
is  present,  it  is  often  only  partially  reduced.  As,  however,  the 
amount  of  barium  sulphate*  present  in  insoluble  sulphides  is 
usually  small,  this  method  serves  for  the  determination  of  the 
total  amount  of  sulphur. 

(6)  The  Tin  Method.} 

Principle. — Almost  all  insoluble  sulphides  on  being  treated 
with  metallic  tin  and  concentrated  hydrochloric  acid  give  off  all 
their  sulphur  as  hydrogen  sulphide.  Harding,  %  who  first  studied 
this  method,  used  tin  and  hydrobromic  acid. 

Procedure. — Into  the  evolution  tube  (Fig.  57),  which  is  20  cm. 
long  and  2.5  cm.  wide,  is  placed  a  layer  of  finely -powdered  tin  (g) 
about  0.5  cm.  thick.  Upon  this  the  substance  is  placed  enclosed 
in  tinfoil  (s)  and  then  a  layer  of  granulated  tin  about  6  cm.  deep 

*  Only  barium   sulphate  is  reduced  with  difficulty,  the  sulphates   of  the 
heavy  metals  are  easily  reduced. 
t  Berichte,  XXV,  p.  2377. 
J  Berichte,  XIV,  p.  2085. 


DETERMINATION  OF  SULPHUR  IN  INSOLUBLE  SULPHIDES.    369 

(Z)  is  added.     A  current  of  pure  hydrogen  is  conducted  through 
the  apparatus  for  about  five  minutes,  after  which  the  step-cock 


FIG.  57. 

is  closed  and  the  tube  connected  with  the  receivers  P  and  V,  as 
shown  in  the  figure.  The  flask  V  contains  an  ammoniacal  solution 
of  hydrogen  peroxide,  while  P  contains  2  to  3  cm.  of  water  in  order 
to  remove  any  stannous  chloride  that  may  be  carried  over  with 
the  gas.  Concentrated  hydrochloric  acid  is  now  added  through 
the  drop-funnel  until  the  tin  is  at  the  most  half  covered  with  the 
acid.  The  contents  of  the  tube  are  then  warmed  slightly,  prefer- 
ably by  placing  it  in  a  small  paraffin  bath.  The  capsule  of  tin 
soon  dissolves,  and  the  substance  is  seen  to  be  floating  in  the  acid. 
It  dissolves  after  about  fifteen  minutes,  and  the  acid  becomes 


370       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

perfectly  clear.  The  heating  is  now  continued  until  there  is  no 
more  yellow  coloration  to  be  detected  in  the  delivery-tube  which 
dips  into  the  receiver  V.  More  acid  is  then  added  to  the  contents 
of  the  tube,  until  the  tin  is  completely  covered  with  it  and  the 
heating  is  continued  for  half  an  hour,  meanwhile  first  heating 
the  contents  of  P  to  boiling  and  passing  a  current  of  hydrogen 
through  a.  By  this  means  all  of  the  sulphur  will  be  driven  over 
into  V  *  and  is  there  held  in  solution  as  ammonium  sulphate  and 
analyzed  as  described  on  p.  350. 

Remark. — This  method  affords  an  accurate  means  for  deter- 
mining the  sulphur  present  in  insoluble  sulphides  as  sulphide  in 
the  presence  of  sulphate.  Thus  the  amount  of  pyrite  in  clay-slate 
that  contains  gypsum  can  be  determined  by  this  method,  although 
usually  the  treatment  with  aqua  regia  or  fusion  with  soda  and 
nitre  is  used.  By  these  last  two  methods,  however,  the  total  sulphur 
is  determined.  More  accurate  values  for  the  pyrite  present  in  such 
cases  may  be  obtained  by  decomposition  in  a  current  of  chlorine 
(see  p.  359),  in  which  case  only  the  sulphide  sulphur  is  determined. 

Finally,  it  may  be  mentioned  that  arsenic  sulphide  may  be  de- 
composed by  the  above  method,  although  a  longer  time  is  required 
than  is  the  case  with  pyrite,  copper,  chalcopyrite,  galena,  cinnabar, 
etc.  Arsenopyrite,  on  the  other  hand,  is  either  unacted  upon  of 
only  decomposed  with  difficulty,  while  the  iron  method  effects  the 
decomposition  with  ease. 

Determination  of  Sulphur  in  Non-electrolytes. 

In  order  to  determine  the  amount  of  sulphur  present  in  organic 
compounds,  it  is  oxidized  to  sulphuric  acid  and  determined  as 
barium  sulphate. 

The  oxidation  is  effected 

(a)  In  the  Wet  Way. 

(b)  In  the  Dry  Way. 

(a)  Oxidation  in  the  Wet  Way  (Carius). 

This  operation  is  conducted  in  precisely  the  same  manner  aa 
was  described  on  p.  325  for  the  determination  of  halogens,  except 

*With  large  amounts  of  sulphur,  one  receiver  is  often  insufficient.  In 
such  cases  the  tube  b  is  connected  with  a  Peligot  tube  containing  ammoniacal 
hydrogen  peroxide  as  shown  in  Fig.  54,  p.  351.  . 


ACETIC  AND    CYANIC  ACIDS.  371 

in  this  case  there  is  no  silver  nitrate  added  to  the  contents  of  the 
tube.  After  the  closed  tube  has  been  heated  and  opened,  its 
contents  are  transferred  to  a  beaker,  hydrochloric  acid  is  added, 
and  the  liquid  is  evaporated  to  a  small  volume  in  order  to  remove 
the  nitric  acid;  it  is  then  diluted  with  water  to  a  volume  of  about  200 
c.c.  and  precipitated  hot  with  a  boiling  solution  of  barium  chloride 
and  weighed  as  barium  sulphate. 

(6)  Oxidation  in  the  Dry  Way  (Liebig). 

A  mixture  of  eight  parts  potassium  hydroxide  (free  from  sul- 
phate) and  one  part  of  potassium  nitrate  is  melted  in  a  large  silver 
crucible  with  the  addition  of  a  little  water.  After  cooling,  a  weighed 
amount  of  the  substance  is  added  and  the  contents  of  the  crucible 
again  heated  very  gradually,  frequently  stirring  the  mixture  with 
a  silver  wire  until  the  organic  substance  is  completely  decomposed. 
After  cooling,  the  melt  is  dissolved  in  water,  acidified  with  hydro- 
chloric acid  and  the  sulphuric  acid  formed  is  precipitated  and 
weighed  as  barium  sulphate. 

This  method  is  particularly  suited  for  the  determination  of  sul- 
phur present  in  difficultly  volatile  substances,  e.g.,  in  wood-cements. 

CH3 

ACETIC  ACID,  |         .    Mol.  Wt.  60.03. 
COOH 

Free  acetic  acid  is  always  determined  volumetrically.  For 
the  analysis  of  acetates,  the  substance  is  heated  with  phosphoric 
acid  when  the  free  acetic  acid  distils  over  and  is  then  titrated 
(cf.  Part  II,  Acidimetry).  The  carbon  and  hydrogen  of  the  acetate 
can  be  determined  by  Elementary  Analysis  (which  see). 

CYANIC  ACID,  HOCN.    Mol.  Wt.  43.02. 

The  only  method  for  examining  cyanates  consists  of  deter- 
mining the  amount  of  carbon  and  nitrogen  present  by  a  combus- 
tion (see  Elementary  Analysis). 

Determination  of  Cyanic  Acid,  Hydrocyanic  Acid,  and  Carbonic 

Acid  in  a  Mixture  of  their  Potassium  Salts. 
In  one  portion  of    the  substance  the  carbonic  acid  is  deter- 
mined by  the  addition  of  calcium  chloride  to  the  ammoniacal  solu- 
tion and  weighing  the  ignited  precipitate  as  calcium  oxide. 


37  2         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

In  a  second  portion  the  cyanogen  of  the  cyanide  is  deter- 
mined as  silver  cyanide  by  treating  the  aqueous  solution  with  an 
excess  of  silver  nitrate,  then  acidifying  with  nitric  acid  and  deter- 
mining the  weight  of  the  silver  cyanide  as  described  on  p.  328. 

In  a  third  portion  the  potassium  is  determined  by  evaporating 
with  sulphuric  acid  and  weighing  the  residue  of  potassium  sulphate 
as  described  on  p.  41.  If  from  the  total  amount  of  potassium 
present  the  amount  present  as  potassium  carbonate  and  potassium 
cyanide  is  deducted,  the  difference  gives  the  amount  of  potassium 
combined  with  the  cyanic  acid. 

HYPOPHOSPHOROUS  ACID,  H3PO2.    Mol.  Wt.  66.02. 

Forms:  Mercurous  Chloride,  Hg2Cl2;  Magnesium  Pyrophosphate, 

Mg2P?07. 

(a)  Determination  as  Mercurous  Chloride. 

The  solution  of  the  salt,  which  is  slightly  acid  with  hydro- 
chloric acid,  is  treated  with  an  excess  of  mercuric  chloride;  by 
this  means  insoluble  mercurous  chloride  is  precipitated.  After 
standing  for  twenty-four  hours  in  a  warm,  dark  place  the  precip- 
itate is  filtered  through  a  Gooch  crucible,  washed  with  water 
dried  at  110°  C.,  and  from  the  weight  of  the  mercurous  chloride  the 
amount  of  hypophosphorous  acid  present  is  calculated  as  follows: 

H8P02  +  2H20  +  4HgCl2  =  2Hg2Cl2  +  4HC1  +  H3P04 
2Hg2Cl2:H3P02=p:* 


= 

~ 


2Hg2Cl2 

in  which  p  is  the  weight  of  the  Hg2Cl2  obtained  in  the  analysis, 
(6)  Determination  as  Magnesium  Pyrophosphate. 

First  of  all,  the  phosphorous  acid  is  converted  into  phosphoric 
acid  by  adding  5  c.c.  of  concentrated  nitric  acid  to  the  aqueous 
solution  of  from  0.5-1  gm.  of  the  substance  in  about  100  c.c.  of 
water,*  evaporating  on  the  water-bath  to  a  small  volume,  adding 
a  few  drops  of  fuming  nitric  acid  and  again  heating.  After  this  the 
phosphoric  acid  is  precipitated  by  magnesia  mixture  and  the  pre- 

*  If  the  hypophosphite  were  at  once  treated  with  nitric  acid,  metaphos- 
phoric  acid  would  be  obtained;  by  the  addition  of  water  the  ortho-salt  is 
formed. 


SULPHUROUS   ACID.  373 

cipitate  is   weighed  as  magnesium  pyrophosphate  as  described 
under  Phosphoric  Acid. 

GROUP  HI. 

SULPHUROUS,  SELENOUS,  TELLUROUS,  PHOSPHOROUS,  CAR- 
BONIC,  OXALIC,  IODIC,  BORIC,  MOLYBDIC,  TARTARIC,  META- 
AND  PYROPHOSPHORIC  ACIDS. 

SULPHUROUS  ACID,  H2SO3.    Mol.  Wt.  82.09. 
Form:  Barium  Sulphate,  BaSO4. 

The  sulphite  or  free  sulphurous  acid  is  first  oxidized  to  sul- 
phuric acid  and  then  precipitated  with  barium  chloride. 

The  oxidation  can  be  accomplished  by  means  of  chlorine, 
bromine,  hydrogen  peroxide,  or  potassium  percarbonate. 

Oxidation  with  Chlorine  or  Bromine. 

Chlorine  or  bromine  water  is  allowed  to  flow  gradually  into 
the  aqueous  solution  of  sulphurous  acid,  or  of  a  sulphite,  the  excess 
of  the  reagent  is  subsequently  removed  by  boiling  and  the  sulphuric 
acid  is  precipitated  with  barium  chloride. 

Oxidation  with  Hydrogen  Per  oxide.* 

The  solution  of  sulphurous  acid  or  of  a  sulphite  is  treated 
with  an  excess  of  ammoniacal  hydrogen  peroxide,  heated  to  boil- 
ing in  order  to  remove  the  excess  of  the  peroxide,  acidified  with 
hydrochloric  acid  and  precipitated  with  barium  chloride  after 
making  acid  with  hydrochloric  acid. 

With  potassium  percarbonate  a  similar  procedure  is  used. 
The  alkaline  solution  of  the  sulphite  is  treated  in  the  cold  with 
solid  potassium  percarbonate,  gently  heated,  after  which  the  tem- 
perature is  gradually  raised  till  the  boiling-point  is  reached. 
The  solution  is  then  acidified  with  hydrochloric  acid  and  pre- 
cipitated with  barium  chloride. 

*  The  hydrogen  peroxide  must  always  be  tested  to  see  if  it  contains  sul- 
phuric acid;  if  it  is  found  to  be  present,  the  amount  is  determined  and 
afterward  an  accurately  measured  quantity  is  used  for  the  oxidation.  The 
amount  of  sulphuric  acid  from  the  peroxide  is  deducted  from  the  total  value 
found  in  the  analysis. 


374       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

Sulphurous  acid  may  be  determined  very  accurately  by  a  volu- 
metric analysis  (cf.  Part  II,  lodimetry). 

Selenous  and  Tellurous  Acids. 

The  analysis  of  these  acids  was  discussed  under  Selenium  and 
Tellurium. 

PHOSPHOROUS  ACID,  H3P03.    Mol.Wt.  82.02. 

Forms:  Mercurous  Chloride,  Hg2Cl2,  and  Magnesium  Pyro- 
phosphate,  Mg2P207. 

This  determination  is  effected  exactly  as  that  of  hypophos- 
phorous  acid  (cf.  page  372). 

In  this  case,  however,  it  is  to  be  noted  that  1  mol.  Hg2Cl2  cor- 
responds to  1  mol.  H3PO3: 

H3P03 + 2HgCl2  +  H20  =  HaPCVr-  2HC1 + HgA. 
Determination  of  Phosphorous  and  Hypophosphoroua  Acids. 

In  this  case  an  indirect  analysis  must  be  made.  Aftei  oxidizing 
one  portion  of  the  substance  to  phosphoric  acid,  the  total  amount 
of  magnesium  pyrophosphate  is  found;  mercuric  chloride  is  allowed 
to  act  upon  a  second  portion  and  the  weight  of  the  mercurous 
chloride  formed  is.  determined.  From  these  data  the  amount  of 
each  acid  present  can  be  calculated  as  follows : 

Suppose  we  have  a  solution  containing  the  two  acids.  Let  us 
denote  by  x  the  amount  of  hypophosphorous  acid  present  in  V  c.c. 
-of  the  solution,  and  let  ox  represent  the  amount  of  mercurous 
chloride  produced  from  it  and  mx  the  amount  of  magnesium  pyro- 
phosphate. Further,  let  y  represent  the  amount  of  phosphorous 
acid  present  in  the  same  volume  of  the  solution  and  vy  the  cor- 
responding amount  of  mercurous  chloride  and  ny  that  of  mag- 
nesium pyrophosphate.  The  total  amount  of  the  mercurous 
chloride  is  q,  while  the  total  amount  of  magnesium  pyrophosphate 
is  p.  We  have  then 


from  which  it  follows 

-i 

on— mv 


CARBONIC  ACID.  375 

and 

y  =  p — q     m      =  p- 1.4733-g.0.1742. 

on—mv      on  —  mv 

In  these  equations,  m,  n,  o,  and  v  have  the  following  values: 

Mg2P207  Mg2P207 

m=  m^T1'687    ^^HsPOT1'358 

2Hg2Cl2_  ^HggO, 

"  H3P02  H3P03~ 

CARBONIC  ACID,  H2CO3.    Mol.  Wt.  62.02. 

Carbonic  acid  is  determined  gravimetrically  as  C02;  but  a 
more  accurate  determination  is  effected  by  expelling  this  gas  and 
measuring  its  volume. 

i.  Gravimetric  Determination  of  Carbon  Dioxide. 

This  analysis  may  be  accomplished  in  two  ways.  First,  we 
may  weigh  the  substance,  expel  the  carbon  dioxide  and  then  weigh 
it  again,  when  the  difference  will  represent  the  amount  of  gas. 
Second,  the  carbon  dioxide  may  be  expelled  from  a  weighed 
amount  of  the  substance  and  then  absorbed  in  a  suitable  appa- 
ratus ;  in  this  case  the  carbon  dioxide  is  r/eighed  directly. 

A.    DETERMINATION    OF   CARBONIC  ACID    BY   DIFFERENCE. 

(a)  Determination  in  the  Dry  Way. 

For  the  analysis  of  a  carbonate,  or  a  mixture  of  carbonates 
which  contains  no  volatile  constituent  other  than  the  carbon 
dioxide,  1  gm.  of  the  substance  is  weighed  into  a  platinum 
crucible  and  gradually  heated  to  a  high  temperature.*  In  case 
calcium,  strontium,  or  magnesium  is  present  a  final  heating  over 
the  blast-lamp  is  necessary,  while  with  other  carbonates  the  heat 
of  a  good  Teclu  burner  is  sufficient ;  even  the  difficultly  decom- 
posable cadmium  carbonate  can  be  analyzed  by  this  method.  The 
carbonates  of  barium  and  the  alkalies,  on  the  other  hand,  do  not 
lose  their  carbon  dioxide  on  ignition. 

*  Carbonic  acid  cannot  be  determined  by  this  method  when  the  residual 
oxide  suffers  change,  as,  for  example,  in  the  case  of  FeCO,  and  MnCOg  where 
an  oxidation  would  take  place. 


376       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 


If  the  substance  contains  water  besides  carbon  dioxide  then 
the  sum  of  the  water  +  carbon  dioxide  is  determined  by  the  loss  on 
ignition,  and  the  amount  of  carbon  dioxide  is  determined  in  a  sec- 
ond portion  by  (6). 

(6)  Determination  in  the  Wet  Way. 

Principle. — The  weighed  carbonate  is  placed  in  an  apparatus 
containing  acid,  but  in  such  a  way  that  the  former  does  not  at  first 
come  in  contact  with  the  latter.  The 
whole  apparatus  is  then  weighed,  after 
which  the  acid  is  allowed  to  act  upon  the 
substance,  when  carbon  dioxide  is  evolved 
and  escapes  from  the  apparatus.  (Care 
must  be  taken  that  no  moisture  escapes 
with  the  gas.)  By  afterward  weighing 
the  apparatus  and  subtracting  this  weight 
from  that  first  obtained,  the  weight  of 
the  carbon  dioxide  is  ascertained. 

Procedure. — This  analysis  is  easily 
accomplished,  and  a  large  number  of 
different  forms  of  apparatus  have  been 
devised  for  this  purpose.  In  this  book, 
however,  only  one  of  these  so-called  alka- 
limeters  will  be  described,  namely,  that 
of  Mohr,  which  in  an  improved  form  is 
shown  in  Fig.  58,  although  it  must  be 
stated  that  many  other  forms  (e.g.,  those 
of  Bunsen,*  Shrotter,  Geissler,  Frese- 
nius-Will,  etc.)  answer  the  purpose  equal- 
ly well. 

The  alkalimeter  consists  of  the  small,  wide-mouthed,  flat-bot- 
tomed flask  F,  which  has  a  ground-glass  connection  with  the  tubes 

*  In  the  German  edition  of  this  book,  Bunsen's  alkalimeter  is  described 
instead  of  Mohr's.  The  above  apparatus  has  the  advantage  of  having  a  stop- 
cock to  separate  the  acid  compartment  from  the  flask,  besides  having  a  flat 
bottom,  upon  which  it  will  rest  unsupported.  It  is  all  made  of  very  thin 
glass  and  weighs  comparatively  little. 


FIG.  58. 


CARBONIC  ACID  BY  DIFFERENCE.  377 

A  and  B.  At  the  bottom  of  B  is  placed  a  loose  wad  of  cotton;  a 
cylinder  of  glazed  paper  about  3  cm.  wide  is  introduced  into  the 
neck  of  the  tube,  and  through  this  cylinder  some  pieces  of  sifted 
calcium  chloride*  are  poured.  The  paper  cylinder  is  removed 
after  the  tube  is  about  three-quarters  full  of  calcium  chloride,  and 
care  is  taken  to  see  that  none  of  the  latter  adheres  to  the  glass 
above  the  filled  portion.  Another  wad  of  cotton  is  then  placed 
in  the  tube,  the  top  is  placed  upon  it,  and  the  tube  is  closed  tempo- 
rarily at  d  by  means  of  a  piece  of  stirring-rod  within  rubber  tubing. 
The  tube  should  be  kept  closed  when  not  in  use  to  prevent  the 
gradual  absorption  of  moisture  from  the  air.  Two  ordinary  cal- 
cium chloride  tubes  are  filled  in  the  same  way  about  two-thirds 
full,  but  in  this  case  softened  cork  stoppers  are  placed  at  the  end 
of  the  tubes  after  the  second  wad  of  cotton.  Through  a  hole  in 
each  stopper  a  short  piece  of  glass  tubing  with  rounded  ends  is 
introduced,  and  the  cork  is  shoved  far  into  the  tube  with  the  help 
of  a  stirring-rod,  leaving  the  outer  2  or  3  mm.  empty.  This  space 
in  the  tube  is  filled  with  molten  sealing-wax,  so  that  a  perfectly 
air-tight  connection  is  made.  These  tubes  are  also  closed,  when 
not  in  use,  by  stirring-rod  within  rubber  tubing. 

Before  beginning  the  determination  the  apparatus  must  be 
clean  and  dry.  It  is  not  advisable  to  dry  the  flask  by  washing 
with  alcohol  and  ether,  but  it  should  be  gently  heated  while  a  cur- 
rent of  dry  air  is  sucked  through  it.  As  aspirator  an  inverted 
wash-bottle  may  be  used,  from  which  the  water  is  allowed  to  run 
out  slowly  through  the  shorter  tube.  During  the  aspiration  the 
small  calcium  chloride  tubes  are  connected  with  c  and  d  respect- 
ively, so  that  no  moisture  can  enter  the  flask. 

When  all  is  ready  the  finely-powdered  substance,  which  has 
been  dried  at  100°  C.  and  cooled  in  a  desiccator,  is  placed  in  a 
weighing-tube,  from  1  to  1.5  gms.  are  transferred  to  the  flask  and 
a  little  water  is  added. t  The  tube  .4  is  now  filled  two-thirds  full 

*  As  commercial  calcium  chloride  always  contains  a  little  free  lime,  some 
carbon  dioxide  will  be  absorbed  by  it  and  consequently  low  results  obtained 
in  the  analysis,  unless  the  calcium  chloride  is  saturated  with  carbon  dioxide 
before  the  analysis  is  made  (see  foot-note,  page  380). 

t  This  method  is  often  used  for  the  determination  of  the  carbonic  acid 
in  baking-powders.  Such  substances  are  decomposed  by  water  so  that  they 
should  be  kept  dry  until  after  the  apparatus  has  been  weighed. 


378      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

with  hydrochloric  acid  (1  part  concentrated  acid  to  4  of  water)  by 
means  of  a  small  funnel  or  thistle  tube,  and  the  stop-cock  T 
must  be  turned  so  that  none  of  the  acid  will  run  into  the  flask. 
The  whole  apparatus,  as  shown  in  Fig.  58,  is  now  placed  upon 
the  balance-pan  and  accurately  weighed.  The  stop-cock  T  is  then 
opened  so  that  the  acid  in  A  slowly  drops  into  the  flask.  As  soon 
as  the  evolution  of  carbon  dioxide  begins  to  take  place  quietly,  the 
apparatus  is  allowed  to  stand  without  watching  for  about  half  an 
hour.  At  the  end  of  this  time  all  of  the  acid  will  have  passed  into 
the  flask,  and  the  decomposition  will  be  nearly  complete  in  most 
cases.  It  now  remains  to  remove  all  carbon  dioxide  absorbed  by 
the  liquid  and  contained  in  the  apparatus.  This  is  effected  by 
gently  heating  the  solution  by  means  of  a  small  flame  until  the  acid 
just  begins  to  boil,  meanwhile  aspirating  a  current  of  dry  air  through 
c  and  out  at  d.  Not  more  than  three  or  four  bubbles  of  air  per 
second  should  be  allowed  to  pass  through  the  flask.  As  soon  as 
the  boiling  begins,  the  flame  is  removed  and  the  slow  current  of 
air  is  still  passed  through  the  apparatus  until  it  is  cold.  It  is  then 
stoppered  and  allowed  to  stand  near  the  balance  for  half  an  hour 
or  more,  after  which  it  is  again  weighed  without  the  stoppers.  The 
loss  in  weight  represents  the  amount  of  carbon  dioxide  originally 
present  in  the  substance  as  carbonate. 

Remark. — This  method  affords  excellent  results  in  the  esti- 
mation of  large  amounts  of  carbonic  acid,  but  it  is  unreliable  for 
the  analysis  of  small  amounts  such  as  are  present  in  cements, 
etc.  In  such  cases  the  Fresenius-Classen  or  Lunge-Marchlewski 
method  is  better.  (See  pp.  380  and  388.) 

The  objection  to  this  method  lies  in  the  fact  that  owing  to 
the  size  and  weight  of  the  apparatus,  there  is  likely  to  be  an 
error  in  making  the  two  weighings.*  On  the  other  hand,  it  is 
unquestionably  true  that  it  is  easier  to  expel  carbon  dioxide 
from  a  solution  than  it  is  to  absorb  it  quantitatively. 

B.    DIRECT   DETERMINATION   OF  CARBON  DIOXIDE. 

Here  again  the  determination  can  be  carried  out  both  in  the 
dry  and  wet  ways. 

*  There  is  some  danger  of  losing  a  little  hydrochloric-acid  gas  during  the 
operation.  To  prevent  this  the  calcium  chloride  may  be  replaced  by  pumice 
impregnated  with  anhydrous  copper  sulphate,  or  the  carbonate  may  be 
decomposed  by  means  of  sulphuric  acid. 


DETERMINATION  OF  CARBONIC  ACID   BY  DIFFERENCE.        379 

(a)  Determination  in  the  Dry  Way. 

From  one  to  two  grams  of  the  substance  are  weighed  out  into  a 
porcelain  boat,  and  the  latter  is  shoved  into  the  middle  of  a  horizon- 
tally held  glass  tube  which  is  about  20  cm.  long  and  1-1.5  cm.  wide, 
and  made  of  difficultly  fusible  glass.  Both  ends  of  the  tube  are 
provided  with  calcium  chloride  tubes  connected  with  it  by  means  of 
tightly-fitting  rubber  stoppers.  Through  one  of  the  calcium  chloride 
tubes  a  slow  stream  of  air  (free  from  carbon  dioxide)*  is  conducted 
and  the  other  is  connected  with  two  weighed  soda-lime  tubes  (cf. 
p.  380) .  The  substance  is  heated  gradually  until  it  glows  strongly, 
meanwhile  passing  a  slow  but  steady  current  of  air  through  the 
apparatus.  When  there  is  no  further  heat  effect  to  be  detected 
in  the  soda-lime  tubes,  the  substance  is  allowed  to  cool  in 
the  current  of  air  and  the  soda-lime  tubes  are  subsequently 
weighed.  The  increase  of  weight  represents  the  amount  of  carbon 
dioxide. 

Remark. — This  method  can  be  employed  for  the  analysis  of 
all  carbonates  with  the  exception  of  those  of  barium  and  the 
alkalies, f  though,  of  course,  no  other  volatile  acid  can  be  present 
at  the  same  time.  Water  is  kept  back  by  the  calcium  chloride 
tube. 

Example:  Analysis  of  White  Lead. — White  lead,  provided  it  is 
free  from  acetate  (which  must  be  shown  by  a  qualitative  test), 
can  be  accurately  and  expeditiously  analyzed  by  the  above 
method.  It  is  a  basic  carbonate  of  lead  and  contains,  therefore, 
lead  oxide,  carbon  dioxide,  and  water,  while  it  is  often  contami- 
nated with  sand. 

The  analysis  is  conducted  as  above  described  except  that  in 
this  case  the  calcium  chloride  tube  which  is  connected  with  the 
soda-lime  tubes  is  weighed.  The  gain  in  weight  of  the  former  repre- 
sents the  amount  of  water  in  the  substance,  the  gain  in  weight  of 
the  latter  shows  the  amount  of  carbonic  acid  present,  while  if  the 


*  The  air  is  passed  through  two  wash-bottles  containing  caustic  potash 
solution. 

t  Even  the  carbonates  of  the  alkalies  and  of  barium  can  be  analyzed  in 
this  way  if  they  are  mixed  with  potassium  bichromate. 


380     GRAVIMETRIC    DETERMINATION  OF  THE  METALLOIDS. 

residue  in  the  porcelain  boat  is  weighed  the  amount  of  lead  oxide 
is  determined.  After  weighing  the  latter  the  lead  oxide  is  treated 
with  hot,  dilute  nitric  acid,  when  it  will  dissolve  to  a  clear  solution 
if  pure,  while  any  sand  will  remain  behind  as  an  insoluble  residue. 
If  there  is  a  residue  it  is  filtered  off,  ignited,  and  weighed.  By 
deducting  the  latter  from  the  original  weight  of  the  residue  in  the 
porcelain  boat,  the  weight  of  the  pure  lead  oxide  is  obtained. 

(b)  Determination  in  the  Wet  Way,  after  Fresenius-Classen. 

The  apparatus  necessary  for  this  determination  is  shown  in 
Fig.  59.  It  consists  of  a  decomposition-flask  of  about  400  c.c. 
capacity  provided  with  a  condenser  and  connected  with  the  drying- 
tubes  a,  6,  and  c  and  with  the  weighed  soda-lime  tubes  d  and  e;  *  /  is 
a  protection  tube  whose  left  arm  is  filled  with  calcium  chloride  and 
whose  right  arm  contains  soda-lime.  The  first  drying-tube,  a, 
contains  glass  beads  wet  with  concentrated  sulphuric  acid,  while 
b  and  c  contain  granular  calcium  chloride.f 

Procedure. — The  substance  is  weighed  out  into  the  dry  flask, 
covered  with  a  little  water  in  order  to  prevent  loss  of  the  substance 
and  a  slow  current  of  air  (free  from  carbon  dioxide)  is  conducted 
through  the  apparatus  in  order  to  remove  any  carbon  dioxide 
that  may  be  present  in  the  flask  or  in  the  three  drying-tubes. 
While  the  air  is  being  led  through  the  apparatus,  the  soda-lime 
tubes  are  carefully  wiped  with  a  linen  cloth  and  weighed.  The 
current  of  air  is  now  stopped,  the  weighed  tubes  are  connected  with  c 


*  The  right  arm  of  the  last  soda-lime  tube  is  one- third  filled  with  calcium 
chloride  in  order  to  absorb  the  water  set  free  by  the  absorption  of  the  carbon 
dioxide  by  the  soda-lime,  2NaOH  +  COj=Na2CO8  +  H,O. 

t  As  commercial  calcium  chloride  always  contains  lime  which  will  absorb 
carbon  dioxide,  it  must  be  saturated  with  this  gas  before  the  determination 
is  made  For  this  purpose  a  dry  current  of  the  gas  is  conducted  through 
the  tubes  for  one  or  two  minutes,  the  outer  end  of  the  tube  is  then  closed 
by  means-  of  a  glass  rod  within  a  piece  of  rubber  tubing  and  the  other  end 
is  kept  connected  with  the  Kipp  generator  for  twelve  hours  At  the  end 
of  that  time  the  excess  of  carbon  dioxide  is  removed  by  passing  air  through 
the  tubes  tor  twenty  minutes  The  air  is  freed  from  carbon  dioxide  and 
dried  by  passing  through  two  bottles  containing  concentrated  caustic  potash 
solution  and  then  through  a  long  tube  filled  with  calcium  chloride. 


DIRECT  DETERMINATION  OF  CARBON   DIOXIDE.  381 

on  the  one  hand  and  with  /  on  the  other,  after  which  a  slow  stream 
of  hydrochloric  acid  (1:3)  is  allowed  to  flow  upon  the  substance 
from  the  funnel  T,  causing  an  immediate  evolution  of  carbon 
dioxide  gas.  The  stream  of  acid  is  regulated  so  that  not  more 
than  3-4  bubbles  per  second  of  gas  pass  through  a.  When  all  of 
the  acid  has  been  added,  the  contents  of  the  flask  are  slowly  heated 
to  boiling  and  while  the  solution  is  boiling  gently,  a  slow  current  of 
air  is  passed  through  the  apparatus  so  that  not  more  than  2-3 


FIG.  59. 

bubbles  per  second  pass  through  a.  During  the  whole  operation, 
cold  water  is  allowed  to  flow  through  the  condenser;  in  this  way 
the  water  vapor  is  condensed  and  flows  back  into  the  flask  instead 
of  reaching  the  sulphuric  acid  in  a;  consequently  the  contents  of 
the  latter  tube  seldom  have  to  be  renewed.  Almost  all  of  the  car- 
bonic acid  is  absorbed  in  the  first  soda-lime  tube,  as  may  be  ascer- 
tained by  the  heat  effect  there.  The  second  tube,  e,  should  remain 
perfectly  cold  provided  not  more  than  0.5-1  gm.  of  the  carbonate 
was  taken  for  the  analysis.  When  all  of  the  carbon  dioxide  has 
been  absorbed  the  tube  d  quickly  cools.  As  soon  as  this  has 
taken  place,  the  flame  is  removed  and  a  somewhat  more  rapid 
current  of  air  is  conducted  through  the  apparatus  for  twenty 
minutes  more.  The  soda-lime  tubes  are  then  removed,  stoppered, 
and  allowed  to  stand  in  the  balance  case  for  twenty  minutes,  in 


3 §2      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

order  to  acquire  the  temperature  of  the  balance;  they  are  then 
weighed. 

Remark. — The  results  obtained  by  this  method  are  perfectly 
satisfactory.  For  the  analysis  of  substances  containing  small 
amounts  of  carbonate,  from  3-10  gms.  are  taken  for  the 
analysis. 

If  the  substance  contains  besides  the  carbonate  a  sulphide 
which  is  decomposable  with  acid,  a  tube  containing  pumice 
impregnated  with  copper  sulphate  *  is  introduced  between  a 
and  b,  and  this  serves  to  absorb  all  of  the  hydrogen  sulphide 
evolved. 

The  Fresenius-Classen  method  is  suitable  not  only  for  the 
determination  of  carbon  dioxide  in  solid  substances,  but  also  for 
the  analysis  of  carbonates  in  solution  provided  little  or  no  free 
carbonic  acid  is  present.  In  case  considerable  amounts  of  the 
latter  are  to  be  estimated,  as  in  the  case  of  many  mineral  waters, 
the  analysis  is  conducted  as  follows: 

Determination  of  the  Total  Amount  of  Carbonic  Acid  in 
Mineral  Waters. 

From  3  to  4  gms.  of  freshly-burnt  lime  f  and  the  same  amount 

*  Sixty  gms.  of  pumice  in  pieces  about  the  size  of  a  pea  are  placed  in  a 
porcelain  dish  and  covered  with  a  concentrated  solution  of  30-35  gms.  of 
copper  sulphate.  The  solution  is  evaporated  to  dry  ness  with  constant  stirring 
and  the  residue  allowed  to  remain  in  the  hot  closet  at  150-160°  C.  for  four 
or  five  hours.  At  this  temperature  the  copper  sulphate  is  partly  dehydrated 
and  in  this  condition  it  absorbs  hydrogen  sulphide  more  readily  than  when 
in  the  hydrous  condition.  It  cannot  be  heated  higher  than  the  above  tem- 
perature as  otherwise  some  sulphur  dioxide  is  formed  which  would  be  absorbed 
by  the  soda-lime. 

t  To  prepare  this  lime  absolutely  free  from  carbonate,  the  lime  is  placed 
in  a  tube  of  difficultly  fusible  glass  and  heated  in  a  small  combustion  furnace, 
meanwhile  passing  a  current  of  dry  air  free  from  carbon  dioxide  over  it. 
In  this  way  4  gms.  of  commercial  lime  can  be  freed  from  carbonate  in 
one-half  to  three-quarters  of  an  hour.  That  the  carbon  dioxide  is  actually 
removed  can  be  shown  at  the  end  of  that  time  by  passing  the  escaping  air 
through  baryta  water;  there  should  be  no  turbidity.  A  blank  experiment 
should  always  be  made  with  this  lime.  If  it  is  desired  to  use  commercial  lime 
for  the  determination,  the  amount  of  carbonate  present  is  determined  and 
an  accurately  weighed  amount  is  used  for  the  analysis. 


DIRECT   DETERMINATION  OF  C.4RBON  DIOXIDE.  383 

of  crystallized  calcium  chloride  *  are  placed  in  each  of  from  four 
to  six  Erlenmeyer  flasks  whose  necks  are  of  such  a  size  that 
they  will  each  fit  the  apparatus  shown  in  Fig.  59.  The  flasks  are 
closed  by  means  of  tightly-fitting  rubber  stoppers  and  accurately 
weighed.  A  double-bored  rubber  stopper  is  taken  of  such  a  size 
that  it  will  fit  into  the  neck  of  each  of  the  above  flasks  and  through 
one  of  the  holes  is  fitted  a  short  glass  tube  which  reaches  about  3  cm. 
above  the  stopper  and  the  same  distance  below,  while  through  the 
other  hole  a  glass  tube  about  50  cm.  long  is  passed  which  likewise 
reaches  about  3  cm.  below  the  stopper.  To  fill  the  weighed  flasks 
with  the  water  to  be  analyzed,  they  are  taken  to  the  spring  and 
are  treated  one  after  another  as  follows:  The  solid  rubber  stopper 
is  quickly  replaced  by  the  one  fitted  with  the  two  tubes,  the  thumb 
is  placed  over  the  shorter  of  the  tubes,  and  the  flask  is  dipped  well 
below  the  surface  of  the  water,  but  so  that  the  longer  tube  still 
reaches  into  the  air  above.  The  thumb  is  now  removed  from  the 
shorter  tube,  when  the  spring- water  will  pass  into  the  flask  and  the 
replaced  air  will  escape  through  the  long  tube.  As  soon  as  the 
flask  is  almost  full,  the  shorter  tube  is  again  closed  with  the  thumb, 
the  flask  is  removed  from  the  water,  and  the  stoppers  are  once 
more  quickly  interchanged.  To  make  sure  that  the  solid  stopper  is 
not  loosened  while  carrying  the  flask  back  to  the  laboratory,  it  is 
covered  by  a  piece  of  parchment  paper,  and  fastened  by  means  of 
string  to  the  neck  of  the  flask.  The  flasks  are  then  allowed  to 
stand  several  days  with  frequent  shaking,  when  the  precipitate  is 
allowed  to  settle  and  the  flask  and  contents  are  weighed.  The  gain 
in  weight  represents  the  amount  of  water  taken  for  the  analysis. 
The  supernatant  liquid  is  quickly  poured  off  through  a  folded  filter, 
the  filter  is  immediately  thrown  into  the  flask,  and  the  latter  is 
now  connected  with  the  apparatus  shown  in  Fig.  59.  The  carbon 
dioxide  is  determined  as  in  the  previous  process. 

This  method  is  capable  of  yielding  excellent  results  provided 
the  flasks  can  be  filled  as  above  described.  Often,  however,  the 
spring  is  not  easily  accessible,  so  that  the  flasks  must  be  filled  by  a 


*  The  addition  of  calcium  chloride  serves  to  decompose  any  alkali  car- 
bonate. This  is  not  quantitatively  decomposed  by  lime  alone,  particularly 
when  magnesium  carbonate  is  present. 


384      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

different  method  and  usually  a  small  amount  of  carbonic  acid  is 
lost  during  the  operation.  A  much  more  expeditious  and  accurate 
procedure  which  can  be  performed  within  one  hour  at  the  spring, 
consists  in  the  determination  of  the  total  amount  of  carbonic 
acid  present  in  mineral  waters  by  measuring  the  volume  of  the 


2.  Gas-volumetric  Determination  of  Carbonic  Acid. 

(a)  Method  of  0.  Fetter  sson.-f 

This  excellent  method,  upon  which  the  two  following  pro- 
cedures are  based,  consists  of  evolving  carbon  dioxide  from 
carbonates  by  the  action  of  acid,  collecting  the  gas  over  mercury 


FIG.  60. 

and  computing  its  weight  from  its  volume.  Petterson's  apparatus 
is  shown  in  Fig.  60,  and  was  used  by  him  for  the  determination 
of  the  carbonic  acid  in  sea-water  (Skagerrak),  in  carbonates,  and 
also  for  the  determination  of  carbon  in  iron  and  steel.  The  method 

*  Cf.  the  modified  method  of  Pettersson  on  p.  393. 
f  Berichte,  23  (1890),  p.  1402. 


GAS-VOLUMETRIC  DETERMINATION  OF  CARBONIC  ACID.     385 

of  determining  the  carbonic  acid  in  a  water  containing  small 
amounts  of  free  carbonic  acid  but  considerable  carbonate  will  suffice 
to  show  how  the  apparatus  is  used.  The  decomposition-flask  K 
is  filled  with  distilled  water  up  to  the  mark  just  below  the  side- 
arm  (the  mark  is  not  shown  in  the  illustration).  By  weighing  the 
flask  both  empty  and  with  this  amount  of  water,  the  volume  of  the 
flask  when  filled  to  the  mark  is  obtained.  The  flask  is  now  filled 
up  to  this  mark  with  the  water  to  be  examined,  a  small  piece  of 
aluminium  wire  *  is  thrown  in,  and  the  flask  is  connected  with  the 
rest  of  the  apparatus  as  shown  in  the  figure.  All  of  the  rubber 
tubing  should  be  firmly  fastened  to  the  glass  by  means  of  wire. 
The  cocks  a,  b,  and  d  are  closed,  c  is  opened,  and  the  air  hi  the 
measuring-tube  is  removed  by  raising  M  until  the  mercury  rises 
in  the  capillary  up  to  the  crossing  point.  After  this  c  is  closed, 
a  is  opened,  M  is  brought  very  low,  and  the  screw-cock  d  is 
slowly  opened.  By  this  means  the  hydrochloric  acid  in  N  is  intro- 
duced into  the  flask  K.  The  acid  is  allowed  to  run  into  the  flask 
until  the  upper  part  of  the  apparatus  is  reached,  when  d  is  closed 
and  then  a.  The  air  in  the  measuring-tube  (which  does  not  con- 
tain an  appreciable  amount  of  carbon  dioxide)  is  removed  by 
opening  c  and  raising  M,  after  which  c  is  again  closed.  Now  a  is 
once  more  opened,  M  is  lowered,  and  the  liquid  hi  K  is  heated  by 
means  of  a  flame. 

A  lively  evolution  of  gas  at  once  ensues.  As  soon  as  the  meas- 
uring-tube is  almost  filled  with  the  gas,  a  is  closed,  the  flame  is 
removed  from  K,  M  is  raised  until  the  mercury  within  it  stands 
level  with  that  in  the  measuring- tube,  and  its  position  in  the  latter 
is  then  read.  At  the  same  time  the  barometer  reading  must  be 
noted  as  well  as  the  temperature  of  the  cold  water  which  surrounds 
the  measuring-tube.  After  this  b  is  opened  and  M  is  raised, 
whereby  the  gas  passes  into  the  Orsat  tube  0  which  contains 
caustic  potash  solution  (1:2).  As  soon  as  the  mercury  has  reached 
the  juncture  of  the  horizontal  and  vertical  tubes,  b  is  closed  and 
the  gas  is  allowed  to  remain  in  the  Orsat  tube  for  three  minutes. 
The  unabsorbed  gas  is  once  more  brought  into  the  measuring- 
tube,  taking  care  that  none  of  the  caustic  potash  solution  comes 
with  it  (the  latter  should  not  quite  reach  the  stop-cock  b).  After 
bringing  the  gas  once  more  to  the  atmospheric  pressure,  its  volume 
*  0.0142  gm.  aluminium  evolves  20  c.c.  of  moist  hydrogen  at  720  mm.  and  15°  C. 


386      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

as  well  as  the  temperature  and  barometer  reading  is  noted.  As  a 
rule,  these  readings  of  the  barometer  and  thermometer  remain 
constant,  otherwise  it  is  necessary  to  reduce  the  gas  volumes  in 
each  case  to  0°  C.  and  760  mm.  pressure.  The  difference  between 
the  two  volumes  represents  the  amount  of  the  carbonic  acid 
gas.  The  unabsorbed  gas  is  removed  through  c  and  this  whole 
operation  of  collecting  the  gas  and  absorbing  the  carbon  dioxide 
is  repeated  until  finally  no  more  gas  is  given  off  from  the  liquid 
inK. 

In  case  it  is  desired  to  determine  the  amount  of  carbonate  in 
a  solid  substance,  a  smaller  decomposition-flask  should  be  used. 
The  aluminium  wire  is  added  to  the  weighed  substance  and  the 
apparatus  is  exhausted  by  repeatedly  lowering  M,  closing  a, 
opening  c,  and  then  raising  M.  Finally  the  acid  is  allowed  to 
act  upon  the  substance  and  the  determination  is  carried  out  exactly 
as  described  above. 

Computation  of  the  Analysis. — Let  us  assume  that  from  a 
gms.  of  substance  V  c.c.  of  carbon  dioxide  were  obtained,  which 
was  measured  moist  at  t°  C.  and  B.  mm.  pressure.  First  of  all 
the  volume  is  reduced  to  0°  C.  and  760  mm.  pressure  by  the 
following  formula : 

7(5-^0-273 
0      760(273+0  ' 

In  this  formula,  w  represents  the  tension  of  aqueous  vapor  ex- 
pressed in  millimeters  of  mercury. 

In  order  to  compute  the  weight  of  the  carbon  dioxide  from  this 
volume,  we  start  with  the  fact  that  the  density  of  carbon  dioxide 
is  1.529  *  referred  to  air  as  unity.  1  c.c.  of  air  at  0°  and  760  mm. 
pressure  weighs  0.001293  gm.t  consequently  at  0°  and  760  mm. 

1  c.c.  C02  weighs  0.001293X1.529  =  0.1977  gm. 

and  V0  c.c.  weigh  FX0.001977  gm.  The  percentage  of  G02  in 
the  original  substance  is  then 

V0X  0.1977 

-  =per  cent.  C02. 


*  Cf.  Lord  Rayleigh,  Proc.  Roy.  Soc,  62,  204  (1897). 
f  Landolt-Bornstein,  Phys.  chem.  Tabellen. 


GAS-VOLb 'METRIC  DETERMINATION  OF  CARBONIC  ACID.       387 

Remark. — The  addition  of  aluminium  is  absolutely  necessary. 
By  boiling   an    acid   solution,   carbonic   acid  is   not  completely 


FIG.  61. 

expelled;  this  is   only   effected  when  a   different  gas  simultane- 
ously passes   through  the  solution.     Formerly  it  was  customary 


388       GRAVIMETRIC   DETERMINATION  OF  THE  METALLOIDS. 

to  pass  air  through  the  apparatus,  but  Petterson  accomplished 
the  same  purpose  by  generating  hydrogen  within  the  liquid 
itself. 

(6)  Method  of  Lunge  and  Marchlewski* 

Lunge  and  Marchlewski  carry  out  the  determination  according 
to  the  same  principle  as  that  of  the  above  process;  i.e.,  by  simul- 
taneously evolving  hydrogen  (aluminium  and  hydrochloric  acid), 
measuring  the  gas,  and  absorbing  the  carbon  dioxide  by  means  of 
caustic  potash  in  an  Orsat  tube. 

The  apparatus  which  they  recommend  is  shown  in  Fig.  61,  6. 
It  consists  of  the  40-c.c.  decomposition-flask  N,  the  140-c.c. 
measuring-tube  A,  the  compensation- tube  C,  and  the  le veiling- 
tube  B;  the  three  last  are  connected  together  as  shown  in  the 
figure. 

In  the  case  of  all  gas-volumetric  methods,  the  volume  of  the 
measured  gas  must  be  reduced  to  0°  C.  and  760  mm.  pressure, 
which  ordinarily  requires  a  knowledge  .of  the  temperature  and  the 
barometric  pressure.  In  this  method  the  reduction  is  accom- 
plished without  paying  any  attention  to  the  actual  readings  of 
the  thermometer  and  barometer  by  means  of  the  compensation- 
tube  C,  which  contains  a  known  amount  of  air,  viz.,  that  amount 
of  air  which  in  a  dry  condition  assumes  a  volume  of  100  c.c.  at 
0°  C.  and  760  mm.  pressure.  If,  therefore,  this  amount  of  air  has 
a  volume  of  V  at  t°  and  atmospheric  pressure  P'  (with  the  mer- 
cury at  the  same  level  in  B  and  C) ,  we  know  that  this  volume  of 
any  gas  would  be  equal  to  100  c.c.  at  0°  C.  and  760  mm.  pressure. 
By  raising  the  le  veiling-tube  B  so  high  that  V  c.c.  is  compressed  to 
100  c.c.,  we  have  accomplished  the  reduction  in  a  mechanical  way. 
If,  however,  there  is  a  gas  volume  V"  in  the  measuring-tube  A 
under  the  same  pressure  as  that  in  the  compensation-tube  (this  is 
the  case  when  the  mercury  level  is  the  same  in  A  and  C),  we  can 
reduce  this  volume  to  the  standard  conditions  by  simply  raising 
B  until  the  volume  of  the  gas  in  C  is  just  100  c.c.,  taking  care  that 
the  mercury  remains  at  the  same  height  in  the  tubes  A  and  C. 
The  volume  of  the  gas  T0"  in  A  corresponds,  therefore,  to  the  vol- 

*Zeitschr.  f.  angew.  Chem.,  1891,  p.  229. 


GAS- VOLUMETRIC  DETERMINATION  OF  CARBONIC  ACID.       389 

ume  of  this  gas  at  0°  C.  and  760  mm.  pressure,  for  it  has  been  com- 
pressed to  the  same  degree  as  the  gas  in  C.  This  is  apparent  when 
we  remember  that  at  a  constant  temperature  the  product  of  the 
pressure  into  the  volume  remains  a  constant  for  any  gas. 

In  the  compensation-tube  we  have  the  volume  V  at  atmos- 
pheric pressure  P' ,  and  after  compression  the  volume  becomes 
F0'  =  100  c.c.  and  the  pressure  is  P0,  from  which  it  follows: 

1.  F'P'  =  F0'P0. 

In  the  measuring-tube  A,  we  have  the  volume  V"  at  the  atmos- 
pheric pressure  Pf,  and  after  compression  this  volume  becomes 
F0",  and  the  pressure  P0,  so  that 

2.  F"-P'  =  F0"P0. 

By  dividing  equation  1  by  equation  2  we  have: 
V'-P'      Fp'.Pp 

F"-P'~F0"-P0 

or 

F':F"-Ff':TV' 

and  F0"  is,  therefore,  the  reduced  gas  volume  that  is  desired. 

Before  using  the  apparatus  for  the  determination,  it  is  necessary 
to  fill  the  compensation- tube  with  the  correct  amount  of  air;  this 
is  accomplished  as  follows: 

First  of  all  a  calculation  is  made  to  determine  what  would  be 
the  volume  of  100  c.c.  of  dry  air  measured  at  0°  C.  and  760  mm. 
pressure  when  measured  moist  at  the  temperature  of  the  laboratory 
and  the  prevailing  barometric  pressure.  To  illustrate,  let  us 
assume 

J=17.5°  C.;  5=731  mm.;    10  =14. 9  (tension  of  aqueous  vapor), 

then 

100X760X290.5 
-273(731-14.9)  ' 

According!}'  112.9  c.c.  of  air  are  introduced  into  the  tube 
C  by  removing  the  stopper  and  lowering  the  levelling-tube  until 
the  mercury  in  the  compensation-tube  stands  at  exactly  112.9  c.c. 
A  drop  of  water  is  added  by  means  of  a  pipette,  the  tube  is  im- 


390        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 


mediately  stoppered,  and  an  air-tight  seal  is  made  by  covering  the 
latter  with  mercury.  A  rubber  stopper  containing  a  glass  tube  is 
then  pressed  down  into  D.  After  this  the  temperature  and  pres- 
sure may  vary  as  much  as  it  will ;  the  reduced  volume  of  the  air 
in  C  will  always  be  equal  to  100  c.c. 

Procedure  for  the  Analysis. — About  0.08  gin.  of  aluminium  wire, 
i.e.,  enough  to  furnish  approximately  100  c.c.  of  hydrogen,  is 
weighed  out  into  the  decomposition- 
flask.  Such  an  amount  of  the  substance 
to  be  analyzed  is  added,  that  at  the 
most  30  c.c.  of  carbon  dioxide  will  be 
generated,  and  the  flask  is  connected 
with  the  funnel  tube  M ,  and  capillary  n. 
Connection  is  also  made  with  the  tube  A 
after  it  has  been  completely  filled  with 
mercury  by  raising  B.  The  air  from  N  is 
now  exhausted  by  lowering  B,  opening  h 
so  that  e  is  connected  with  A,  then  clos- 
ing h  by  turning  it  90°,  and  carefully 
raising  B  until  the  mercury  stands  at 
an  equal  height  in  A  and  B;  after  this 
h  is  turned  so  that  A  is  connected 
with  the  capillary  d,  and  the  air  in 
A  is  expelled.  After  repeating  this 
process  three  or  four  times  until  finally 
only  two  or  three  centimeters  of  air  re- 
main in  A,  B  is  lowered,  the  hydro- 
chloric acid  (1:3)  is  added  to  M,  h  is 

carefully  opened,  then  m  until  10  c.c.  of  the  acid  have  run 
into  the  flask  .V,  when  m  is  once  more  closed.  The  carbon 
dioxide  evolution  begins  at  once  and  the  mercury  level  quickly 
falls  in  A.  The  contents  of  the  flask  are  heated  to  boiling  over  a 
flame  and  kept  at  this  temperature  until  all  of  the  aluminium  has 
dissolved.  During  the  whole  operation  the  mercury  level  in  B 
must  be  kept  lower  than  that  in  A.  In  order  to  transfer  the  gas 
remaining  in  the  flask  N  into  the  tube  A,  M  is  filled  with  distilled 
water,  m  is  slowly  opened  and  the  water  is  allowed  to  run  into  N 
until  the  stop-cock  h  is  reached,  when  the  latter  is  immediately 


FIG.  62. 


GAS-VOLUMETRIC  DETERMINATION  OF  CARBONIC  ACID.     391 

closed.  The  gas  is  then  compressed  by  raising  the  tube  B  until 
the  mercury  stands  at  the  same  height  in  A  and  C  and  so  that  the 
level  in  the  latter  tube  is  exactly  at  the  100-c.c.  mark.  The  reduced 
volume  is  then  read.  After  this  the  capillary  d  is  connected 
with  an  Orsat  tube  filled  with  caustic  potash  (1:2)  (Fig.  62),  the 
gas  in  A  is  driven  over  into  the  latter,  allowed  to  stand  there  for 
three  minutes,  and  again  transferred  to  A,  where  its  volume  at 
0°  C.  and  760  mm.  pressure  is  determined  as  before.  The  difference 
in  the  two  readings  represents  the  volume  of  the  carbon  dioxide, 
and  the  per  cent,  nan  be  computed  according  to  the  formula 

Per  cent.  CO2  =  0.1977.—, 

in  which  F  is  the  amount  of  carbon  dioxide  absorbed  in  theOrsat  tube 
and  a  represents  the  amount  of  substance  taken  for  the  analysis. 

Remark. — This  is  the  most  exact  of  all  methods  for  the  deter- 
mination of  carbon  dioxide  in  solid  substances  and  is  accom- 
plished most  quickly.  It  is  particularly  to  be  recommended 
where  carbon  dioxide  determinations  must  be  made  daily,  as,  for 
example,  in  cement  factories.  It  is  necessary,  however,  to  test 
the  volume  of  the  gas  in  the  compensation-tube  from  time  to  time 
in  order  to  make  sure  that  it  really  corresponds  to  100  c.c.  of  air 
under  the  standard  conditions  of  temperature  and  pressure. 

For  a  single  determination  the  author  prefers  to  dispense 
with  the  compensation- tube.  In  this  case,  however,  the  collected 
gas  must  be  kept  surrounded  by  water  at  a  definite  temperature, 
as  in  the  Petterson  method,  and  the  temperature  and  pressure 
must  be  observed.  It  is  also  well  to  make  these  readings  in  the 
above-described  procedure,  in  order  to  be  sure  that  the  volume  in 
the  compensation-tube  has  remained  constant. 

(c)   Method  of  Lunge  and  Rittener.  * 

In  the  decomposition  flask  K,  Fig.  63,  is  placed  0.14-0.15  g.  of 
calcite,  or  a  corresponding  amount  of  any  other  carbonate,  and 
a  small  piece  of  aluminium  wire,  weighing  about  0.015  g.,  is  fas- 
tened to  the  neck  of  the  flask.  About  1  c.c.  of  water  is  allowed 
to  flow  through  the  funnel,  T,  and  then  the  capillary  is  connected 
*  Z.  angew.  Chem.,  1906,  1849. 


392        GRAVIMETRIC  DETERMINATION  OF   THE    METALLOIDS. 


with  the  dry  Bunte  burette.  The  stop-cock  of  the  funnel  T  is 
closed  and  the  two  cocks  of  the  Bunte  burette  are  opened.  The 
lower  stop-cock,  hl}  is  now  connected  with  the  suction  pump, 
and  a  partial  vacuum  is  produced  in  the  burette  by  letting  the 
pump  run  two  or  three  minutes,  after  which  h:  is  closed.  Now 

from  the  funnel  T,  hydrochloric  acid 
|T'  (1:4)  is  allowed  to  flow  upon  the  sub- 
stance until  the  latter  is  decomposed 
completely;  then  the  liquid  is  heated 
to  boiling,  *  taking  care  that  no  water 
gets  into  the  burette.  Acid  is  then 
added  from  T  until  the  aluminium 
wire  is  reached  and  the  flask  is  heated 
again.  The  hydrogen  now  evolved 
serves  to  expel  the  last  traces  of  carbon 
dioxide  from  the  flask.  As  soon  as  all 
the  aluminium  is  dissolved,  hydro- 
chloric acid  is  added  through  the  funnel 
until  the  liquid  reaches  the  stop-cock  hy 
which  is  then  closed  at  once.  The 
lower  end,  a,  of  the  burette  is  now 
connected  by  rubber  tubing  with  the 
levelling  tube  N,  which  contains  a 

saturated  solution  of  common  salt.  By  carefully  opening  the  lower 
stop-cock  h^  the  salt  solution  is  allowed  to  rise  in  the  burette  until 
the  liquid  there  stands  at  the  same  height  as  in  the  levelling  tube, 
whereupon  the  stop-cock  ht  is  closed.  The  apparatus  is  allowed  to 
stand  for  20-25  minutes  so  that  the  temperature  of  the  gas  will 
be  that  of  the  surroundings  and  then,  by  suitably  raising  or  low- 
ering the  levelling  tube  with  ^i  open,  the  burette  reading  is  taken 
and  the  temperature  as  well  as  barometer  reading  noted.  The 
funnel  T  |  of  the  burette  is  filled  with  strong  caustic  potash 

*  In  the  case  of  carbonates,  such  as  magnesite,  dolomite,  or  siderite, 
they  are  decomposed  so  slowly  by  cold,  dilute  acid  that  it  may  be  added 
much  more  quickly  than  prescribed  above. 

t  Provided  the  temperature  and  pressure  are  the  same  as  before  the 
absorption  of  the  CO2.  If  this  is  not  the  case,  both  volumes  must  be  reduced 
to  0°  and  760  mm.  pressure  before  the  difference  is  found. 


FIG.  63. 


GAS-yQLU METRIC  DETERMINATION  OF  CARBONIC  ACID.     393 

solution  (1:2)  and  a  partial  vacuum  is  produced  in  the  burette, 
by  lowering  the  levelling  tube  and  opening  the  stop-cock  h^ 

The  caustic  potash  solution  is  allowed  to  run  into  the  burette 
by  opening  the  upper  stop-cock  h,  which  is  closed  before  the  last 
few  drops  of  liquid  leave  the  funnel,  and  the  contents  of  the 
burette  mixed  by  shaking.  By  repeating  the  operation  it  is 
easy  to  tell  whether  the  absorption  of  carbon  dioxide  has  been 
complete.  The  residual  volume  is  read  with  the  usual  precau- 
tions and  the  difference  between  the  two  readings  *  gives  the 
volume  of  the  carbon  dioxide. 

The  computation  of  the  weight  of  carbon  dioxide  is  carried 
out  exactly  as  described  on  p.  386,  except  that  the  vapor  tension 
of  the  saturated  salt  solution  only  amounts  to  80  per  cent,  of  the 
tension  of  pure  water  at  the  same  temperature. 

Example:     Weight  of  substance  =  a        Tempera ture  =  £°, 
Volume  of  hydrogen  +  air +  CO2=Vt      Barometer =B  mm. 

Hydrogen + air  =  TV     Tension  of  aqueous  vapor 

=  w  mm. 

CO2=  Vt-Vt'    Tension  of  salt  solu- 
tion =  0.8w  mm. 

The  volume  reduced  to  0°  and  760  mm.  is,  therefore: 

_  (Ff- TV)- (£-0.8uQ273 

760(273  +  0 

and  the  percentage  of  CO2  in  the  substance  (see  p.  386)  is, 

=  per  cent.  CO2. 


F0X0.1977 


For  the  determination  of  carbon  dioxide  in  mineral  waters 
this  apparatus  is  not  suited;  for  this  purpose  the  author  has  modi- 
fied the  Pettersson  apparatus  as  shown  in  Fig.  64. 

(d)   The  Modified  Method  of  Pettersson. 

For  decomposition-flasks,  Erlenmeyers  of  from  70-200  c.c. 
capacity  are  used  (according  to  the  supposed  amount  of  carbonic 


394        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

acid)  and  the  exact  capacity  of  each  flask  is  etched  upon  it.  To 
determine  this,  each  flask  is  provided  with  a  tightly-fitting  stopper 
of  gray  (not  red)  rubber  containing  one  hole,  through  which  the 
small  tube  R  is  introduced.  The  bottom  of  R  is  fused  together, 
but  near  the  bottom  a  small  opening  is  blown. 


The  tube  R  is  shoved  into  the  stopper  until  the  small  opening 
can  be  seen  just  below  the  bottom  of  the  rubber  stopper,  and  the 
latter  is  pressed  as  far  as  possible  into  the  Erlenmeyer  flask  full  of 
water.  By  this  means  some  of  the  water  passes  from  the  flask 
into  the  tube  R.  The  latter  is  then  raised  as  is  shown  in  Fig. 
64,  b,  and  in  this  way  an  air-tight  seal  is  made. 

The  water  in  R  is  now  removed  by  filter-paper  and  the  flask 
and  contents  weighed  to  the  nearest  centigram.  By  deducting 


GAS-VOW  METRIC  DETERMINATION  OF  CARBONIC  ACID.      395 

from  this,  the  weight  of  the  empty  flask  together  with  the  rubber 
stopper  and  R,  the  weight  of  the  water,  i.e.  the  volume  of  the 
flask,  is  obtained.  By  means  of  a  piece  of  gummed  paper  fast- 
ened to  the  flask,  the  position  of  the  lower  edge  of  the  rubber 
stopper  is  noted.  The  flask  is  emptied,  dried,  and  the  neck  of  the 
flask  as  well  as  the  paper  strip  is  covered  with  a  thin  coating  of 
wax.  Along  the  edge  of  the  paper  where  the  bottom  of  the  rubber 
stopper  came  on  the  flask,  a  sharp  line  is  cut  in  the  wax  by  means 
of  a  knife  and  the  capacity  is  written  upon  the  wax  wTith  a 
pointed  file.  These  lines  are  etched  upon  the  flask  by  exposing 
them  to  the  action  of  hydrofluoric  acid  for  two  minutes.  The 
excess  of  the  latter  is  then  washed  off,  the  flask  dried,  and  the  wax 
melted  and  wiped  off  with  filter-paper.  The  flask  is  now  ready  to 
be  used  for  the  analysis. 

About  0.04  gm.  of  aluminium  is  placed  in  the  flask,  which  is 
then  filled  by  dipping  into  the  spring.  When  this  is  not  possible, 
a  piece  of  rubber  tubing  is  placed  in  the  bottle  containing  the 
water  to  be  analyzed  so  that  it  reaches  to  the  bottom  and  the  water 
is  siphoned  off  into  the  flask  for  two  or  three  minutes.  After  this 
the  filled  flask  is  closed  by  the  rubber  stopper  with  the  tube  R  so 
that  the  bottom  of  the  stopper  reaches  just  to  the  mark  again. 
The  tube  R  is  raised  (Fig.  64,  6)  and  the  spring-water  within  the 
tube  is  washed  out  by  a  stream  of  distilled  water  from  a  wash- 
bottle.*  The  flask  is  then  connected  with  the  bulb-tube  P  (of  about 
40  c.c.  capacity),  which  in  turn  is  connected  with  the  measuring- 
tube  B.  The  latter  is  placed  in  a  condenser  through  which  a  stream 
of  ordinary  water  constantly  flows.  The  reservoir  N'  is  now  con- 
nected with  the  flask  as  shown  in  the  figure  and  the  screw-cock  H  is 
closed.  All  rubber  connections  must  be  tightly  fastened  with  wire. 

The  bulb  P  is  exhausted  by  lowering  N  so  that  the  air  passes 
into  B,  from  whence  it  is  driven  into  the  Orsat  tube  0  by  turning 
the  stop-cock  M  and  raising  N.  This  operation  is  repeated  four 
times.  The  air  is  then  removed  from  the  Orsat  tube  by  suction 
through  the  right-hand  capillary  and  the  stop-cock  is  changed  to 
its  original  position  as  shown  in  the  figure. 

The  tube  R  is  now  pressed  into  the  flask  so  that  the  small 
opening  reaches  below  the  lower  surface  of  the  stopper. 

*  With  water  containing  much  carbonic  acid,  the  flask  and  its  contents 
are  cooled  by  ice  in  order  to  prevent  the  glass  breaking. 


396        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

Usually  carbon  dioxide  is  immediately  evolved  and  the  mercury 
in  B  at  once  begins  to  sink  slowly.  The  evolution  of  the  gas  is 
hastened  by  gently  heating  the  contents  of  the  flask.  As  soon  as 
the  measuring-tube  is  almost  entirely  rilled  with  gas,  the  flame 
is  removed,  M  is  closed,  and  the  contents  of  B  are  brought  under 
atmospheric  pressure  by  raising  N  until  the  mercury  in  the  two 
tubes  is  at  the  same  height,  after  which  its  position  in  B  is  noted. 
The  temperature  of  the  water  surrounding  B  is  taken,  the  barometer 
is  read,*  and  the  gas  is  driven  over  into  the  Orsat  tube  and  allowed 
to  remain  there.  This  boiling,  measuring,  and  driving  over  of 
the  gas  is  repeated  until  only  a  slight  gas  evolution  can  be  made 
to  take  place.  In  this  way  all  the  free  carbon  dioxide  and  a  part 
of  that  present  as  bicarbonate  is  driven  off,  while  that  present  as 
normal  carbonate  together  with  the  rest  of  the  bicarbonate  remains 
in  the  flask;  the  liquid  in  the  latter  is  usually  turbid  at  this  point 
owing  to  the  precipitation  of  alkaline-earth  carbonates.  The 
reservoir  Nf  is  now  filled  with  hydrochloric  acid  (1:2)  and  the 
air  removed  from  the  rubber  tubing  by  raising  N'  high  and  pinch- 
ing the  tubing  with  the  fingers.  The  le veiling-tube  N  is  placed  in 
a  low  position,  H  is  opened,  and  a  small  amount  of  acid  is  allowed 
to  run  into  K,  after  which  H  is  again  closed.  As  soon  as  the  acid 
reaches  the  contents  of  K,  a  lively  evolution  of  carbon  dioxide 
ensues,  which  is  afterward  hastened  by  gentle  warming.  When 
the  measuring-tube  B  is  nearly  filled,  its  contents  are  read  and 
driven  over  into  the  Orsat  tube  as  before.  The  addition  of  the 
acid,  etc.,  is  repeated  until  finally  the  liquid  in  K  clears  up  and 
the  aluminium  begins  to  evolve  a  steady  stream  of  hydrogen,  when 
the  contents  of  the  flask  are  heated  to  boiling ;  but  care  is  taken  that 
none  of  the  liquid  in  the  flask  is  carried  over  with  the  escaping 
gas.  As  soon  as  the  aluminium  has  completely  dissolved,  N  is 
lowered,  H  is  opened,  so  that  the  flask  is  filled  with  the  hydrochloric 
acid  solution  and  the  last  portions  of  the  gas  are  carried  over 
into  the  measuring-tube  B.  As  soon  as  the  acid  has  reached 
the  stop-cock  M,  this  is  closed,  and  after  reading  the  volume  of 
the  gas  as  before  it  is  led  into  the  Orsat  tube.  After  remaining 


*  If  this  analysis  is  made  at  the  spring,  it  is  necessaiy  to  have  a  sensitive 
aneroid  barometer  at  hand. 


DETERMINATION   OF  CARBONIC  ACID.  397 

there  three  minutes  the  unabsorbed  gas  is  once  more  transferred 
to  B  and  its  volume  subtracted  from  the  total  amount  of  gas  which 
has  been  expelled  from  the  water  that  was  analyzed.  This  differ- 
ence represents  the  volume  of  the  carbon  dioxide  gas.  By  cor- 
rectly adjusting  the  current  of  water  flowing  through  the  condenser, 
the  temperature  at  which  the  gas  is  measured  will  remain  constant 
during  the  entire  experiment. 

From  the  volume  of  the  absorbed  carbon  dioxide  the  per  cent, 
present  is  computed  as  was  shown  under  the  Pettersson  method. 

Remark. — By  this  method  the  author  has  determined  success- 
fully at  the  spring  the  amount  of  carbonate  in  a  great  many  of  the 
most  important  waters  of  Switzerland.  For  a  single  determination 
more  than  one  hour  is  seldom  required.  The  apparatus  *  can 
be  readily  transported.  The  author  has  travelled  with  an  outfit 
during  the  last  six  years  over  the  highest  mountain  passes  under 
many  disadvantages  of  weather,  both  in  winter  and.  summer, 
without  its  meeting  with  any  accident.  In  order  to  maintain 
the  tube  N  at  any  desired  height  it  is  well  to  fasten  it  to  a  ring- 
stand. 

Determination  of  Carbonic  Acid  in  the  Air. 

See  Part  II,  Acidimetry. 

Determination  of  Carbonic  Acid  in  the  Presence  of  Other 
Volatile  Substances. 

(a)  Determination  of  Carbonic  Acid  in  the  Presence  of  Chlorine. 

If  it  is  desired  to  determine  the  amount  of  carbonate  present 
in  commercial  chloride  of  lime,  chlorine  will  be  evolved  with  the 
carbonic  acid  on  treatment  of  the  solid  substance  with  hydrochloric 
acid,  so  that  neither  the  direct  nor  the  indirect  method  will  give 
correct  results.  The  determination  can  easily  be  effected  by  the 
following  procedure: 

The  chloride  of  lime  is  decomposed  with  hydrochloric  acid  and 
the  gases  evolved  (CO2-f  C12)  are  passed  into  an  ammoniacal  solu- 

*  It  can  be  purchased  from  Bender  and  Hobein  of  Zurich. 


398         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

tion  containing  calcium  chloride.*  After  standing  several  hours 
in  a  warm  place,  the  precipitate  is  filtered  off  quickly,  washed  with 
water,  and  the  carbonate  determined  in  the  precipitated  calcium 
carbonate  by  one  of  the  usual  methods. 

Remark. — On  conducting  the  mixture  of  chlorine  and  carbon 
dioxide  into  the  ammoniacal  solution  of  calcium  chloride,  the 
chlorine  is  changed  into  ammonium  chloride  with  evolution  of 
nitrogen : 

4NH3+  3C1=  3NH4C1+  N, 

while  the  carbon  dioxide  is  absorbed  by  the  ammonia  forming 
ammonium  carbonate,  and  the  latter  is  precipitated  by  the  calcium 
chloride  as  calcium  carbonate. 

(6)    Determination  of  Carbon  Dioxide  in  the  Presence  of  Alkali 
Sulphides,  Sulphites,  or  Thiosulphates. 

The  solution  to  be  analyzed  is  treated  with  an  excess  of  a  solu- 
tion of  hydrogen  peroxide  containing  potassium  hydroxide,  but 
free  from  carbonate.  It  is  heated  to  boiling  to  destroy  the  excess 
of  the  hydrogen  peroxide,  concentrated,  and  the  carbonate  deter- 
mined preferably  by  the  Fresenius-Classen  method  (p.  380). 

DETERMINATION  OF  CARBON. 

(1)  In  Iron  and  Steel. 

(2)  In  Organic  Compounds. 

(1)    DETERMINATION   OF  CARBON   IN  IRON   AND   STEEL. 

Carbon  exists  in  two  forms  in  iron  and  steel: 

(a)  As  Carbide  Carbon. 

(b)  As  Graphite. 

On  treating  an  iron  containing  carbide  carbon  with  dilute  hydro- 
chloric or  sulphuric  acid,  only  a  part  of  it  is  evolved  in  the  form  of 


*  One  part  of  crystallized  calcium  chloride  is  dissolved  in  five  parts  of 
water,  ten  parts  of  ammonia  (sp.  gr.  0.96)  are  added,  and  the  mixture  allowed 
to  stand  at  least  four  weeks  before  using.  . 


DETERMINATION    OF  CARBON.  399 

characteristic  smelling  hydrocarbons.  This  carbon  is  called  by 
Ledebur  *  "hardening  carbon"  to  distinguish  it  from  "ordinary 
carbide  carbon"  which  is  left  behind  as  a  brown  or  gray  mass  when 
the  iron  is  treated  with  dilute  hydrochloric  or  sulphuric  acid;  but 
on  boiling  with  strong  hydrochloric  acid  the  latter  is  also  changed 
to  volatile  hydrocarbons. 

Graphite  is  unattacked  by  acids  under  all  circumstances. 

In  the  analysis  of  iron  and  steel  it  is  customary  to  determine 
directly  the  total  carbon  and  the  graphite,  in  which  case  the  differ- 
ence represents  the  carbide  carbon. 

Determination  of  Total  Carbon. 

Principle. — The  carbon  is  oxidized  to  carbon  dioxide,  and  the 
latter  is  either  absorbed  in  a  suitable  apparatus  or  its  volume 
is  measured. 

For  the  oxidation  of  the  carbon  a  great  many  methods  have 
been  proposed ;  they  can  be  classified  as  follows : 

(a)  Those  in  which  the  oxidation  is  effected  with  the  un- 
changed substance  itself. 

(£)  Those  in  which  the  greater  part  of  the  iron  is  removed, 
and  the  residue  subjected  to  combustion. 

The  Chromic-Sulphuric  Acid  Method. 

In  this  method  the  borings,  which  should  be  as  fine  as  possible 
and  free  from  grease,  are  treated  with  a  mixture  of  chromic  and 
sulphuric  acids  and  heated  to  boiling.  Thereby,  the  iron  goes 
into  solution  and  the  carbon  is  oxidized  to  carbon  dioxide.  In 
spite  of  a  large  excess  of  chromic  acid,  however,  a  considerable 
amount  of  the  carbon  is  likely  to  escape  in  the  form  of  hydro- 
carbons and  carbon  monoxide,  unless  precautions  are  taken. 
To  prevent  such  losses,  Sarnstrom  f  recommended  leading  the 

*  Stahl  und  Eisen,  1888,  p.  742. 

t  Sarnstrom,  Berg-  und  Hiittenm.  Ztg.,  1885,  52,  and  Corleis,  Stahl  u. 
Eisen,  1894,  581.  With  ferromanganese  the  loss  amounts  to  22.5  per  cent, 
of  the  total  carbon,  with  steel  9  per  cent.  With  ferromanganese  the  escaping 
gases  contain,  besides  carbon  dioxide  and  traces  of  heavy  hydrocarbons,  18 
per  cent,  methane,  76  per  cent,  hydrogen,  3  per  cent,  oxygen,  and  2  per 
cent,  carbon  monoxide. 


400       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

escaping  vapors  over  copper  oxide  in  a  combustion  tube,*  80  cm. 
long,  which  is  heated  in  a  combustion  furnace.  Many  experiments 
have  shown  that  the  method  of  Sarnstrom  gives  exact  results, 
although  objection  has  been  raised  to  the  long  combustion  tube 
that  is  required. 

Corleis  has  succeeded  in  simplifying  the  method  by  showing 
that  a  very  short  combustion  tube,  filled  with  copper  oxide,  heated 
by  a  single  Bunsen  flame,  suffices  if  the  sample  is  covered  with  a 
coating  of  copper  during  the  treatment  with  chromic  and  sulphuric 


FIG.  65. 

acids.  In  fact  the  use  of  the  combustion  tube  is  unnecessary  in  an 
ordinary  steel  anlaysis,  because  only  2  per  cent,  of  the  total  amount 
of  carbon  present  is  lost  in  this  case.  In  the  analysis  of  ferro- 
manganese,  and  similar  alloys,  however,  the  use  of  the  hot  tube  is 
to  be  recommended. 

Ledebur  f  even  found  that  the  results  obtained  with  irons  rich 
in  graphite  were  a  little  too  high  on  account  of  the  formation  of 
some  sulphur  dioxide,  but  this  error  can  be  overcome  by  passing 
the  gases  through  chromium  trioxide  just  before  they  enter  the 
combustion  tube. 

The  apparatus  required  is  shown  in  Fig.  65  and  consists  of  a 
Corleis  decomposition  flask  A  with  condenser. 


*  A  small  platinum  tube  heated  to  glowing  also  suffices, 
t  Leitfaden  fur  Eisenhiitten-Laborat. 


DETERMINATION  OF  CARBON.  4°i 

The  flask  is  connected,  as  shown  in  the  drawing,  on  one  side 
with  a  soda-lime  tower,  W,  at  the  bottom  of  which  is  placed  a 
little  concentrated  caustic  potash  solution,  and  on  the  other  side 
with  a  system  of  tubes.  The  tube  B  is  about  10  cm.  long  and 
contains  chromium  trioxide  between  two  asbestos  plugs.  The 
tube  C  is  15  cm.  long,  is  made  of  difficultly  fusible  glass,  and 
filled  with  granular  cupric  oxide.  It  is  placed  in  a  little  box  made 
of  asbestos  paper.  The  tubes  a,  b,  c,  d,  e,  and  /  are  filled  exactly 
as  described  on  p.  380.  Tubes  a,  6,  and  c  are  drying  tubes,  the 
first  containing  glass  beads  wet  with  concentrated  sulphuric  acid, 
the  other  two  containing  calcium  chloride;  d  and  e  are  glass- 
stoppered  soda  lime  tubes,  the  upper  third  of  the  right-hand  arm 
of  each  containing  calcium  chloride.  The  tube  /  is  a  safety  tube 
which  is  not  weighed,  but  is  used  to  avoid  any  chance  of  carbon 
dioxide  or  moisture  entering  the  weighed  tubes  from  the  air. 

Reagents. — 1.  A  saturated  solution  of  ordinary  chromic  acid 
containing  some  sulphate.  It  is  not  advisable  to  use  chemically- 
pure  chromic  acid  for  this  purpose,  for  the  latter  often  contains 
organic  substances. 

2.  A  solution  of  copper  sulphate  made  by  dissolving  200  gms. 
of  the  salt  in  1  liter  of  water. 

Procedure. — The  ground-glass  stopper  a  is  removed,  and 
through  the  opening  25  c.c.  of  chromic  acid  solution,  150  c.c.  of 
copper  sulphate  solution  and  200  c.c.  of  concentrated  sulphuric 
acid  are  poured  into  the  flask,  A,  and  mixed.  The  mixture  in  the 
flask  is  heated  to  boiling  and  kept  at  this  temperature  for  ten 
minutes.  The  flame  is  then  removed  and  a  current  of  air  free 
from  carbon  dioxide  is  passed  through  the  apparatus  for  ten 
minutes  at  the  rate  of  about  three  bubbles  per  second.  The  flask  is 
then  connected  with  the  tube  B,  the  red-hot  copper  oxide  tube, 
and  with  the  U  tubes,*  while  the  current  of  air  is  continued  for  five 
minutes  more.  The  soda-lime  tubes  d  and  e  are  removed,  closed, 
and  allowed  to  stand  ten  minutes  in  the  balance  room.  They  are 
opened  for  a  moment,  quickly  closed,  rubbed  off  with  a  piece  of 
chamois  skin  or  a  clean  linen  cloth,  allowed  to  stand  five  minutes 
in  the  balance-case,  and  then  weighed. 

*  Corleis  used  phosphorus  pentoxide  for  a  drying  agent,  but  calcium 
chloride  is  satisfactory. 


402       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

By  means  of  this  preliminary  boiling,  traces  of  organic  matter 
contained  in  the  apparatus  are  removed. 

After  weighing  the  soda-lime  tubes,  they  are  connected  with 
the  apparatus  again,  the  decomposition-flask  is  opened,  and  the 
weighed  substance  (from  0.5  to  5  gms.  according  to  the  amount  of 
carbon  present)*  is  introduced  quickly  from  a  glass-stoppered 
weighing  tube,  which  is  subsequently  weighed  again  to  determine 
the  amount  of  sample.  The  flask  is  immediately  closed  and  the 
copper  oxide  tube  heated  to  glowing,  after  which  the  contents  of 
the  flask  are  slowly  heated  so  that  after  from  15-20  minutes  the 
liquid  begins  to  boil.  The  solution  is  kept  boiling  for  one  or  two 
hours  while  a  slow  current  of  air  is  conducted  through  the  apparatus. 
The  flame  is  then  removed,  and  about  two  liters  more  of  air 
are  passed  through  the  apparatus,  when  the  soda-lime  tubes  are 
removed  and  subsequently  weighed  as  before. 

Since  the  use  of  the  copper  sulphate  solution  prevents  the  loss 
of  more  than  about  2  per  cent,  of  the  total  amount  of  carbon  pres- 
ent, it  is  evident  that  the  combustion-tube  can  be  dispensed  with 
for  technical  purposes. 

Combustion  of  Carbon  in  the  Wet  Way  and  Measuring  the 
Volume  of  the  Carbon  Dioxide. 

This  operation  is  best  carried  out  by  means  of  the  Lunge- 
Marchlewski  method. 

The  apparatus  necessary  is  shown  in  Fig.  61.  In  this  case, 
however,  the  decomposition-flask  is  larger  and  there  should  be 
a  ground-glass  connection  between  the  flask  and  a  condenser. 
Furthermore,  a  funnel-tube  is  fused  into  the  neck  of  the  flask, 
and  runs  along  the  side  of  the  flask  on  the  inside  ending  in  a 
quite  fine  point  near  its  bottom.  The  upper  end  of  the  condenser  is 
connected  with  the  measuring-tube  by  means  of  a  capillary  tube 
about  36  cm.  long,  ground  to  fit  the  condenser-tube. 

Reagents. — 1.  A  saturated,  neutral  solution  of  copper  sulphate. 

2.  A  chromic  acid  solution  (100  gms.  CrO3  in  100  c.c.  of  water). 

3.  Sulphuric   acid  of  specific  gravity  1.65  and  saturated  with 
chromic  acid. 

*  For  cast  iron  0.5  gms.  suffices  but  for  steel  from  1  to  2  gin.  and  for  wrought 
iron  5  gms.  should  be  used. 


COMBUSTION  OF  CARBON  IN   THE   WET   WAY. 


403 


4.  Sulphuric  acid  of  specific  gravity  1.71,  also  saturated  with 
chromic  acid. 

5.  Pure  sulphuric  acid  of  specific  gravity  1.10. 

6.  Commercial  hydrogen  peroxide  solution. 

Procedure. — The  amount  of  iron  or  steel  to  be  weighed  out 
and  the  necessary  quantities  of  the  reagents  are  shown  in  the 
following  table: 


Per  Cent.  C. 

Weigh 
Out. 
Grams. 

c.c. 
Copper 
Sulphate 
Solution. 

c.c. 
Chromic 
Acid 
Solution. 

c.c. 
Acid 
So.  Gr. 
1.65. 

c.c. 
Acid 
Sp.  Gr. 
1.71. 

c.c'. 
Acid 
Sp.  Gr. 
1.10. 

c  .c. 
H202. 

Over  1  5 

0.4-0.5 

5 

5 

135 

30 

1 

0  8-1  5 

1 

10 

10 

130 

25 

2 

0  5-0  8 

2 

20 

20 

130 

5 

2 

0  25-0  5 

3 

50 

45 

75 

5 

2 

Less  than  0  25 

5 

50 

50 

70 

5 

0 

The  substance  is  treated  with  the  copper  sulphate  solution  in 
the  decomposition-flask  at  the  ordinary  temperature.  Malleable 
iron  is  allowed  to  stand  for  at  least  one  hour,  while  pig  iron  re- 
quires at  least  six  hours.  The  flask  is  then  connected  with  the 
measuring-tube,  which  is  filled  with  mercury,  and  the  air  in  the 
flask  is  exhausted  as  was  described  on  p.  390.  After  this  is  ac- 
complished, the  levelling-tube  is  placed  in  a  low  position  and  the 
proper  amount  of  the  chromic  acid  solution  is  added  through  the 
funnel,  followed  first  by  the  proper  amount  of  the  stronger  acid  and 
then  by  that  of  the  weaker  acid,  after  which  the  stop-cock  in  the 
funnel  is  quickly  closed.  The  communication  between  the  meas- 
uring-tube and  the  flask  remains  open.  With  The  levelling-tube 
remaining  in  its  low  position,  the  contents  of  the  flask  are  heated 
to  gentle  boiling,  which  is  continued  for  one  hour,  and  the  flame 
is  then  removed.  Now,  in  order  to  remove  the  last  traces  of 
carbon  dioxide  from  the  solution,  the  prescribed  amount  of  hydro- 
gen peroxide  is  added  to  the  contents  of  the  flask  and  the  flask 
is  af  terwards  completely  filled  with  hot  water  until  all  of  the  gas  is 
driven  over  into  the  measuring-tube.  The  stop-cock  b  is  then 
closed,  the  gas  is  reduced  to  the  volume  corresponding  to  0°  C. 
and  760  mm.  pressure  as  described  on  p.  388  and  this  volume 
read.  It  is  then  driven  over  into  the  Orsat  tube  and  the  volume 
of  the  unabsorbed  gas  is  determined  as  before.  The  difference 
between  the  two  readings  represents  the  amount  of  carbon  dioxide 


404       GRAVIMETRIC  DETERMINATION   OF   THE  METALLOIDS, 


measured  under  the  standard  conditions  of  temperature  and 
pressure.  If  this  is  multiplied  by  the  factor  0.0005392  the  amount 
of  carbon  present  will  be  obtained. 

After  the  analysis  has  been  completed,  a  blank  determination 
must  be  made,  using  the  same  amounts  of  each  reagent,  in  order 
to  determine  small  amounts  of  organic  matter  which  are  invariably 
present  in  them.  The  amount  of  carbon  dioxide  found  under 
these  conditions  must  be  subtracted  from  that  obtained  in  the 
analysis  proper. 

Method  of  HempeL* 

Hempel  objects  to  the  above  procedure  on  the  ground  that  by 
dissolving  the  iron  in  the  mixture  of  chromic  and  sulphuric  acids 
some  hydrocarbon  is  likely  to  escape  oxidation.  He  found  that 
by  dissolving  iron  in  chromic-sulphuric  acid  under  diminished  pres- 
sure in  the  presence  of  mercury  all  of  the  carbon  would  be  readily 
oxidized  to  its  dioxide.  Fig.  66  represents  the  apparatus  used. 

Reagents  Required. 

1.  Chromic  acid  solution.     100  gms.  of  chromic  acid  are  dis- 
solved   in    300    c.c.    of    water    and 

30  gms.  of  sulphuric  acid,  sp.  gr. 
1.704,  are  added.  The  resulting 
solution  has  a  specific  gravity  of 
1.2. 

2.  Sulphuric  acid.     1000  c.c.  of 
concentrated     sulphuric     acid     are 
mixed  with  500  c.c.  of  water  and 
10  gms.  of  chromic  acid  and  heated 
for  an  hour  in  a  large  flask  upon 
a  sand-bath  in  order  to  completely 
destroy   any   dust,   etc.,   that   may 
be    present.     The    flame    is    then 
taken  away  and  a  current  of  air  is 
slowly  conducted  through  the  solu- 
tion in  order  to  remove  any  carbon 
dioxide  that  may  have  been  formed. 

After  cooling  the  solution  is  diluted  with  water  until  it  has  a 
specific  gravity  of  1.704. 

*  Verhandlg.  d.  Vereins  z.  Beford.  d.  Gewerbefleisses,  1893. 


FIG.  66. 


HEMP  EL'S  METHOD  FOR  THE  COMBUSTION  OF  C4RBON.    405 


Procedure. 

About  0.5  gm.  of  the  iron  or  steel  is  placed  in  the  decomposi- 
tion-flask B,  about  2.3  gms.  of  mercury  are  added  by  means  of 
a  small  pipette,  and  the  apparatus  is  connected  together  as  is 
shown  in  the  drawing. 

By  raising  the  levelling-bulb  N,  the  measuring- tube  M  is  entirely 
filled  with  mercury,  the  stop-cock  is  closed,  and  the  apparatus  is 
connected  at  p  with  a  suction-pump,  by  means  of  which  the  air  in  the 
flask  B  is  exhausted  as  completely  as  possible.  In  order  to  make 
sure  that  the  ground-glass  connection  between  the  flask  and  the 
condenser  is  perfectly  air-tight,  a  little  water  is  poured  into  the 
cup  there.  Into  the  funnel  C  are  placed  30  c.c.  of  chromic 
acid  solution,  the  stop-cock  at  p  is  closed,  and  by  carefully  lifting 
the  latter  a  little  the  chromic  acid  is  allowed  to  run  into  the  flask, 
which  is  immediately  heated  to  boiling  over  a  small  flame.  After 
boiling  for  half  an  hour,  120  c.c.  of  sulphuric  acid  are  added  through 
C,  the  stop-cock  at  M  is  now  opened  for  the  first  time  and 
the  contents  of  the  flask  boiled  for  half  an  hour  longer.  (At  the 
start  only  carbon  dioxide  is  generated,  in  proportion  to  the  tem- 
perature of  the  solution,  but  toward  the  end  of  the  operation  there 
is  a  fairly  lively  evolution  of  oxygen.)  The  flame  is  removed,  the 
gas  in  the  flask  is  carried  over  into  M  by  pouring  water  into  C  and 
lifting  the  tube  p  until  the  gas  in  the  flask  is  entirely  expelled. 
The  total  volume  of  the  gas  is  read,  after  which  the  carbon  dioxide 
is  absorbed  in  a  HempeFs  potash  pipette  and  the  volume  of  the 
unabsorbed  gas  is  determined.  The  difference  represents  the 
amount  of  carbon  dioxide  formed  by  the  oxidation.  From  this 
the  amount  of  carbon  present  can  be  computed. 

The  measuring  of  the  gas  in  this  apparatus  will  be  described 
more  in  detail  in  Part  III,  Gas  Analysis. 

Other  methods  for  the  determination  of  the  volume  of  the  car- 
bon dioxide  formed  from  the  carbon  in  iron  or  steel  are  those  of  J. 
Wiborgh,*  Otto  Petterssoii  and  August  Smitt.f 

*  Zeit.  f.  anal.  Chem.,  XXIX  (1890),  p.  198. 
t  Ibid..  XXXII  (1893),  p.  385. 


4°6        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

The  methods  already  described  are  suitable  for  the 
determination  of  carbon  in  wrought  iron,  cast  iron,  steel, 
etc.,  but  not  in  alloys  such  as  ferro-silicon,  ferro-chrome  or 
tungsten  steel.  For  such  materials,  the  following  method  is 
used. 


Wohler's  Chlorine  Process.* 

Principle. — The  sample  of  iron  or  steel  is  heated  in  a  stream 
of  pure  chlorine  gas  whereby  iron,  silicon,  phosphorus,  and  sulphur 
are  volatilized  while  the  carbon  remains  behind  in  the  presence  of 
small  amounts  of  non-volatile  chlorides.  The  silicon  present 
as  silica,  due  to  inclosed  slag,  is  not  affected  by  the  treatment. 
The  residue  is  filtered  through  asbestos,  the  chlorides  washed  out 
by  water,  and  the  carbon  burned  to  carbon  dioxide  either  in  the 
wet  or  in  the  dry  way. 

The  principal  requisite  for  the  success  of  the  process  is  pure 
chlorine.  This  must  not  contain  oxygen,  water,  nor  carbon 
dioxide,  because  these  substances  all  tend  to  convert  a  part  of 
the  carbon  into  carbon  monoxide,  whereby  low  results  are  ob- 
tained in  the  carbon  determination. 

Procedure. — The  specimen  is  subjected  to  the  action  of  chlorine 
in  an  apparatus  like  that  shown  in  Fig.  67. 

B  is  the  liter  flask  in  which  the  chlorine  is  generated;  it 
contains  about  200  gms.  of  pyrolusite  and  500  c.c.  of  concentrated 
hydrochloric  acid.  The  contents  of  the  flask  are  heated  over  a 
very  low  flame  and  in  this  way  a  continuous  stream  of  chlorine 
is  evolved.  When  the  current  begins  to  slacken,  more  hydro- 
chloric acid  is  needed  which  is  allowed  to  flow  into  the  flask  through 
a  Bulk's  f  dropping  funnel. {  To  regulate  the  current  of  gas,  the 


*  Z.  anal.  Chem.,  8,  401  (1869),  cf.  A.  Ledebur,  Leitfaden  fiir  Eisenhiitten 
Laboratorien . 

f  Z.  anal.  Chem.,  16,  467  (1892). 

J  The  flow  of  the  acid  is  regulated  by  raising  the  tube  S.  Instead  of  S 
a  glass  rod  covered  with  rubber  tubing  may  be  used. 


WOHLERS   CHLORINE  PROCESS. 


407 


flask  is  connected  with  the  right-angled  tube,  h,  which  is  provided 
with  a  stop-cock  and  leads  to  a  cylinder,  A,  containing  caustic 
soda  solution.  If  the  stream  of  chlorine  becomes  too  strong,  the 
stop-cock  is  opened  a  little  so  that  the  excess  of  chlorine  is  absorbed 
by  the  sodium  hydroxide.  The  chlorine  is  purified  by  means  of 
the  tubes  a,  b,  c,  C,  and  d;  a  contains  water,  b  concentrated 
sulphuric  acid,  c  glass  beads,  or  pumice,  moistened  with  sulphuric 
acid.  C  is  a  tube  40  cm.  long  and  1  cm.  wide,  made  of  difficultly- 


FIG.  67. 

fusible  glass.  It  contains  a  layer,  15  cm.  long,  of  coarse  charcoal 
which  has  previously  been  well  ignited  and  cooled  in  a  desiccator. 
The  charcoal  is  placed  in  the  tube  between  two  loose  plugs  of 
ignited  asbestos.  The  tube  is  heated  to  dark  redness  in  a  small 
combustion  furnace.  If  the  chlorine  gas  contains  small  amounts 
of  oxygen  (air)  or  carbon  dioxide,  these  are  changed,  on  coming 
in  contact  with  the  hot  charcoal,  to  carbon  monoxide,  which  is 
unaffected  by  the  carbon  in  the  iron  or  steel.  The  last  traces  of 
moisture  are  removed  by  passing  the  gas  through  the  tube  d 
containing  glass  beads  moistened  with  concentrated  sulphuric 
acid. 

The  chlorine  gas  is  next  passed  into  the  combustion  tube  D. 


4o8      G  Ml/I  METRIC  DETERMINATION  OF  THE  METALLOIDS. 

This  is  about  40  cm.  long  by  1.5  cm.  wide,  is  bent  into  a  right 
angle  and  leads  into  concentrated  sulphuric  acid  in  the  flask  e. 
The  sulphuric  acid  serves  as  a  seal  and  prevents  air  from  getting 
into  the  tube. 

The  substance,  which  should  be  as  fine  as  possible,  is  sprinkled 
as  a  thin  layer  *  upon  a  previously  ignited  porcelain  boat.  Of 
ferro-chrome  about  0.5  gm.  should  be  taken,  and  of  ferro- 
silicon  about  1  gm.  The  boat  is  shoved  into  the  combustion  tube 
and  the  evolution  of  chlorine  is  started  as  described  above.  The 
tube  is  not  heated  at  all  until  after  about  twenty  minutes,  when 
the  air  will  have  all  been  expelled ;  then  the  heating  is  begun  very 
gradually,  lighting  the  burners  one  at  a  time  from  right  to  left. 
The  formation  and  volatilization  of  the  ferric  chloride  takes  place 
at  a  relatively  low  temperature. 

As  soon  as  no  more  brown  vapors  escape  from  the  tube,  the 
heat  is  gradually  raised  until  the  tube  begins  to  get  red  and  then 
the  residue  in  the  tube  is  allowed  to  cool  in  the 
stream  of  chlorine. 

The  boat  is  removed  from  the  combustion 
tube,  and,  in  the  case  of  ferro-silicon,  the  con- 
tents are  rinsed  with  cold  water  into  a  beaker. 
From  the  beaker  the  insoluble  residue  is  washed 
into  an  asbestos  filter  prepared  as  follows:  In 
the  funnel  R,  Fig.  68,  which  is  about  1  cm.  wide 
and  5  cm.  long,  is  placed  a  little  glass  wool, 
and  upon  this  a  suspension  of  ignited  asbestos 
fibers  in  water  is  poured  until,  with  the  aid  of  YIG.  68. 

light  suction,  the  filtrate  comes  through  perfectly 
free  from  asbestos  fibers.     The  residue  is  washed,  on  such  a  filter, 
with   cold  water  until   no    chloride    can    be    detected    in    the 
filtrate. 

The   carbonaceous   residue   can  be  oxidized  in  the  apparatus 


*  This  is  especially  important  in  the  case  of  ferro-chrome,  because 
otherwise  the  metal  will  become  covered  with  a  coating  of  non-volatile 
chromic  chloride  which  prevents  it  from  being  acted  upon  by  the 
chlorine. 


COMBUSTION  OF  CARBON  IN  THE  DRY  WAY.  4°9 

shown  in  Fig.  65,  p.  400  but  in  this  case  the  flask  A.  should  contain 
5  c.c.  of  a  saturated,  aqueous  solution  of  chromic  acid,  and  60  c.c. 
of  sulphuric  acid,  sp.  gr.  1.71  which  is  likewise  saturated  with 
chromic  acid. 

In  the  analysis  of  ferro-chrome,  there  is  always  some  insoluble 
chromic  chloride  in  the  boat  which  cannot  be  removed  by  washing. 
In  this  case,  therefore,  the  substance  after  being  heated  in  a  stream 
of  chlorine,  is  heated  in  an  atmosphere  of  hydrogen,  whereby  the 
insoluble  chromic  chloride  is  converted  into  soluble  chromous 
chloride.  The  contents  of  the  boat  are  then  treated  exactly  as 
described  above. 


Combustion   of   Carbon   in   the   Dry    Way.* 

(a)   Solution  of  the  Iron. 

A  number  of  methods  have  been  proposed  for  dissolving 
away  the  iron  and  leaving  the  carbon  behind  in  the  form  of 
an  insoluble  residue.  For  this  purpose  a  solution  of  potas- 
sium-cupric  chloride  containing  300  gms.  of  the  double  salt 
(2KCl.CuCl2.2H20)  and  75  c.c.  of  concentrated  hydrochloric 
acid  to  the  liter  has  proved  most  satisfactory.  Before  using, 
the  solution  is  filtered  through  ignited  asbestos  and  preserved 
in  a  glass-stoppered  bottle.  The  solution  of  the  borings  takes 
place  very  slowly  unless  the  solution  is  stirred,  which  is  best 
accomplished  by  means  of  a  mechanical  stirrer.  Warming 


*  The  dry  combustion  methods  are  much  used  in  the  steel  laboratories  of 
the  United  States  and  by  the  Bureau  of  Standards  at  Washington,  D.  C.,  for 
analyzing  special  samples  of  iron  and  steel  which  are  available  for  distribution 
and  serve  as  standard  samples  of  known  chemical  composition.  Further- 
more, the  Committee  on  Standard  Methods  of  the  American  Foundrymen's 
Association  (Chemical  Engineer,  5,  416  (1097)  )  have  recommended  the  use 
of  a  dry  combustion  method  for  settling  all  cases  of  dispute. 


410       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

the  solution  also  helps,  but  it   should  never  be  heated  above 
60°  to  70°.     The   following  reactions  take  place: 


Fe+CuCl2==FeCl2 
2Cu  +  2CuCl2  =  2Cu2Cl2. 

The  presence  of  potassium  chloride  aids  the  solution  of 
the  copper,  probably  on  account  of  the  formation  of  a  double 
salt. 

The  residue  is  filtered,  dried  and  usually  burned  in  a  current 
of  oxygen,  the  carbon  dioxide  being  absorbed  in  a  weighed  bulb 
containing  potassium  hydroxide  solution.  To  make  sure  that 
the  oxygen  used  contains  no  carbon  in  any  form,  it  is  advisable  to 
make  use  of  a  preheating  tube,  such  as  for  example  a  short  porce- 
lain tube  filled  with  copper  oxide;  this  serves  to  convert  any 
carbon  to  carbon  dioxide  which  is  then  absorbed  in  potassium 
hydroxide  solution  before  coming  in  contact  with  any  of  the 
carbon  from  the  sample. 

The  combustion  may  take  place  in  a  porcelain  or  platinum 
tube,  or  in  a  special  form  of  crucible,  such  as  the  jacketed  crucible 
devised  by  Shimer,  or  the  tubulated  one  of  Gooch.  These,  how- 
ever, are  made  of  platinum  and  are  expensive  but  Ruppel  *  has 
shown  that  one  of  nickel  answers  the  purpose  equally  well. 

The  following  directions  are  taken  from  the  Report  of  the 
Committee  on  Standard  Methods  of  the  American  Foundrymen's 
Association  and  corresponds  closely  to  the  method  used  by  the 
Bureau  of  Standards  at  Washington,  D.  C.,  in  preparing  standard 
samples  for  distribution.  They  apply  equally  well  for  the  deter- 
mination of  the  total  carbon  in  steel  except  that  in  the  latter 
case  usually  3  gms.  of  the  sample  and  200  c.c.  of  the  potassium- 
cupric  chloride  solution  are  used. 

*  J.  Ind.  Eng.  Chem.  1,  415  (1909). 


DETERMINATION  OF  TOTAL   CARBON  IN   CAST  IRON.       41  * 

Determination  of  Total  Carbon  in  Cast  Iron. 

The  train  used  consists  of  a  pre-heating  furnace  containing 
copper  oxide,  followed  by  a  bulb  containing  potassium  hydroxide 
solution  (sp.  gr.  1.27),  then  one  containing  granular  calcium 
chloride;  the  latter  is  connected  with  a  ten-burner  combustion 
furnace  in  which  either  a  porcelain  or  platinum  tube  is  placed. 
This  combustion  tube  contains  four  or  five  inches  of  copper  oxide 
between  plugs  of  platinum  gauze.  The  plug  at  the  front  end  * 
of  the  furnace  should  be  at  about  the  point  where  the  tube  leaves 
the  furnace.  A  roll  of  silver  foil,f  about  two  inches  long,  is  placed 
in  front  of  this  plug.  The  combustion  tube  is  connected  at  this 
end  with  a  tube  connecting  anhydrous  cupric  sulphate,  one  of 
anhydrous  cuprous  chloride,  and  one  of  granular  calcium  chloride. 
A  Geissler  or  Liebig  bulb  is  connected  with  this  last  tube  and 
contains  potassium  hydroxide  solution  (sp.  gr.  1.27)  with  a  pro- 
long of  calcium  chloride.  A  calcium  chloride  tube  is  used  between 
this  last  tube  and  the  aspirator  bottle  to  prevent  any  moisture 
working  backward. 

In  filling  the  calcium  chloride  tubes,  especially  the  prolong  of 
the  absorption  bulb,  care  must  be  taken  to  press  down  the  calcium 
chloride  lumps  well  against  one  another,  as  when  the  tube  is 
loosely  filled,  some  moisture  is  likely  to  get  by.  A  negative 
blank  is  often  obtained  by  beginners  for  this  reason. 

Before  using  the  apparatus,  the  contents  of  the  tube  should 
be  heated  for  half  an  hour  in  a  stream  of  oxygen,  then  the  weighed 
absorption  bulb  should  be  connected  with  the  train  and  a  blank 
determination  made.  The  bulb  should  not  gain  in  weight  more 
than  0.5  mgm. 

One  gram  of  the  well-mixed  drillings  are  meanwhile  dissolved 
in  100  c.c.  of  the  double  chloride  solution,  and  the  solution  event- 
ually filtered  on  ignited  asbesots.  The  beaker  in  which  the 

*  In  describing  a  combustion  train  it  is  customary  to  follow  the  direction 
of  the  flow  of  gas.  The  back  or  rear  end  is  considered  the  end  toward  the  gas 
reservoir,  and  the  front  or  forward  end  is  that  nearest  to  the  weighed  potash 
bulb. 

t  The  silver  foil  is  unnecessary  if  the  carbonaceous  residue  is  washed 
free  from  hydrochloric  acid. 


4*3       GRAVIMETRIC   DETERMINATION  OF  THE.  METALLOIDS. 

solution  took  place  is  washed  thoroughly  with  20  c.c.  of  dilute 
hydrochloric  acid  (1:1),  and  transferred  to  the  filter  by  means  of 
more  of  the  same  acid  from  a  wash-bottle.  Finally  the  residue  is 
washed  on  the  filter  with  hot  water  until  free  from  chlorides. 
The  filter  is  then  dried  at  a  temperature  between  95°  and  100°. 

Blair  recommends  the  use  of  a  perforated  platinum  boat  for 
the  filtering.  This  unquestionably  works  well,  but  where  many 
determinations  are  being  made  it  involves  considerable  expense. 
An  excellent  substitute  can  be  prepared  from  a  funnel  such  as 
was  described  for  use  with  a  Gooch  crucible,  although  it  is  well 
to  shorten  the  sides  somewhat.  A  tight  coil  of  copper  wire  is 
placed  in  the  bottom  of  the  funnel  with  a  long  free  end  of  wire 
reaching  down  below  the  bottom  of  the  stem.  Loose  ignited 
asbestos  is  placed  upon  the  coil  of  wire,  followed  by  a  suspension  of 
the  same  asbestos  in  water.  After  applying  suction,  the  asbestos 
is  gently  tamped  down  with  the  flattened  end  of  a  stirring  rod. 
The  finished  pad  is  about  0.75  of  an  inch  in  diameter  and  0.25 
of  an  inch  thick. 

The  dried  asbestos,  with  the  carbon  upon  it,  is  pushed  into 
back  end  of  the  furnace  and  the  funnel  wiped  out  with  dry,  ignited 
asbestos.  Care  should  be  taken  that  the  carbon  on  the  asbestos 
reaches  far  enough  into  the  tube  to  get  the  full  heat  from  the 
furnace.  The  burners  under  the  pre-heating  furnace  are  now 
lighted,  the  oxygen  turned  on  and  allowed  to  pass  through  the 
Absorption  bulb  at  the  rate  of  three  bubbles  per  second,  but  no 
faster.  The  two  forward  burners  under  the  combustion  tube  are 
lighted,  at  first  turning  them  low  and  gradually  raising  the  heat 
until  the  tube  is  hot.  As  soon  as  this  end  of  the  tube  is  hot,  the 
third  burner  from  the  forward  end  is  lighted  and  a  few  minutes 
later  the  fourth  burner,  which  should  be  near  the  forward  end 
of  the  carbon  residue.  Light  each  burner  in  succession  until 
finally  all  are  lighted  and  turned  high  enough  to  heat  the  tube  red 
hot.  After  twenty  minutes  have  elapsed  from  the  time  the  last 
burner  is  turned  on  full,  the  oxygen  is  stopped  and  a  current  of 
air  swept  through  the  tube  at  the  same  rate  for  twenty  minutes 
longer,  gradually  turning  down  the  burners  under  the  com- 
bustion tube.  The  potassium  hydroxide  bulb  at  the  front  of 
the  train  is  then  detached  and  weighed  with  the  usual  precautions. 


DETERMINATION  OF   TOTAL   CARBON  IN  CAST  IRON.       4*3 

When  the  Shinier  or  similar  crucible  is  used  for  the  combustion, 
it  should  be  followed  by  a  copper  tube  ^  of  an  inch  inside  diameter 
and  10  inches  long  with  its  ends  cooled  by  water  jackets.  In  the 
center  of  this  tube  is  placed  a  disc  of  platinum  gauze  and  for  three 
or  four  inches  in  the  side  toward  the  crucible  a  roll  of  silver 
foil,  and  for  the  same  distance  on  the  other  side  some  copper  oxide. 
The  ends  of  this  tube  are  plugged  with  glass  wool  and  the  tube 
heated  with  a  fish-tail  burner  before  the  aspiration  of  air  is 
started. 

b.  Direct  Combustion  of  the  Sample. 

In  most  cases  it  is  possible  to  effect  the  combustion  by  heating 
the  finely  divided  substance  itself  in  a  current  of  oxygen.  In 
fact,  according  to  Blair,*  this  is  true  not  only  of  ordinary  steels 
and  pig  iron,  but  experiments  have  shown  that  with  chrome- 
tungsten  steels  the  direct  method  is  capable  of  giving  exact 
results,  whereas  those  obtained  by  dry  combustion  after  solution 
of  the  iron  in  potassium-cupric  chloride  are  from  10  to  40  per 
cent,  too  low. 

It  has  been  claimed,  however,  that  there  is  difficulty  in  burn- 
ing all  of  the  carbon  on  account  of  the  sample  becoming  coated 
superficially  with  oxide,  but  according  to  Schneider  f  this  may 
be  overcome  by  mixing  the  finely  divided  sample  with  three  parts 
of  lead  and  one  of  copper. 

The  combustion  may  be  carried  out  in  a  platinum  tube,  in 
one  of  the  special  forms  of  crucible,  {  in  a  porcelain  tube,  or  in 
one  of  fused  quartz.  When  platinum  is  used  it  is  advisable  to 
place  the  drillings  in  a  little  depression  of  sand,  the  layer  of  which 
being  not  less  than  0.25  in.  deep. 

According  to  Campbell  and  Gott,§  if  a  combustion  boat  con- 
taining the  borings  of  sample  is  placed  in  a  cold,  platinum-lined 

*  A.  A.  Blair,  The  Chemical  Analysis  of  Iron,  7th  ed.  (1908). 

t  Oesterr.  Zeitschr.,  1894,  No.  21. 

J  P.  W.  Shimer,  J.  Ind.  Eng.  Chem.,  1,  738. 

§  J.  Ind.  Eng.  Chem.,  1,  739. 


4H      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

porcelain  tube  and  then  heated  at  a  temperature  of  about  900° , 
complete  combustion  will  take  place  without  endangering  the 
platinum  by  any  spattering  of  the  oxides  formed. 

It  is  very  convenient  to  heat  the  tube  by  means  of  an  electric 
furnace.*  Such  a  furnace  can  be  constructed  at  a  cost  not 
exceeding  $30. 

Determination  of  Graphite. 

One  gram  of  pig  iron  is  treated  with  35  c.c.  of  nitric  acid 
(sp.  gr.  1.13)  in  a  300-c.c.  beaker  and  heated  gently  until  there 
is  no  further  evolution  of  gas.  By  this  means  the  carbide  carbon 
is  dissolved  while  the  graphite  is  not  attacked.  The  solution  is 
filtered  through  an  ignited  asbestos  filter  and  the  residue  washed 
with  hot  water,  then  with  a  hot  solution  of  potassium  hydroxide 
(sp.  gr.  1.1),  followed  by  hot  water,  dilute  hydrocohloric  acid, 
and  finally  with  hot  water  again  until  free  from  chloride.  After 
drying  at  110°,  the  asbestos  and  graphite  are  transferred  to  a 
combustion-tube  and  the  carbon  burned  in  a  current  of  pure 
oxygen  as  described  above.  From  the  weight  of  carbon  dioxide 
absorbed,  the  amount  of  graphite  is  calculated. 


Determination  of  Carbon  and  Hydrogen  in  Organic  Substances, 
according  to  Liebig. 

(Elementary  Analysis.) 

Principle. — The  organic  substance  is  burned  in  air  or  in  oxygen 
and  the  products  of  the  combustion  are  passed  over  glowing  cop- 
per oxide,  which  oxidizes  all  of  the  carbon  to  carbon  dioxide 
and  the  hydrogen  to  water.  The  latter  is  collected  in  a  weighed 
calcium  chloride  tube,  the  former  in  a  weighed  vessel  which  con- 
tains either  caustic  potash  solution  or  dry  soda-lime. 

The  combustion  is  performed. 

(a)  In  an  open  tube] 

(b)  In  a  closed  tube. 

*  J.  Ind.  Eng.  Chem.,  1,  741.  Campbell  and  Gott,  loc.  cit.  W.  H.  Keen, 
J.  Ind.  Eng.  Chem.,  1,  741. 


DETERMINATION  OF    GRAPHITE,  ETC.  415 

(a)  Combustion  in  an  Open  Tube. 

By  far  the  greater  majority  of  all  combustions  are  carried  out 
in  this  way. 

Requirements. — 1.  An  open  tube  made  of  difficultly-fusible  glass 
which  is  from  12-15  mm.  wide.  The  length  of  the  tube  depends 
upon  that  of  the  combustion-furnace;  it  must  be  10  cm.  longer 
than  the  furnace. 

2.  350  gms.  of  coarse  and  50  gms.  of  fine  copper  oxide. 

3.  A  drying  apparatus  (Fig.  69,  on  the  left). 

4.  A  calcium  chloride  tube  (Fig.  70). 

5.  A  Geissler  potash  bulb   (Fig.  71)  or  two  soda-lime  tubes 
(see  p.  381,  d  and  e). 

6.  A  screw-cock. 

7.  Dry  rubber  tubing. 

8.  Two  plates  of  asbestos  board  to  protect  the  rubber  stoppers 
in  the  two  ends  of  the  tube  from  the  heat  of  the  furnace. 


FIG.  60. 

Procedure  for  the  Combustion  of  Organic  Substances  Free  from 
Nitrogen,  Halogen,  Sulphur,  and  Metals. 

Preparation  and  Combustion. 

1.  The  calcium  chloride  tube  (Fig.  70)  is  filled  from  the  left 
side  as  was  described  on  p.  377,  closed  with  a  plug  of  glass-wool 
and  the  end  of  the  tube  fused  together,  as  shown  in  the  figure.* 
It  is  more  practical  to  use  a  calcium  chloride  tube  fitted  with 
ground-glass  stoppers.  After  filling  the  tube,  its  contents  are  satu- 
rated with  carbon  dioxide,  as  described  on  p.  380,  in  the  foot-note. 

The  outside  of  the  tube  is  rubbed  with  a  piece  of  chamois  skin 
or  old  linen,  and  the  two  ends  are  stoppered  with  short  pieces 
of  rubber  tubing  each  containing  a  piece  of  stirring-rod.  It  is 

*  Or  the  tube  is  stoppered  and  an  air-tight  seal  made  by  covering  it 
neatly  with  sealing-wax. 


4i 6        GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

allowed  to  stand  in  the  balance-case  for  fifteen   minutes   and  is 
then  weighed  without  the  stoppers. 

2.  The  Geissler  bulb  (Fig.  71)  is  filled  with  caustic  potash  solu- 
tion (two  parts  solid  KOH  in  three  parts  of  water)  as  follows: 
The  small  soda-lime  tube  d  is  replaced  by  a  piece  of  rubber  tubing, 
c  is  dipped  into  the  solution  of  caustic  potash,  and  the  bulbs  are 
filled  two-thirds  full  by  sucking  through  the  rubber  tubing.  Tha 


FIG.  70.  FIG.  71. 

end  of  the  tube  c  is  then  cleaned  by  means  of  a  piece  of  filter- 
paper,  the  soda-lime  tube  d  is  again  connected  (its  right  half  is 
filled  with  soda-lime  and  the  outer  half  with  calcium  chloride), 
and  the  two  ends  are  closed  with  pieces  of  rubber  tubing  each 
containing  a  piece  of  stirring-rod  with  rounded  ends.  The  apparatus 
is  wiped  with  wash-leather  and  weighed  without  the  stoppers, 
after  taking  the  same  precautions  as  with  the  weighing  of  the 
large  calcium  chloride  tube. 

3.  The  drying  apparatus  (Fig.  69,  on  the  left),  which  serves  to 
free  the  air  and  oxygen  used  for  the  combustion  from  carbon  diox- 
ide and  water  vapor,  consists  of  a  wash-bottle,  D,  containing  con- 
centrated caustic  potash  solution,  the  soda-lime  tube  a,  and  the 
two  calcium  chloride  tubes  6  and  c. 


ad  k'  K  k 

FIG.  72. 

4.  The  Combustion-tube  (Fig.  72),  both  ends  of  which  are 
rounded  by  heating  in  the  blast-lamp;  after  cooling,  the  tube  is 
washed,  dried,  and  filled  as  follows:  First  a  short  roll,  k,  of  copper 
gauze  is  introduced  into  the  right-hand  end  of  the  tube  so  that 


COMBUSTION  OF  ORGANIC  SUBSTANCES.  417 

5-6  cm.  of  the  latter  are  left  empty.  This  roll  serves  as  a  plug  and 
must,  therefore,  fit  tightly  in  the  tube.  A  layer  of  coarse  copper 
oxide,  K,  about  45  cm.  long,  is  next  added,  and  after  this  is  placed 
another  plug  of  copper  gauze,  k' '.  Finally  another  roll  of  copper 
gauze,  d,  about  10  cm.  long  and  large  enough  to  fill  the  tube  loosely 
is  placed  so  that  a  space  of  about  10  cm.  is  left  on  the  right  and 
about  5  cm.  on  the  left.  The  tube  is  now  placed  in  a  combus- 
tion-furnace, so  that  about  5  cm.  extend  beyond  the  furnace 
at  each  end,  as  shown  in  Fig.  69.  The  left  end  of  the  tube  is  closed 
with  a  tightly  fitting  rubber  stopper  through  which  a  glass  tube 
passes,  and  is  connected  with  the  drying  apparatus  by  means  of 
a  short  piece  of  rubber  tubing.  (The  tube  should  be  provided 
with  a  glass  stop-cock,  as  is  shown  in  Fig.  72,  a,  but  which  is 
lacking  in  Fig.  69.)  The  right  end  of  the  tube  is  left  open  for  the 
time  being. 

A  slow  current  of  oxygen  *  is  passed  through  the  apparatus 
and  the  furnace  is  lighted.  At  first  the  flame  is  turned  low  and 
the  whole  tube  is  heated  equally.  Gradually  the.  temperature 
is  raised,  until,  with  the  tiles  covering  the  tube,  the  copper  oxide 
is  at  a  dull-red  heat. 

Usually  some  water  condenses  in  the  right-hand  end  of  the 
tube;  this  is  expelled  by  carefully  holding  a  hot  tile  under  the  tube. 
When  all  the  water  is  removed,  and  when  the  presence  of  oxygen 
can  be  detected  at  the  right  end  of  the  tube  (by  its  igniting  a 
glowing  splinter),  this  end  of  the  tube  is  closed  with  a  rubber 
stopper  through  which  an  open  calcium  chloride  tube  is  placed. 
The  burners  are  now  turned  down  and  the  oxygen  current  dis- 
continued. All  of  the  flames  are  extinguished  after  some  time 
except  those  under  the  right  half  of  the  tube. 

While  the  tube  is  cooling,  the  calcium  chloride  tube  and  the 
potash  bulb  (or  soda-lime  tubes)  are  weighed  (the  stoppers  being 
replaced  immediately  after  the  weighing)  and  from  0.15-0.2  gm.  of 
the  substance  is  weighed  into  a  porcelain  or  platinum  boat. 

If  the  substance  is  a  difficultly-volatile  oil  it  is  weighed  into  a 

*  The  oxygen  must  be  free  from  hydrogen.  Commercial  oxygen  often 
contains  the  latter,  in  which  case  it  is  necessary  to  pass  the  gas  through  a 
"preheating"  furnace  before  using  it.  The  gas  should  come  from  a  gas- 
ometer, never  from  the  bomb  itself. 


41 8         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS.  ( 

small  glass  tube  open  at  one  end.  If  it  is  readily  volatile,  a  bulb 
is  blown  into  a  small  capillary  tube;  this  is  weighed,  the  bulb  is 
warmed,  and  the  capillary  is  introduced  into  the  liquid  to  be  ana- 
lyzed, so  that  the  liquid  rises  in  the  bulb  as  it  cools.  The  bulb  is 
then  turned  so  that  the  capillary  lies  in  a  horizontal  position,  the 
latter  is  warmed  slightly  to  expel  a  little  liquid  that  adheres  to  the 
sides  of  the  tube,  the  end  is  melted  together,  and  the  tube  is  again 
weighed.  Care  must  be  taken  that  there  is  no  liquid  in  the  capil- 
lary. Everything  is  now  ready  for  the  combustion.  The  stopper 
is  removed  from  the  left  (and  now  cold)  end  of  the  combustion- 
tube,  the  long  copper  roll  is  removed  by  means  of  a  piece  of  wire 
with  a  hook  in  the  end  of  it,  the  boat  with  the  substance  in  it  is 
placed  in  the  tube  and  the  copper  roll  right  after  it.  Connection 
is  made  with  the  drying  apparatus  on  the  left  and  with  the  absorp- 
tion-tubes on  the  right,  as  is  shown  in  Fig.  69.  In  case  the  sub- 
stance is  a  liquid,  the  tube  containing  it  is  placed  so  that  its  capillary 
is  pointed  towards  the  left,  and  in  the  case  of  a  volatile  liquid  the 
end  of  the  capillary  is  broken  off  with  a  file  just  before  introducing 
it  into  the  combustion-tube.  The  stop-cock  between  the  tube  and 
the  drying  apparatus  is  closed,  the  latter  is  connected  with  an  air- 
gasometer,  and  the  stop-cock  in  the  drying  apparatus  is  wholly 
opened,  while  that  between  the  drying  apparatus  and  the  com- 
bustion-tube is  opened  just  enough  to  permit  two,  or  at  the  most 
three,  bubbles  of  gas  per  second  to  pass  through  the  apparatus. 
The  two  outer  burners  on  the  left  are  now  lighted  and  the  copper 
oxide  spiral  d  (the  copper  was  changed  to  the  oxide  by  the  ignition  in 
oxygen)  is  slowly  heated  just  to  redness.  The  tube  is  now  gradu- 
ally heated  from  right  to  left,  taking  care  that  the  gas  evolution  is 
never  greater  than  four  bubbles  per  second  through  the  potash 
bulb ;  this  can  be  easily  regulated  by  means  of  the  stop-cock  or  by 
turning  the  gas-burners.  When  the  contents  of  the  entire  tube 
have  been  brought  to  redness,  with  the  tiles  in  place,  and  the  boat 
is  empty,  the  combustion  is  usually  complete.  It  is  well,  however, 
to  pass  oxygen  through  the  hot  tube  until  the  gas  can  be  detected 
at  the  right-hand  end  of  the  combustion  train  (a  glowing  splinter 
ignites  at  n).*  The  flames  are  then  turned  down  and  a  current  of 

*  To  prevent  moisture  from  getting  into  this  tube  from  the  air,  it  is  well  to 
connect  it  with  an  unweighed  calcium  chloride  tube. 


COMBUSTION   OF  ORGANIC  SUBSTANCES.  419 

air  passed  through  the  apparatus  until  the  oxygen  is  completely 
expelled.  A  little  water  always  collects  in  the  front  (right)  end 
of  the  tube,  and  this  must  be  driven  over  into  the  calcium  chloride 
tube  by  holding  a  hot  tile  under  it.  The  calcium  chloride  tube 
and  the  potash  bulbs  are  now  removed,  wiped  off  with  a  piece  of 
chamois  skin  or  a  clean  linen  cloth,  allowed  to  stand  in  the  balance- 
room  for  twenty  minutes,  after  which  time  they  are  weighed 
without  the  stoppers.  The  gain  in  weight  represents  the  amount 
of  water  and  carbon  dioxide  respectively,  and  from  this  the  amount 
of  hydrogen  and  carbon  can  be  calculated  as  follows : 

If  a  represents  the  weight  of  the  substance,  p  that  of  the  water, 
and  p'  that  of  the  carbon  dioxide,  then 

H2O:H2    =p:x  and      CO2:C  =p':z/ 

18.02: 2.02= p:x  44:12=p/:x/ 

2.02  ,     12    ,     3     , 

X==WVP  X=44P=liP 

and  in  per  cent. 

101    p  300   p' 

—  ---per  cent.H  -^--p*  Cent'C 

Determination  of  Carbon  and  Hydrogen  in  Nitrogenous  Organic 

Substances. 

By  the  combustion  of  many  organic  substances  containing  nitro- 
gen, especially  nitroso-  and  nitro-compounds,  oxides  of  nitrogen  are 
formed  which  are  partly  absorbed  in  the  calcium  chloride  tube  and 
partly  in  the  potash  bulb,  so  that  if  such  substances  were  analyzed 
according  to  the  previous  process,  both  the  carbon  and  hydrogen 
results  will  be  too  high.  If,  however,  an  unreduced  copper  spiral 
is  introduced  in  the  front  (right)  end  of  the  combustion-tube, 
this  serves  to  reduce  the  oxides  of  nitrogen  to  nitrogen  itself,  and, 
as  the  latter  is  not  absorbed,  correct  results  will  be  obtained. 

The  copper  spiral  is  prepared  by  rolling  together  a  piece  of 
copper  gauze  about  10  cm.  wide,  making  it  as  large  as  will  con- 
veniently pass  into  the  combustion-tube.  The  spiral  is  heated 
till  it  glows  by  holding  it  in  a  large  gas  flame,  and  while  still  hot 
it  is  thrown  into  a  test-tube  containing  one  or  two  cubic  centi- 
meters of  methyl  alcohol.  The  alcohol  quickly  boils  away,  but 


420       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

some  of  it  is  oxidized  to  aldehyde  by  the  hot  copper  oxide,  and 
the  latter  is  reduced  .  completely  to  bright  metallic  copper.  The 
spiral  is  taken  out  with  a  pair  of  crucible  tongs  and  dried  by 
quickly  passing  it  through  a  flame  a  few  times,  and  while  still 
warm  it  is  introduced  into  the  front  end  of  the  combustion-tube, 
which  has  been  previously  burned  out  as  described  in  the  pre- 
vious analysis. 

To  carry  out  the  combustion,  the  stop-cock  between  the  com- 
bustion-tube and  the  drying  apparatus  (Fig.  69)  is  closed,  the 
substance  introduced  into  the  tube,  and  the  copper  oxide  spiral  at 


FIG.  73. 


FIG.  74. 


FIG.  75. 


d  is  first  heated  and  then  the  reduced  spiral  at  the  other  end  of  the 
tube.     Then  beginning  at  k  (Fig.  72),  one  burner  is  lighted  after 


COMBUSTION  OF  ORGANIC  SUBSTANCES.  421 

another,  until  finally  the  entire  contents  of  the  tube  are  heated  to 
dull  redness  and  no  more  bubbles  escape  through  the  potash 
bulb.  Xow  for  the  first  time  the  stop-cock  is  opened  somewhat 
and  oxygen  is  passed  through  the  tube  until  it  can  be  detected  at 
n;  by  a  test  with  a  glowing  splinter.  The  flames  are  then  gradually 
turned  down,  the  oxygen  replaced  by  air,  and  the  analysis  com- 
pleted as  in  the  previous  case. 

Substances  hard  to  burn  are  treated  somewhat  differently. 
First  of  all  the  left  end  of  the  combustion  tube  (Fig.  69)  is  filled 
with  the  aid  of  the  funnel  T  (Fig.  73) ,  with  finely  granular,  but 
not  pulverized,  copper  oxide,  and  this  is  ignited  in  a  stream  of 
oxygen.  The  oxygen  is  then  replaced  by  air  and  the  tube  allowed 
to  cool  until  it  can  be  held  in  the  hand.  The  finely  granular 
copper  oxide  is  next  transferred  to  the  small  flask  K,  Fig.  74,  and 
the  flask  closed  by  inserting  a  tin-foil  covered  cork  which  is  fitted 
with  a  calcium  chloride  tube.  While  the  copper  oxide  in  the 
flask  is  becoming  perfectly  cold,  the  substance  to  be  analyzed  is 
weighed  into  the  glass-stoppered  mixing  tube  M,  Fig.  75.  From 
one-sixth  to  one-fifth  of  the  copper  oxide  in  the  flask  is  transferred 
to  the  mixing  tube,  which  is  then  stoppered  and  its  contents  well 
ghaken,  whereby  the  substance  becomes  intimately  mixed  with 
the  copper  oxide  to  which  it  adheres.  The  mixture  is  then 
transferred  to  the  combustion  tube,  and  the  mixing  tube  is 
shaken  repeatedly  with  small  portions  of  the  remaining  copper 
oxide  in  the  flask  until  finally  it  can  be  assumed  that  all  of  the 
substance  has  been  transferred  to  the  combustion-tube.  The 
combustion  is  then  carried  out  in  the  usual  manner.* 

Combustion  of  Organic  Substances  Containing  Halogens. 

The  analysis  is  conducted  exactly  the  same  as  in  the  case  of 
nitrogenous  substances,  except  instead  of  a  reduced  copper  spiral 
one  of  silver  is  used  to  keep  back  any  halogen  set  free.  The 
silver  spiral  should  not  be  heated  to  redness,  but  only  to  about 
180-200°  C.  In  case  a  silver  spiral  is  not  at  hand,  a  long  copper 
spiral  is  used,  its  end  reaching  outside  the  furnace. 

*  For  another  method  of  conducting  a  combustion  in  an  open  tube,  consult 
M.  Dennstedt,  Z.  anal.  Chem.  40,  611  (1903). 


422      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 


Combustion  of  Organic  Substances  Containing  Sulphur. 

Sulphur  compounds  cannot  be  burned  in  a  tube  containing 
copper  oxide,  for  the  sulphur  dioxide  escapes  and  is  partly  ab- 
sorbed by  the  water  in  the  calcium  chloride  tube  and  partly  in 
the  potash  bulb,  so  that  absolutely  worthless  results  are  obtained. 
Instead  of  the  long  layer  of  copper  oxide,  one  of  ignited  lead  chro- 
mate  is  used.  The  latter  oxidizes  the  sulphur  dioxide  to  sulphur 
trioxide,  forming  difficultly-volatile  lead  sulphate  which  remains 
in  the  tube.  When  lead  chromate  is  used,  the  combustion  must 
take  place  at  a  lower  temperature  than  with  copper  oxide,  for 
the  former  melts  easily,  and  by  adhering  to  the  glass  is  likely  to 
cause  the  tube  to  break. 

Combustion  of  Organic  Substances  Containing  Metals. 

•If  the  substance  contains  alkalies,  alkaline  earths,  or  cadmium, 
a  part  of  the  carbon  will  remain  in  the  tube  as  carbonate.  In 
this  case  the  substance  is  mixed  in  the  boat  with  a  mixture  of  ten 
parts  of  powdered  lead  chromate  and  one  part  of  potassium  chro- 
mate, and  the  combustion  is  conducted  as  is  the  case  when  sul- 
phur is  present. 

Dumas'  Method  for  Determining  Nitrogen  in  Organic 
Substances. 

This  determination  should  really  be  discussed  under  Part  III, 
but  it  will  be  described  here  on  account  of  its  being  an  analysis 
by  combustion. 

Principle. — The  substance  is  burned  in  a  combustion-tube, 
free  from  air,  which  contains  copper  oxide  and  copper  spirals 
exactly  as  in  the  determination  of  the  hydrogen  and  carbon  in 
substances  containing  nitrogen,  but  in  this  case  the  nitrogen 
evolved  is  measured. 

Procedure. — This  determination  may  be  carried  out  in  either 
a  closed  or  open  tube. 


DUMPS'  METHOD  FOR  DETERMINING   NITROGEN.  4*3 


(a)  Determination  in  a  Closed  Tube. 

The  necessary  apparatus  is  shown  in  Fig.  76.  The  combus- 
tion-tube is  closed  at  one  end  and  is  about  75  cm.  long.  It 
contains  at  M  a  layer  of  magnesite  15  cm.  long,  in  pieces  about  the 
size  of  a  pea,  followed  by  a  loose  plug  of  ignited  asbestos  and 
then  a  10-cm.  layer  of  coarse  copper  oxide,  S.  The  substance 
is  added  at  a  in  a  boat  and  mixed  with  powdered  copper  oxide 
by  means  of  a  spiral  wire  (cf.  p.  421),  after  which  a  layer  of 
coarse  copper  oxide  *  about  40  cm.  long  is  added,  and  finally 
the  reduced  copper  spiral  (prepared  as  described  on  p.  419).  The 
tube  is  then  placed  in  a  combustion-furnace  and  connected  with 
an  azotometer,  f  as  shown  in  the  figure,  which  is  filled  with  mer- 
cury to  a  little  above  the  lower  end  of  r,  and  upon  this  rests  a 
liberal  amount  of  caustic  potash  solution  (300  gms.  KOH  dis- 
solved in  a  liter  of  water). 

The  experiment  is  begun  (with  the  le veiling-bulb  low  and  the 
stop-cock  of  the  azotometer  open)  by  heating  the  left  half  of  the 
magnesite  layer,  whereby  the  air  in  the  tube  is  expelled  by  the 
carbon  dioxide  and  passes  through  the  azotometer.  From  time 
to  time  a  test  is  made  to  see  whether  the  air  has  all  been  expelled. 
The  levelling-bulb  is  raised,  and  the  stop-cock  closed  with  the 
azotometer  tube  completely  filled.  If  all  the  air  has  been  replaced 
by  carbon  dioxide  gas,  the  bubbles  of  gas  will  all  be  absorbed  by  the 
caustic  alkali.  When  this  is  the  case  the  flame  is  put  out  under  M. 
The  tube  is  heated  first  at  R  and  the  burners  are  lighted  one  after 
another  toward  the  left  until  about  three-quarters  of  the  layer  of 
coarse  copper  oxide  is  heated  to  a  dull  redness.  The  tube  is  then 
heated  at  S  and  the  process  is  continued  as  in  an  ordinary  com- 
bustion until  the  whole  tube  (with  the  exception  of  the  part  where 
the  magnesite  is  found)  is  heated  to  a  uniform  temperature  and 
finally  no  more  nitrogen  is  evolved. 

The  heating  must  be  accomplished  so  that  there  will  be  a  slow 
but  steady  evolution  of  nitrogen.  When  the  combustion  is  com- 
plete, the  magnesite  layer  is  once  more  heated  and  the  nitrogen 

*  The  copper  oxide  must  be  previously  ignited,  as  described  on  p.  417. 
t  H.  Schiff,  Berichte,  XIII,  p.  885. 


•424        GRA  VIME  TRIC  DE  TERM  IN  A  TION  OF  THE  ME  TALLOIDS. 

remaining  in  the  tube  is  completly  driven  over  into  the  azotom- 
eter  by  the  carbonic  acid  set  free.  As  soon  as  the  volume  of 
the  gas  in  the  azotometer  remains  constant,  the  experiment  is 
ended  and  it  remains  only  to  measure  the  nitrogen. 

For  this  purpose  the  azotometer  together  with  the  connecting 
piece  of  rubber  tubing  is  removed  from  the  combustion-tube  and 


FIG.  76. 


the  tubing  closed  by  means  of  a  pinch-cock.  The  apparatus  is  then 
set  aside  for  at  least  thirty  minutes  at  a  place  where  a  uniform 
temperature  prevails,  after  which  the  gas  levelling-tube  is  raised 
until  the  solution  in  it  stands  at  exactly  the  same  height  as  that 
in  the  tube,  when  the  volume  is  read.  At  the  same  time  the 
barometer  and  thermometer  *  readings  are  noted. 

The  weight  of  the  nitrogen  present  is  computed  as  follows: 
Let  us  assume  that  a  gms.  of  the  substance  were  used  for  the 

*  An  accurate  thermometer  should  hang  at  the  side  of  the  azotometer. 


DUMAS'  METHOD  FOR    DETERMINING  NITROGEN.          425 

analysis  and  V  c.c.  of  nitrogen  were  obtained  at  t°  C.  and  B  mm. 
barometric  pressure.  In  order  to  obtain  the  weight  of  the  nitrogen, 
its  volume  must  be  first  reduced  to  0°  C.  and  760  mm.  pressure. 
If  the  gas  had  been  measured  over  pure  water  the  formula 


_  V(B0  -10).  273 
0      760(273+0 


would  hold  in  which  B0  represents  the  observed  barometer 
reading  reduced  to  a  temperature  of  0°  and  w  is  the  tension  of  water 
vapor  measured  in  millimeters  of  mercury.  The  nitrogen,  how- 
ever, was  not  measured  over  pure  water  but  over  a  solution  of 
potassium  hydroxide,  and  the  vapor  tension  of  this  solution  is 
less  than  that  of  pure  water.  In  fact,  with  potassium  hydroxide 
of  the  concentration  used,  the  diminution  of  the  vapor  tension  as 
compared  with  pure  water  almost  exactly  compensates  the 
correction  which  would  be  applied  in  reducing  the  barometer 
reading  to  0°.  Consequently  the  following  formula  holds  with 
sufficient  accuracy: 

=  V(B-w)-273 
0      760(273+0* 

As  1  c.c.  of  nitrogen  at  0°  and  760  mm.  has  been  found  to  weigh 
0.0012506  gm.,*  then  VQ  c.c.  of  nitrogen  will  weigh 

0.0012506  XF0  gms. 

and  in  per  cent., 

a  =  0.0012506  -F0  =  100  :x 

_  0.1 2506  -V0 

X  —  . 

a 


*  Cf.  Nitrogen  under  Gas  Analysis. 


426     GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

If  the  value  for  VQ  is  inserted  in  this  last  equation,  and  the 
constant  values  are  united,  it  becomes 


x  =  0.04493          .  .       =per  cent.  N. 


(b)    Determination  of  Nitrogen  in  an  Open  Tube. 

The  determination  is  carried  out  in  practically  the  same  way 
as  before,  except  that  in  this  case  the  carbon  dioxide  is  generated 
outside  of  the  tube.  If  the  combustion-tube  of  Fig.  76  is  imag- 
ined to  be  cut  off  at  M  and  connected  by  means  of  the  two-bulbed 
tube  with  a  long  test-tube,  as  shown  in  the  upper  part  of  the  fig- 
ure, the  apparatus  necessary  for  this  determination  will  be  seen. 

The  long  test-tube  contains  sodium  bicarbonate,  and  it  is  cov- 
ered with  a  piece  of  copper  gauze  in  order  that  it  may  be  heated 
more  uniformly. 

At  S  is  a  long  copper  oxide  spiral,  this  is  followed  by  a  copper 
boat  containing  the  substance  mixed  with  powdered  copper  oxide, 
then  the  long  layer  of  coarse  copper  oxide,  and  finally  the  reduced 
copper  spiral.  After  the  connection  with  the  azotometer  has  been 
made,  the  tube  containing  the  sodium  bicarbonate  is  heated  and 
the  air  removed  from  the  combustion-tube  by  means  of  the  carbon 
dioxide  evolved.  The  greater  part  of  the  water  that  is  simul- 
taneously set  free  collects  in  the  tw^o-bulbed  tube.  Otherwise 
the  procedure  is  exactly  the  same  as  in  the  former  case. 

Remark. — The  advantage  of  this  method  over  the  former  lies 
in  the  fact  that  the  combustion-tube  can  be  used  for  a  large  num- 
ber of  nitrogen  determinations  without  refilling  it  each  time. 

With  difficultly-combustible  substances  the  author  prefers  to 
work  with  the  closed  tube,  for  in  this  way  it  is  possible  to  get 
a  very  intimate  mixture  of  the  substance  with  the  powdered 
copper  oxide. 


OXALIC  ACID.  427 

OXALIC  ACID,  H2C2O4.    Mol.  Wt.  90.02. 

Forms:   Calcium  Oxide,  CaO,  and  Carbon  Dioxide, 
C02. 

Determination  as  Calcium  Oxide. 

The  neutral  solution  of  an  alkali  oxalate  is  treated  with  a  few 
drops  of  acetic  acid,  heated  to  boiling,  and  precipitated  with  boil- 
ing calcium  chloride  solution.  After  standing  twelve  hours  the 
precipitate  is  filtered  off,  washed  with  hot  water,  ignited  wet  in 
a  platinum  crucible,  and  from  the  weight  of  the  calcium  oxide  the 
amount  of  oxalic  acid  is  calculated  as  follows  : 


_HAO 


Determination  as  Carbon  Dioxide. 

Principle.  —  The  method  is  based  upon  the  fact  that  oxalic  acid 
on  being  heated  with  manganese  dioxide  and  dilute  sulphuric 
acid  is  quantitatively  oxidized  to  carbon  dioxide: 

H2C2O4+  MnO2-f  H2SO4  -  MnSO4+  2H2O+  2CO2. 

Procedure.  —  A  weighed  amount  of  the  oxalate  is  treated  with 
one  and  a  half  times  as  much  manganese  dioxide  (free  from  car- 
bonate) either  in  the  apparatus  shown  on  page  376  (Fig.  58),  or 
in  that  of  Fresenius-Classen  (Fig.  59,  p.  381).  The  procedure  is 
exactly  the  same  as  was  described  for  the  determination  of  carbon 
dioxide.  If  p  gm.  of  carbon  dioxide  were  found,  this  corresponds  to 

p-  1.023  gm.  =  Oxalic  Acid,  H2C204. 

Remark.  —  Both  methods  give  good  results,  but  oxalic  acid 
can  be  much  more  conveniently  determined  by  a  volumetric 
process  (see  Part  II,  Volumetric  Analysis). 

The  free  acid  crystallizes  with  two  molecules  of  water  and  its 
molecular  weight  is  then  126.05.  This  should  be  borne  in  mind  in 
determining  the  purity  of  a  commercial  sample. 


428       GRAVIMETRIC    DETERMINATION  OF  THE  METALLOIDS. 

BORIC  ACID,  HBO2.*    Mol.  Wt.  44.01. 

Determination  as  Boron  Trioxide,  B203,  by  the  Method  of 
Rosenbladt-Gooch.f 

Principle. — Alkali  and  alkaline-earth  borates,  on  being  dis- 
tilled with  absolute  methyl  alcohol  (free  from  acetone)  and  acetic 
acid,  give  up  all  their  boron  in  the  form  of  methyl  borate,  a  liquid 
which  boils  at  65°  C.  If  the  methyl  borate  is  passed  over  a 
weighed  amount  of  lime  in  the  presence  of  water,  it  is  completely 
saponified : 

B(OCH3)3+  3H20  =  3CH3OH+  B(OH)3. 

The  boric  acid  set  free  combines  with  the  lime  to  form  calcium 
borate.  If  the  paste  of  water  and  lime  is  evaporated  to  dryness, 
the  gain  in  weight,  therefore,  represents  the  amount  of  B2O3. 

Procedure. — About  1  gm.  of  the  purest  lime  J  obtainable  is  ig- 
nited to  a  constant  .weight  over  the  blast-lamp,  and  as  much  of  it  as 
possible  is  transferred  to  the  dry  Erlenmeyer  flask  (Fig.  77)  which 
serves  as  a  receiver.  The  crucible,  with  some  of  the  lime  adhering 
to  it,  is  placed  in  a  desiccator  and  set  aside  for  the  present. 

The  lime  in  the  flask  is  slaked  by  the  careful  addition  of  about 
10  c.c.  of  water,  and  the  flask  is  connected  with  the  distillation- 
flask  as  shown  in  the  figure. § 

The  aqueous  solution  of  the  alkali  borate  (containing  not  more 
than  0.2  gm.  B2O3)  is  treated  with  a  few  drops  of  either  litmus  or 
lakmoid  solution,  and  hydrochloric  acid  is  added  drop  by  drop 
until  the  solution  turns  red.  A  drop  of  dilute  sodium  hydroxide 
is  added  and  then  a  few  drops  of  acetic  acid. II  The  slightly  acid 

*  This  is  the  formula  of  meta-boric  acid. 

f  Zeit.  f.  anal.  Chem.,  27  (1887,  pp.  18,  364). 

%  Instead  of  lime,  Gooch  and  Tones  use  4  to  7  gms.  of  sodium  tungstate 
which  is  fused  with  about  0.5  gm.  WO3  in  a  platinum  crucible  to  expel  any 
carbonic  acid.  The  fused  mass  is  cooled  and  weighed. 

§  To  permit  the  escape  of  air  from  the  flask,  a  cut  is  made  in  the  side  of  the 
cork  stopper,  at  s. 

||  It  is  absolutely  necessary  to  neutralize  the  greater  part  of  the  alkali 
with  hydrochloric  acid  and  then  the  last  of  it  with  acetic  acid.  If  the  alkali 
were  all  neutralized  with  acetic  acid,  little  or  none  of  the  boric  acid  would 
pass  over  into  the  receiver  during  the  subsequent  distillation  with  alcohol. 


DETERMINATION   OF  BORIC  ACID. 


429 


solution,  prepared  in  this  way,  is  added  by  means  of  the  funnel  T  to 
the  pipette-shaped  retort,  R,  of  about  200  c.c.  capacity.  The  funnel 
is  washed  three  times  by  the  addition  of  2  or  3  cubic  centimeters 
of  water  and  the  stop-cock  is  closed.  The  liquid  is  distilled 
by  placing  R  in  a  paraffin  bath  at  not  over  140°  C.,  and  the 
distillate  collected  in  the  Erlenmeyer  flask  containing  the  lime 
When  all  of  the  liquid  has  distilled  over,  the  paraffin  bath  is  lowered, 
and  after  R  has  cooled  somewhat,  10  c.c.  of  methyl  alcohol  (free 
from  acetone)  are  added  through  the  funnel  and  the  contents 
of  R  are  again  distilled  off.  This  process  is  repeated  three  times. 


FIG.  77. 

Then  2-3  c.c.  of  water  are  added  to  the  retort,  also  a  few  drops 
of  acetic  acid  until  the  liquid  becomes  distinctly  red  again,* 
and  the  distillation  with  10  c.c.  of  methyl  alcohol  is  repeated 

*  By  the  repeated  distillation,  the  contents  of  the  retort  become  alkaline,, 
as  shown  by  the  blue  color  of  the  solution. 


43°        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

three  times  more.  At  the  end  of  this  time  all  of  the  boric  acid 
will  be  found  in  the  receiver.*  The  stoppered  flask  is  thoroughly 
shaken  and  allowed  to  stand  for  an  hour  or  two  in  order  to  make 
sure  that  all  of  the  methyl  borate  is  saponified.  The  contents 
of  the  receiver  are  then  poured  into  a  platinum  dish  of  about 
200  c.c.  capacity  and  evaporated  on  the  water-bath  to  dry  ness  at 
as  low  a  temperature  as  possible.  During  this  process  the  alcohol 
must  not  be  allowed  to  boil  under  any  circumstances.  Then, 
in  order  to  remove  the  small  amount  of  lime  that  remained  adher- 
ing to  the  sides  of  the  flask,  a  few  drops  of  dilute  nitric  acid  are 
added  to  the  receiver,  and,  by  carefully  inclining  the  flask,  its  entire 
inner  surface  is  wet  by  the  acid,  after  which  the  contents  are  washed 
into  the  platinum  dish  and  evaporated  to  dry  ness  again.  This 
time  the  water  in  the  bath  may  boil,  as  there  is  now  no  danger  of 
losing  the  boric  acid,  the  alcohol  being  all  removed  by  the  first 
evaporation.  The  residue  in  the  dish  is  then  gently  ignited  over 
a  small  flame  in  order  to  destroy  the  calcium  acetate  f  present ; 
it  is  allowed  to  cool  and  is  transferred  by  means  of  a  little  water 
to  the  crucible  in  which  it  was  originally  weighed.  The  dark-colored 
lime  and  carbon  remaining  on  the  sides  of  the  dish  are  dissolved 
in  a  little  nitric  or  acetic  acid  and  washed  into  the  crucible. 
The  contents  of  the  latter  are  evaporated  to  dryness  on  the  water- 
bath,  and,  with  the  cover  upon  it,  the  crucible  is  ignited  at  first 
gently  and  finally  more  strongly  until  a  constant  weight  is  obtained. 
The  increase  in  weight  represents  the  amount  of  B2O3. 

Remark. — This  method  affords  faultless  results,  even  in  the 
presence  of  considerable  amounts  of  other  salts.  Free  halogen 
hydride  or  sulphuric  acid  must  not  be  present,  for  these  acids 
form  compound  ethers  with  the  methyl  alcohol  and  distil  over 
with  the  boric  acid,  with  which  they  would  be  weighed.  Instead 
of  using  lime  in  the  receiver,  the  methyl  borate  can  be  distilled 
into  a  dilute  solution  of  ammonium  carbonate,  and  the  latter 
evaporated  with  slaked  lime  in  a  platinum  dish  immediately  after 
the  distillation.  The  author,  however,  prefers  the  above  method. 

*  When  the  distillation  is  over,  the  retort  should  be  removed  from  the 
paraffin  bath,  by  lowering  the  latter.  If  this  is  not  done,  the  retort  is  likely 
to  break  when  the  paraffin  solidifies. 

f  Due  to  the  excess  of  the  acetic  acid  added. 


DETERMINATION  OF  BORIC  ACID    IN  SILICATES,  ETC.       431 

If  one  possesses  a  large  platinum  crucible  (with  a  capacity  of 
from  80  to  100  c.c.),  the  first  evaporation  can  take  place  in  this 
and  it  is  then  advisable  to  place  the  crucible  within  a  ring-shaped 
copper  or  tin  tube  through  which  steam  passes  (Fig.  17,  page  32). 
In  this  way  the  calcium  acetate  does  not  creep  up  over  the  sides 
of  the  dish,  and  there  is  no  danger  of  any  bumping. 

Determination  of  Boric  Acid  in  Silicates,  Enamel,  etc. 

The  finely-powdered  substance  is  fused  with  four  times  as 
much  sodium  carbonate,  the  melt  is  extracted  with  water,  and 
the  aqueous  solution  containing  the  boric  acid*  is  evaporated 
to  a  small  volume,  acidified  with  acetic  acid,  and,  without  regard 
to  any  separation  of  silica,  the  solution  is  transferred  to  the  Gooch 
retort  and  analyzed  as  above  directed. 

Remark. — This  determination  can  be  performed  in  the  presence 
of  fluorine  provided  acetic  and  not  nitric  acid  is  used  to  set  free 
the  boric  acid;  but,  for  that  matter,  it  is  in  no  case  advisable 
to  use  nitric  acid  and  it  is  not  permissible  when  chlorides  are 
present. 

Determination  of  Boric  Acid  in  Mineral  Waters. 

If  the  water  contains  considerable  boric  acid  (0.1  gm.  or  more 
of  B2O3  in  a  liter),  a  weighed  amount  (from  200  to  300  c.c.)  is  evap- 
orated to  a  small  volume,  f  the  precipitated  calcium  and  magnesium 
carbonates  are  filtered  off,  the  filtrate  concentrated,  slightly  acidi- 
fied with  acetic  acid,  and  analyzed  as  described  on  page  428. 

If  the  water  contains  only  a  little  boric  acid,  as  is  true  in  the 
great  majority  of  cases,  a  large  amount  must  be  taken  for  the 
determination.  From  10  to  15  liters  are  evaporated  in  a  large 
porcelain  dish  to  about  1  liter,*  the  deposited  salts  are  filtered 
off  (these  never  contain  any  borate),  washed  thoroughly  with 
hot  water,  and  the  filtrate  and  washings  are  evaporated  on  the 
water-bath  until  a  moist  residue  is  obtained.  If  this  residue  does 

*  Sometimes  the  insoluble  residue  contains  appreciable  amounts  of  borie 
acid.  In  the  method  given  under  Volumetric  Analysis,  this  fact  will  be 
taken  into  consideration. 

f  If  the  water  reacts  alkaline,  it  is  at  once  evaporated ;  otherwise  enough 
sodium  carbonate  solution  is  added  to  make  it  so. 


432       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

not  amount  to  more  than  5  or  6  gms.  it  is  redissolved,  acidified 
with  acetic  acid,  transferred  to  the  Gooch  retort,  and  distilled  as 
described  on  page  428.  Usually  a  larger  residue  is  obtained,  such 
that  it  cannot  be  conveniently  analyzed  directly,  in  which  case  the 
boric  acid  is  extracted  from  it.  For  this  purpose  the  residue  is 
acidified  with  a  little  hydrochloric  acid,  thoroughly  stirred  with 
absolute  alcohol,  and  by  means  of  more  of  the  latter  it  is  trans- 
ferred to  a  flask,  corked  up,  and  allowed  to  stand  twelve  hours  with 
frequent  shaking.  The  boric  acid  will  then  be  found  in  the  alco- 
holic solution.  The  residue  is  filtered  off,  washed  with  96  per  cent, 
alcohol,  diluted  largely  with  water,  1  gm.  of  sodium  hydroxide  is 
added,  the  alcohol  distilled  off  (see  Remark),  and  the  liquid  evap- 
orated until  a  moist  residue  is  obtained.  This  is  again  acidified 
with  hydrochloric  acid  and  the  above  extraction  with  alcohol,  and 
subsequent  distillation  of  the  alcohol,  after  the  addition  of  water 
and  1  gm.  of  sodium  hydroxide,  is  repeated.  If  the  residue  now  ob- 
tained is  not  too  large,  it  is  gently  ignited  in  order  to  destroy  the  or- 
ganic matter;  after  extracting  with  water,  the  carbonaceous  residue 
filtered  off,  and  the  filtrate  is  acidified  with  hydrochloric  acid.  It  is- 
then  made  slightly  alkaline  with  sodium  hydroxide,  after  which  just 
enough  acetic  acid  is  added  to  make  the  solution  react  acid  again. 
The  solution  thus  prepared  is  analyzed  as  described  on  page  42X. 

Remark. — Unless  a  large  amount  of  water  and  the  sodium 
hydroxide  are  added,  some  of  the  boric  acid  will  be  volatilized 
with  the  alcohol.  It  is  always  best  to  test  the  alcoholic  distillate 
for  boric  acid  as  follows :  A  few  pieces  of  turmeric  root  are  extracted 
with  alcohol,  2-3  drops  of  the  yellow  solution  are  placed  in  a  porce- 
lain dish,  the  alcoholic  solution  to  be  tested  for  boric  acid  and  a  few 
drops  of  acetic  acid  are  added,  after  which  the  solution  is  diluted 
with  water  and  evaporated  to  dryness  on  the  water-bath.  Accord- 
ing to  F.  Henz,  if  as  much  as  ToVo-  rngm.  of  boric  acid  is  present, 
a  faint  but  distinct  coloration  will  be  evident,  while  the  presence 
of  T-§  Q-  mgm.  will  cause  a  strong  reddish-brown  coloration,  which 
en  being  treated  with  sodium  hydroxide  is  turned  to  the  charac- 
teristic blue-black  color. 

If  boric  acid  is  found  in  the  alcoholic  distillate,  it  must  be 
again  treated  with  water  and  sodium  hydroxide,  and  the  alcohol 
once  more  distilled  ofT. 


MOLYBDIC  ACID,  TARTARIC  ACID,  IODIC  ACID,  ETC.        433 

MOLYBDIC  ACID,  H2MoO4.     Mol.  Wt.  162.02. 

The  determination  of  molybdic  acid  has  already  been  con- 
sidered on  page  284. 

TARTARIC  ACID,  H2C4H4O6.    Mol.  Wt.  150.05. 

The  composition  of  free  tartaric  acid  as  well  as  that  of  the  tar- 
trates  is  determined  by  an  elementary  analysis,  see  page  4 14  etseq. 

META-  AND  PYRO PHOSPHORIC  ACIDS. 

These  acids  are  changed  to  phosphoric  acid  and  determined 
as  described  on  page  434. 

IODIC  ACID,  HIO3.    Mol.  Wt.  175.93. 
Form :  Silver  Iodide,  Agl. 

For  the  determination  of  iodic  acid  as  silver  iodide,  the  solu- 
tion of  the  alkali  iodate  is  acidified  with  sulphuric  acid,  and  sul- 
phurous acid  is  added  until  the  solution,  which  at  first  becomes 
yellow  on  account  of  the  separation  of  iodine,  is  again  colorless. 
After  this  an  excess  of  silver  nitrate  and  a  considerable  amount  of 
nitric  acid  are  added.  The  solution  is  heated  to  boiling  and  the 
precipitated  silver  iodide  determined  as  described  on  page  330. 

It  is  not  permissible  to  change  the  iodate  to  iodide  by  ignition, 
for  the  decomposition  takes  place  at  a  temperature  above  that 
at  which  the  iodide  itself  begins  to  volatilize.  The  transforma- 
tion is  therefore  not  quantitative.  This  is  especially  true  of 
sodium  iodate,  which  is  only  changed  to  iodide  upon  heating  to  a 
white  heat.  Potassium  and  silver  iodates  are  much  more  readily 
decomposed,  but  even  then  some  iodide  is  lost.  Both  iodic  and 
periodic  acids  may  be  more  accurately  determined  by  a  volumetric 
process  (see  Part  II,  lodimetry). 

For  the  determination  of  the  metal  present  in  an  iodate  it  is 
first  changed  to  the  chloride  by  repeated  evaporation  with  con- 
centrated hydrochloric  acid: 

KI03+  6HC1  =  KC1+  3H2O+ 203+ IC1. 


434         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 


GROUP  IV. 

PHOSPHORIC,  ARSENIC,  ARSENIOUS,  THIOSULPHURIC, 
CHROMIC,  VANADIC,  AND  PERIODIC  ACIDS. 

PHOSPHORIC  ACID,  H3PO4.    Mol.  Wt.  98.02. 

Forms  :  Magnesium  Pyrophosphate,  Mg2P207 ;  Ammonium 
Phosphomolybdate,  (NH4)3P04- i2Mo03;  Phosphomolybdic 
Anhydride,  P205  •  24Mo03. 

i.  Determination   as   Magnesium   Pyrophosphate,   according   to 

B.  Schmitz. 

Until  recently,  it  was  the  usual  practice  to  precipitate  phos- 
phoric acid  in  the  cold  with  "  magnesia  mixture  "  and  ammonia, 
but  according  to  the  experiments  of  Neubauer  *  and  of  Gooch  f 
it  is  evident  that  it  is  very  difficult  to  obtain  a  pure  precipitate  of 
magnesium  ammonium  phosphate  in  this  way ;  sometimes  it  is  con- 
taminated with  Mg3(PO4)2  and  sometimes  with  Mg(NH4)4(PO4)2. 
If,  however,  the  precipitation  takes  place  in  a  hot  solution,  as 
recommended  by  Schmitz, J  Jarvinen,§  and  Jorgensen,||  a  very 
pure,  coarsely  crystalline  precipitate  of  Mg(NH4)PO4-6H2O  is 
obtained. 

Procedure. — The  solution  of  alkali  phosphate  is  treated  with 
a  little  hydrochloric  acid,  a  considerable  excess  of  "  magnesia 
mixture,"  1  and  10-20  c.c.  of  a  saturated  solution  of  ammonium 
chloride.  After  heating  the  mixture  to  boiling,  some  2.5  per  cent, 
ammonia  is  added  very  slowly,  while  constantly  stirring,  until  a 
precipitate  begins  to  form,  and  then  the  addition  of  the  ammonia 
is  regulated  so  that  about  four  drops  are  added  in  a  minute.  If 
a  milky  turbidity  appears,  it  must  be  redissolved  in  hydrochloric 

*  H.  Neubauer,  Z.  angew.  Chem.,  1896,  439. 

f  F.  A.  Gooch,  Z.  anorg.  Chem.,  20,  135. 

j  B.  Schmitz,  Z.  anal.  Chem.,  45,  512  (1906). 

§  K.  K.  Jarvinen,  Z.  anal.  Chem.,  43,  279  (1904),  44,  333  (1905). 

||  G.  Jorgensen,  Z.  anal.  Chem.  45,  278  (1906). 

1  The  "magnesia- mixture"  is  prepared,  according  to  Schmitz,  by  dissolv- 
ing 55  gms.  of  crystallized  magnesium  chloride  and  105  gms.  of  ammonium 
chloride  in  water  adding  a  little  hydrochloric  acid  and  diluting  to  a  volume 
of  one  liter. 


PHOSPHORIC  ACID.  435 

acid.  It  is  important  that  the  precipitate  which  first  forms 
shall  be  crystalline.  As  the  precipitate  increases  in  amount,  the 
addition  of  the  ammonia  may  be  quickened,  until  finally  the 
liquid  smells  of  ammonia,  after  blowing  away  the  vapors  on  top 
of  the  liquid.  The  solution  is  then  allowed  to  cool,  one-fifth  of 
its  volume  of  concentrated  ammonia  is  added,  and  at  the  end  of 
ten  minutes  more  it  is  ready  to  filter.  The  precipitate  is  washed 
three  times  by  decantation  with  2.5  per  cent,  ammonia,  then 
transferred  to  a  filter  and  washed  free  from  chloride.  It  is  dried, 
ignited  and  weighed  as  described  on  p.  67-8.  It  is  best  to  use  a 
Munroe  crucible  and  an  electric  oven. 

If  the  weight  of  the  precipitate  is  p  gms.,  then  the  amount  of 
PC>4,  5,  can  be  computed  according  to  the  proportion. 


2PO 


Solution  and  Reprecipitation  of  the  Ignited  Magnesium 
Pyrophosphate. 

If  it  is  desired  to  dissolve  the  ignited  precipitate  and  to  repre- 
cipitate  the  phosphoric  acid,  the  crucible  together  with  its  cover, 
is  placed  in  a  beaker,  enough  water  is  added  to  cover  the  crucible, 
and  then  an  excess  of  concentrated  hydrochloric  acid.  The  beaker 
is  covered  with  a  watch-glass  and  its  contents  are  heated  on  the 
water-bath,  the  liquid  in  the  beaker  being  occasionally  rotated. 
When  the  precipitate  has  dissolved,  the  heating  is  continued  for 
three  or  four  hours  longer  in  order  to  make  sure  that  the  pyro- 
phosphoric  acid  is  completely  changed  to  orthophosphoric  acid. 
This  change  is  always  complete  at  the  end  of  this  time  if  the  weight 
of  the  magnesium  pyrophosphate  was  not  over  0.2  gm.  The  time 
necessary  to  effect  this  transformation  is  proportional  to  the 
amount  of  nitric  acid  used. 

After  the  liquid  has  been  sufficiently  heated,  the  crucible  and 
its  cover  are  removed,  washed  off,  from  2  to  5  c.c.  of  magnesia 
mixture  are  added,  and  the  solution  is  treated,  as  described  above, 
with  2^  per  cent,  ammonia,  etc. 

The  method  described  on  page  434  for  the  precipitation  of 
phosphoric  acid  is  not  applicable  when  the  substance  contains 
alkaline  earths  or  heavy  metals.  In  such  cases  the  phosphoric 


436        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

acid  should  be  precipitated  first  as  ammonium  phosphomolybdate 
and  the  phosphoric  acid  in  this  precipitate  determined  by  one  of 
the  following  methods. 

I.  Determination  of  Phosphoric  Acid  as  Magnesium  Pyro- 
phosphate  after  Previous  Precipitation  as  Ammonium 
Phosphomolybdate. 

This  method,  first  proposed  by  Sonnenschein,  has  experienced, 
in  the  course  of  time,  a  great  many  modifications,  and  of  these,  that 
of  Woy  *  will  be  described,  for  it  is  one  of  the  quickest  and  most 
accurate.  It  may  be  mentioned  that  the  molybdate  method  is 
always  applicable  when  the  phosphoric  acid  is  present  as  ortho- 
phosphate,  irrespective  of  what  metals  are  in  solution. 

Principle. — If  a  solution  containing  phosphoric  acid,  in  the  pres- 
ence of  ammonium  nitrate  and  sufficient  nitric  acid,  is  treated  with 
a  slight  excess  of  ammonium  molybdate  and  heated  just  to  the 
boiling-point,  all  of  the  phosphoric  acid  is  immediately  precipitated 
as  yellow  ammonium  phosphomolybdate.  According  to  Hunde- 
shagen,  the  precipitate  possesses  the  following  composition: 
(NH4)3PO4  -  12MoO3  •  2HNO3  •  H2O, 

and  always  contains,  when  sufficient  molybdic  acid  is  present  24 
mols.  of  MoO3  to  1  mol.  PaO5.  It  never  contains  more  molybdic  acid 
than  corresponds  to  the  above  formula,  but  is  always  some  what 
contaminated  with  small  amounts  of  the  bases  in  solution,  even 
when  only  alkalies  are  present.  If,  however,  after  decanting  off 
the  supernatant  liquid,  the  precipitate  is  dissolved  in  ammonia,  a 
little  more  ammonium  molybdate  added,  and  the  boiling  solution  re- 
precipitated  by  the  addition  of  nitric  acid,  it  is  then  obtained  pure. 
It  must  also  be  noted  that  the  solution  may  contain  neither 
silicic  acid  nor  organic  substances  f  and  only  a  small  amount  of 

*  Chem.  Zeit.,  21,  p.  442. 

f  According  to  Hundeshagen,  (Zeit.  f.  anal.  Chem.,  28,  p.  164)  and  Eggertz 
(Jour.  f.  prak.  Chem.,  79,  p.  496)  the  presence  of  tartaric  and  oxalic  acids  hin- 
ders the  formation  of  the  yellow  precipitate,  and  in  some  cases  prevents 
it  entirely.  According  to  Hans  v.  Jiiptner  (Oesterr.  Zeit.  fur  Berg-  u. 
Hiittenw.,  1894,  p.  471)  this  is  not  the  case;  he  even  recommends  that 
tartaric  acid  be  added  for  the  determination  of  phosphorus  in  iron,  on  the 
ground  that  it  prevents  the  precipitate  being  contaminated  with  molybdie 
acid  and  ferric  oxide.  ,er. — 


PHOSPHORIC  ACID.  437 

chloride  (best  none  at  all),  but  there  must  be  considerable  free 
nitric  acid  present;  1  gm.  of  PaO5  requires  11.6  gms.  of  HNO3, 
but  as  much  as  35.5  gm.  of  the  latter  acid  does  no  harm.*  The 
precipitate  will  dissolve  somewhat  if  more  nitric  acid  than  the 
above  quantity  is  used,  but  the  addition  of  ammonium  molyb- 
date  decreases  the  solubility  of  the  precipitate  in  nitric  acid ;  1  gm. 
of  ammonium  molybdate  makes  55.7  gms.  of  nitric  acid  inactive. 
The  presence  of  ammonium  nitrate  not  only  facilitates  the  forma- 
tion of  the  precipitate,  but  its  presence  is  absolutely  necessary, 
although  about  5  per  cent,  is  sufficient. 

Solutions  Required. 

1.  A  3  per  cent,  solution  of  ammonium  molybdate  obtained 
by  the  solution  of  120  gms.  commercial  ammonium  molybdate, 
(NH4)6Mo7O24+4H2O,  in  4  liters  of  water  (1  c.c.  of  this  solution 
will  precipitate  0.001  gm.  P205). 

2.  A  solution  of  ammonium  nitrate,  obtained  by  dissolving 
340  gms.  of  ammonium  nitrate  in  1  liter  of  water. 

3.  Nitric  acid,  sp.  gr.  1.153  (containing  25  per  cent.  HN03). 

4.  As  wash  liquid,  200  gms.  ammonium  nitrate  and  160  c.c. 
of  nitric  acid  dissolved  in  4  liters  of  water. 


Woy's  Method  of  Precipitation. 

In  all  cases  50  c.c.  of  the  solution  are  taken,  containing  at 
the  most  0.1  gm.  P2O5.  If  the  solution  contains  more  than  this 
amount  of  phosphoric  acid,  an  aliquot  part  is  used  for  the 
analysis. 

This  amount  of  the  neutral  or  slightly  acid  (HNOa)  solution 
is  placed  in  a  400-c.c.  beaker  and  to  precipitate  0.1  gm.  of  P2Os 
30  c.c.  of  ammonium  nitrate  solution  and  10-20  c.c.  of  nitric  acid 
are  added  and  the  solution  is  heated  until  bubbles  begin  to  rise. 
At  the  same  time  the  required  amount  of  ammonium  molybdate 


*  These  figures  are  taken  from  experimental  data  furnished  by  Hun- 
deshagen.  They  do  not  refer  to  the  formula  on  p.  436  given  the  yellow 
precipitate. — [Translator.]  I 


43$         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

solution  (in  this  case  120  c.c.*)  is  likewise  heated  until  it  begins 
to  boil,  and  then  transferred  to  a  separatory  funnel  and  allowed 
to  run  in  a  thin  stream  into .  the  middle  of  the  phosphate 
solution,  which  is  rotated  while  the  molybdate  solution  is  being 
added.  The  yellow  ammonium  phosphomolybdate  is  at  once 
thrown  down  and  the  separation  is  quantitative.  The  contents 
of  the  beaker  are  kept  in  motion  for  about  one  minute  more  and 
then  allowed  to  stand  for  fifteen  minutes,  when  the  clear  liquid 
is  poured  through  a  filter,  the  precipitate  is  washed  once  by  decanta- 
tion  with  50  c.c.  of  the  wash  liquid  and  then  dissolved  in  10  c.c. 
of  8  per  cent,  ammonia.  To  this  solution  20  c.c.  of  the  ammonium 
nitrate  solution,  30  c.c.  of  water,  and  1  c.c.  of  ammonium  molybdate 
are  added.  It  is  heated,  as  before,  until  bubbles  begin  to  rise, 
when  the  phosphoric  acid  is  reprecipitated  by  the  addition  of  20  c.c. 
of  hot  nitric  acid,  added  drop  by  drop  through  the  same  funnel 
that  was  used  for  the  molybdate  solution,  the  solution  being  rotated 
as  before.  The  precipitate  is  immediately  formed  and  is  now 
pure.  After  standing  ten  minutes  it  is  filtered  off  and  dissolved 
in  warm  2£  per  cent,  ammonia,  after  which  hydrochloric  acid  ia 
added  until  the  yellow  precipitate  produced  dissolves  only  slowly 
on  being  mixed  with  the  solution.  Now,  according  to  Schmitz,f 
an  excess  of  an  acid  solution  of  "  magnesia  mixture  "  is  added, 
and  the  solution  heated  to  boiling.  A  few  drops  of  phenol- 
phthalein  are  added,  and  an  approximately  2.5  per  cent,  ammonia 
solution  introduced  as  quickly  as  possible  from  a  burette,  while 
stirring  the  solution,  until  the  liquid  becomes  slightly  red  in  color. 
It  is  allowed  to  cool  and  then  one-fifth  of  its  volume  of  concen- 
trated ammonia  is  added.  After  ten  minutes,  the  precipitate 
of  magnesium  ammonium  phosphate  is  ready  to  filter. 

*AMOUNTS    OF   REAGENTS    REQUIRED. 

Nitric 
Acid. 

19  C.C, 

10  " 

10  « 

5  a 

5  " 
f  Loc.  tit. 


Amount  of  PzOs 
Present  in  Grams. 

Ammonium 
Molybdate. 

Ammonium 
Nitrate. 

0.1 

120  c.c 

30  c.c 

0.01 

15  " 

20  " 

0.005 

15  '• 

20  " 

0  002 

10  " 

15  « 

0.001 

10  " 

15  " 

PHOSPHORIC  ACID.  439 

2.  Direct   Determination    of   Phosphoric   Acid    as   Ammonium 
Phosphomolybdate  (Finkener) .  * 

The  precipitate  produced  as  described  under  1,  having  the 
following  composition, 

(NH4)3PO4  •  12MoO,  •  2HNO3  •  H2O, 

is  transformed  by  heating  for  a  long  time  at  160-180°  C.  into  pure 
ammonium  phosphomolybdate  of  the  composition 

(NH4)3P04.12MoCV 

Theoretically  this  substance  contains  3.782  per  cent,  of  P2O5. 

If,  therefore,  the  amount  of  yellow  precipitate  (dried  until  its 
weight  is  constant)  is  multiplied  by  0.0378,  the  actual  amount  of 
P2O5  present  should  be  obtained.  The  results  obtained  by  Finkener, 
however,  were  accurate  only  when  the  factor  0. 03794 f  was  used. 
Hundeshagen;J  on  the  other  hand,  found  that  the  factor  0.03753 
should  be  used,  and  this  has  been  confirmed  by  experiments  per- 
formed in  the  author's  laboratory .§ 

Procedure. — The  phosphoric  acid  is  precipitated  twice,  accord- 
ing to  the  directions  of  Woy  (p.  437),  with  ammonium  moly fa- 
date ;  the  precipitate  is  filtered  through  a  Gooch  crucible,  washed 
with  the  prescribed  mixture  until  no  further  brown  coloration 
is  produced  by  K4Fe(CN)6,  and  dried  in  a  current  of  air  at  160° 
C.  in  a  Paul's  drying  oven,  until  a  constant  weight  is  obtained. 
If  the  precipitate  should  become  slightly  greenish,  a  small  crystal 
of  ammonium  nitrate  and  one  of  ammonium  carbonate  are  added 
and  the  contents  of  the  crucible  again  heated,  whereby  the  pre- 
cipitate will  at  once  assume  a  homogeneous  yellow  color. 

Remark. — The  results  of  Hundeshagen  and  Steffan  show  that 
this  method  gives  very  exact  results.  Steffan  worked  precisely 
according  to  the  directions  of  Finkener,  precipitating  the  phos- 
phoric acid  in  the  cold  with  a  33£  per  cent,  solution  of  ammonium 

*  Berichte,  11  (1878),  p.  1640. 

f  Loc.  dt, 

t  Zeit.  f.  anal.  Chem.,  XXXII  (1893),  p.  144. 

§  A.  Steffan,  using  50  c.c.  of  a  potassium  phosphate  solution  containing 
0.0989  gm.  P2O5,  in  four  experiments  found  0.0994,  0.0994,  0.0995,  0.0992 
gm.  P205. 


44°       GRAVIMETRIC   DETERMINATION  OF  THE  METALLOIDS. 

molybdate  and  filtering  after  standing  twenty-four  hours.  It  is, 
however,  not  necessary,  as  Hundeshagen  has  shown,  to  work  with 
such  a  concentrated  solution  of  ammonium  molybdate;  the  pre- 
cipitation from  a  hot  solution  with  a  3  per  cent,  molybdate  solu- 
tion yields  just  as  accurate  results  and  the  solution  does  not  have 
to  stand  so  long  before  filtering.  Even  when  iron  is  present  this 
method  gives  good  results,  so  that  it  is  to  be  recommended  for  the 
determination  of  phosphorus  in  iron  and  steel. 

3.  Determination  of  Phosphoric  Acid  as  Phosphomolybdic 
Anhydride  (Woy). 

The  precipitate,  produced  in  the  same  way  as  before,  is  gently 
ignited,  whereby  a  greenish-black  residue  remains  of  the  com- 
position 24MoO3-P2O5,  with  3.946  per  cent,  of  P2O5.  The  pre- 
cipitate is  ignited  as  follows:  Upon  the  bottom  of  a  nickel  cru- 
cible is  placed  a  disk  of  ignited  asbestos  paper  about  2  mm.  thick, 
or  the  porcelain  plate  of  a  Gooch  crucible  may  be  used.  Upon 
this  is  placed  the  Gooch  crucible  containing  the  precipitate,  which 
is  covered  with  a  watch-glass  and  heated  at  first  gently  and  finally 
until  the  bottom  of  the  nickel  crucible  is  at  a  dull-red  heat.  When 
the  precipitate  has  become  of  a  homogeneous,  bluish-black  color, 
it  is  allowed  to  cool  in  a  desiccator,  after  which  the  covered  cru- 
cible is  weighed. 

This  method  is  rapid  and  gives  good  results  in  the  presence 
of  iron  and  aluminium.* 

Determination  of  Phosphorus  and  Silicon  in  Iron  and  Steel. 

The  determination  of  these  two  elements  is  often  effected  in 
the  same  sample,  and  in  all  cases  it  is  best  to  remove  the  silicic 
acid  before  precipitating  the  phosphoric  acid. 

Since  phosphorus  and  silicon  are  present  in  the  iron  as  phos- 
phide and  silicide,  a  too  dilute  nitric  acid  must  not  be  used  for  dis- 
solving the  sample  or  there  will  be  a  loss  of  volatile  phosphides 
and  silicides. 

*  Steffan  found,  in  the  analysis  of  50c.c.  of  a  potassium  phosphate  solu- 
tion containing  0.0989  gm,  P2O5,  0.0988,  0.0992,  0.0986  gm.  P2O5;  and  in 
a  solution  of  5  gms.  of  iron  in  the  form  of  its  nitrate,  0.0099  gm.  P2O5,  this 
method  gave  0.0099  gm.  and  the  same  result  was  obtained  by  the  method  of 
Finkener. 


PHOSPHORUS  AND  SILICON  IN  7/?O.V  AND  STEEL.  441 

Determination  of  the  Silicon. 

About  5  gms.  of  the  iron  borings,  after  having  been  washed  with 
ether  (cf.  p.  236,  foot-note),  are  placed  in  a  500-c.c.  beaker  under  a, 
good  hood,  covered  with  60  c.c.  of  nitric  acid  (1  vol.  concentrated 
acid,  sp.  gr.  1.4,  and  1  vol.  of  water)  and  a  watch-glass  placed 
upon  the  beaker.  A  violent  reaction  at  once  takes  place  and 
brown  vapors  are  evolved.  As  soon  as  the  action  slackens, 
the  beaker  is  placed,  upon  wire  gauze  and  its  contents 
boiled  gently  until  all  the  iron  is  dissolved  and  no  more  brown 
vapors  are  evolved.  The  contents  of  the  beaker  are  then  washed 
into  a  250-c.c.  porcelain  casserole,  evaporated  on  the  water- 
bath  to  a  syrupy  consistency,  and  then  heated  over  a  free  flame 
,to  dryness,  constantly  stirring  with  a  glass  rod.  Care  is  taken 
during  this  operation  that  a  cake  of  basic  ferric  nitrate  does  not 
adhere  to  the  bottom  of  the  dish,  as  in  this  case  the  latter  will 
surely  break  during  the  subsequent  ignition.  The  dry  mass 
should  at  the  end  be  reduced  to  a  loose  powder.  When  this  point 
is  reached,  the  contents  of  the  dish  are  ignited  until  all  of  the  ferric 
nitrate  is  changed  to  oxide,  which  is  accomplished  when  no  more 
brown  fumes  are  expelled.  By  this  procedure  all  organic  matter 
formed  by  the  oxidation  of  hydrocarbons  is  destroyed  and  the 
silicic  acid  is  dehydrated.  After  cooling,  the  residue  is  covered 
with  50  c.c.  of  concentrated  hydrochloric  acid  and  heated  with 
constant  stirring  almost  to  the  boiling-point.  This  dissolves  the 
ferric  oxide  and  phosphate,  while  the  silicic  acid  remains  behind.* 

When  all  of  the  iron  oxide  has  dissolved,  the  solution  is  evap- 
orated to  dryness,  moistened  with  2-3  c.c.  of  hydrochloric  acid, 
allowed  to  stand  for  twenty  minutes,  after  which  water  is  added. 
After  heating  the  liquid  to  boiling  the  silicic  acid  is  filtered  off 
through  a  small  filter,  washed  with  water  containing  hydrochloric 
acid  and  finally  with  pure  hot  water.  The  silica  is  ignited  wet  in  a 
platinum  crucible  and  weighed.  The  silica  thus  obtained  usually 
contains  ferric  oxide,  so  that  its  purity  must  be  tested  in  all  cases. 
For  this  purpose  it  is  covered  with  1  c.c.  of  water,  a  drop  of  dilute 
sulphuric  acid  and  2  c.c.  of  pure  hydrofluoric  acid  are  added,  and 
after  evaporating  on  the  water-bath  as  far  as  possible,  the  excess  of 

*  If  graphite  were  originally  present  it  remains  with  the  silica. 


442     GRAVIMETRIC   DETERMINATION  OF  THE  METALLOIDS. 

sulphuric  acid  is  removed  by  placing  the  crucible  on  a  triangle  in 
an  inclined  position  and  carefully  heating  by  means  of  a  moving 
flame.  As  soon  as  no  more  vapors  of  sulphuric  acid  are  given  off, 
the  contents  of  the  crucible  are  more  strongly  ignited  and  the 
residue  of  ferric  oxide  is  weighed.  This  amount  deducted  from 
the  weight  of  impure  silica  gives  the  amount  of  pure  silica,  pt 
from  which  the  amount  of  silicon,  x,  can  be  calculated  as  follows: 


Si 


and  in  per  cent.,  where  a  is  the  amount  of  iron  taken  for  the  analysis, 
100  Si 


^-^-' 

SiO     a 


=   er  cent.  Si. 


Remark.  —  If  the  impure  silica  was  grayish  colored  (as.  is  always 
the  case  when  graphite  is  present)  it  is  not  weighed,  but  a  little 
pure  sodium  carbonate  and  potassium  nitrate  are  added  to  the 
contents  of  the  crucible  and,  by  fusing,  the  graphite  is  completely 
oxidized.  The  melt  is  placed  in  a  small  porcelain  dish  and  dissolved 
in  water.  .  The  solution  is  acidified  with  hydrochloric  acid,  evap- 
orated to  dryness  on  the  water-bath,  moistened  with  a  little  con- 
centrated hydrochloric  acid,  diluted  with  water  and  filtered.  The 
residual  silica  is  ignited  wet;  a  further  purification  of  the  silica 
is  unnecessary. 


Drown  Method  for  Determining  Silicon  in  Iron  and  Steel. 

This  method  has  come  into  very  general  use,  and  is  much  more 
rapid  than  the  above  method,  though  quite  as  exact.  It  is 
recommended  by  the  American  Foundrymen's  Association  for  the 
analysis  of  cast  iron  and  has  been  used  by  the  Bureau  of  Standards 
at  Washington,  D.  C.,  for  analyzing  samples  of  steel. 

One  gram  of  borings  is  treated  in  a  platinum  or  porcelain  dish 
with  20  c.c.  of  nitric  acid,  sp.  gr.  1.2.  When  all  action  has  ceased 


PHOSPHORUS   AND  SILICON  IN  IRON  AND  STEEL.          443 

20  c.c.  of  50  per  cent,  sulphuric  acid  are  added  and  the  solution 
evaporated  until  copious  fumes  are  evolved.  The  liquid  is  then 
cooled,  diluted  with  150  c.c.  of  water,  and  heated  until  all  the 
ferric  sulphate  has  dissolved.  The  hot  solution  is  at  once  filtered, 
washed  with  dilute  hydrochloric  acid,  sp.  gr.  1.1,  and  then  with 
hot  water.  The  residue  is  placed  in  a  platinum  crucible  without 
drying,  ignited  and  weighed.  The  contents  of  the  crucible  are 
then  treated  with  4  or  5  c.c.  of  hydrofluoric  acid  and  a  few  drops  of 
sulphuric  aicd,  evaporated  to  dryness,  and  the  crucible  again 
ignited  and  weighed.  The  difference  in  the  two  weights  is  the 
silica. 

Determination  of  Phosphorus. 

In  the  hydrochloric  acid  filtrate  from  the  silicic  acid  all  the 
phosphorus  is  present  in  the  form  of  phosphoric  acid.  The  latter 
is  determined  according  to 

(a)  The  Acetate  Method  or 

(b)  The  Molybdate  Method. 

Both  methods  give  equally  good  results,  judging  from  experiment/ 
performed  in  the  author's  laboratory. 

(a)  The  Acetate  Method  of  A.  A.  Blair. 

The  filtrate  from  the  silicic  acid  is  diluted  in  a  beaker  to  a  volume 
of  about  400  c.c.  and  ammonia  is  added  until  a  permanent  precipi- 
tate of  ferric  hydroxide  is  produced.  The  liquid  is  then  treated 
with  200  c.c.  of  a  saturated,  aqueous  solution  of  sulphurous  acid 
and  slowly  heated  to  boiling.  The  precipitate  of  ferric  hydrox- 
ide soon  dissolves  and  the  liquid  assumes  a  dark  reddish-brown 
color,  which  on  further  heating  becomes  a  light  green,  or  almost 
colorless.  As  soon  as  this  point  is  reached,  10-20  c.c.  of  concen- 
trated hydrochloric  acid  are  added  and  a  current  of  carbon  dioxide 
is  conducted  into  the  colorless  solution  until  the  excess  of  sul- 
phurous acid  is  removed.  The  solution  is  now  cooled  by  placing  the 
beaker  in  cold  water,  after  which  1  or  2  c.c.  of  chlorine  or  bromine 
water  is  added  to  oxidize  a  part  of  the  iron.  To  this  solution 
ammonia  is  added  very  slowly  with  constant  stirring  until  the 
greenish  precipitate  of  ferrous-ferric  hydroxide  dissolves  with 
difficulty.  The  addition  is  then  continued  drop  by  drop  until  a 
distinct  brown  precipitate  is  formed,  which  on  stirring  becomes 
green.  If  before  this  occurs  the  precipitate  does  not  appear 


444      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

decidedly  red  in  color,  it  is  dissolved  in  a  drop  or  two  of  hydro-, 
cjiloric  acid  and  1  or  2  c.c.  more  of  chlorine  or  bromine  water 
is  added,  and  the  addition  of  ammonia  is  repeated  until  the  per- 
manent green  or  brownish  precipitate  is  obtained.  Acetic  acid 
is  now  added  drop  by  drop  until  the  precipitate  redissolves,  or  at 
any  rate  becomes  white  in  color,  when  the  solution  is  heated  to 
boiling  and  kept  at  this  temperature  for  one  minute.  All  of 
the  phosphoric  acid  is  thus  precipitated  as  basic  ferric  phosphate 
while  the  excess  of  the  ferric  salt  is  thrown  down  as  basic  ferric 
acetate.  The  solution  is  filtered  through  a  large  filter  and  washed 
once  with  hot  water.  The  precipitate  filters  readily  and  the 
filtrate  is  at  first  clear,  but  becomes  turbid  on  standing  in  the  air. 
The  precipitate  adhering  to  the  sides  of  the  beaker  is  dissolved 
by  warming  with  a  mixture  of  hydrochloric  acid  (1:1)  and  10  c.c. 
of  bromine  water.  Should  this  not  be  sufficient  to  effect  com- 
plete solution  (as  is  usually  the  case)  enough  concentrated  hydro- 
chloric acid  is  added  to  accomplish  this.  The  solution  is  then 
poured  upon  the  filter  containing  the  precipitate  and  the  filtrate 
received  in  a  small  beaker.  The  filter  is  washed  well  with  hot 
water  and  the  solution  is  evaporated  nearly  to  dryness  to  get 
rid  of  the  excess  of  hydrochloric  acid,  5  c.c.  of  a  50  per  cent,  citric 
acid  solution  are  added,  an  equal  amount  of  magnesia  mixture 
and  enough  ammonia  to  make  the  solution  faintly  alkaline.  When 
perfectly  cold,  one-half  of  the  liquid's  volume  of  strong  ammonia 
is  added  and  the  mixture  well  stirred.  After  standing  twelve 
hours,  the  precipitate  is  filtered  off  and  washed  with  2*>  per  cent, 
ammonia  containing  2.5  gms.  of  ammonium  nitrate  in  each  100  c.c. 
This  precipitate  of  magnesium  ammonium  phosphate  always  con- 
tains a  small  amount  of  iron  and  silicic  acid  (the  latter  from  the 
glass)  so  that  it  is  dissolved  in  hydrochloric  acid,  the  solution  evapo- 
rated to  dryness,  the  residue  moistened  with  concentrated  hydro- 
chloric acid,  taken  up  in  a  little  water,  filtered  through  a  small 
filter  and  the  residual  silica  washed  with  hot  water.  The  filtrate, 
amounting  to  not  over  20  c.c.  at  the  most,  is  treated  with  1  c.c. 
of  the  citric  acid  solution  and  two  drops  of  magnesia  mixture 
and  the  precipitation  with  ammonia  is  repeated  as  above.  In 
this  way  a  precipitate  is  obtained  which  yields  pure  magnesium 
pyrophosphate  on  ignition. 


DETERMINATION  OF  PHOSPHORIC  ACID  IN  SILICATES.      445 

Remark. — Blair  recommends  the  use  of  ammonium  bisulphite 
(XH4HSO3)  instead  of  sulphurous  acid  for  the  reduction  of  the 
ferric  salt.  Much  of  the  ammonium  bisulphite  of  commerce,  how- 
ever, contains  phosphoric  acid,  so  that  it  seems  safer  to  use  sul- 
phurous acid  for  this  purpose.  Again,  Blair  suggests  that  hydro- 
gen sulphide  be  passed  into  the  solution  after  the  excess  of  the 
sulphurous  acid  has  been  removed,  in  order  to  precipitate  any 
arsenic  as  the  trisulphide.  The  filtrate  from  the  arsenic  precipitate 
is  heated  to  boiling,  the  excess  of  hydrogen  sulphide  expelled  by 
means  of  a  current  of  carbon  dioxide,  and  the  solution  then  partly 
oxidized  as  above  described. 

(b)  The  Molybdate  Method. 

The  filtrate  from  the  silica  (see  p.  441)  is  evaporated  to  dry- 
ness  in  a  porcelain  dish,  the  dry  residue  is  dissolved  in  as  little 
nitric  acid  as  possible,  30  c.c.  of  ammonium  nitrate  solution  and 
10  c.c.  of  nitric  acid  are  added,  and  the  phosphoric  acid  is  precipi- 
tated according  to  the  procedure  of  Woy,  p.  437,  by  the  addition 
of  75  c.c.  of  ammonium  molybdate.  After  decanting  off  the  clear 
Liquid,  the  precipitate  is  washed  once  by  decantation  with  10-20 
c.c.  of  the  prescribed  wash  liquid  and  redissolved  in  a  little  ammo- 
nia. To  this  solution  6  c.c.  of  molybdate  solution  and  30  c.c. 
of  water  are  added;  it  is  heated  just  to  the  boiling-point  and  re- 
precipitated  by  the  addition  of  20  c.c.  of  hot  nitric  acid.  The 
precipitate  is  then  analyzed  by  the  method  of  Finkener  (p.  439) 
or  by  that  of  Woy  (p.  440) . 

1  gm.  Mg2P2O7  =  0.27848  gm.  P 

(XH4)3PO4-12MoO3=  0.01639    "    " 
P2O5-24MoO3  =0.01723   "    " 

Remark. — According  to  the  above  directions,  some  difficulty 
is  likely  to  be  encountered  at  the  stage  where  the  dry  residue  is 
taken  up  in  nitric  acid.  If  the  residue  is  overheated  at  all,  it 
dissolves  very  slowly  in  the  nitric  acid  owing  to  the  formation  of 
basic  ferric  salts.  For  this  reason  many  chemists  prefer  to  carry 
out  the  analysis  in  accordance  with  the  directions  of  the  American 
Foundrymen's  Association,  which  are  as  follows: 

A  2-gm.  sample  of  the  borings  is  dissolved  in  50  c.c.  of  nitric 
acid,  sp.  gr.  1.13,  and  10  c.c.  of  hydrochloric,  sp.  gr.  1.2.  In  case 


446         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

the  sample  contains  a  fairly  high  percentage  of  phosphorus,  it  is 
advisable  to  use  half  the  above  quantities  of  sample  and  reagents. 
The  solution  is  evaporated  to  dryness  and  the  residue  baked  until 
free  from  acid,  at  a  temperature  of  about  200°.  This  baking 
serves  to  oxidize  carbonaceous  matter  which  otherwise  interferes 
with  the  precipitation  of  the  phosphorus.  The  residue  is  dis- 
solved by  heating  it  with  25-30  c.c.  of  concentrated  hydrochloric 
acid;  the  solution  is  diluted  to  about  60  c.c.  and  filtered.  The 
filtrate  is  evaporated  to  about  25  c.c.,  20  c.c.  of  concentrated  nitric 
acid  are  added,  and  the  evaporation  is  repeated  until  a  film  begins 
to  form.  At  this  point  30  c.c.  of  nitric  acid,  sp.  gr.  1.2,  are  added 
and  once  more  the  solution  is  evaporated  until  a  film  forms.  It  is 
then  diluted  with  hot  water  to  a  volume  of  about  150  c.c.  and 
allowed  to  cool  somewhat.  When  at  a  temperature  between  70° 
and  80°  C.,  50  c.c.  of  ammonium  molybdate  solution  are  added 
and  the  solution  agitated  for  a  few  minutes.  The  precipitate  is 
then  filtered  on  a  tared  Gooch  crucible  which  has  a  paper  disc 
at  the  bottom.  The  precipitate  is  washed  three  times  with  3  per 
cent,  nitric  acid  and  twice  with  alcohol.  It  is  then  dried  at  a 
temperature  between  100°  and  105°  to  constant  weight.  The 
precipitate  contains  1.63  per  cent,  of  phosphorus. 

The  ammonium  molybdate  solution  used  in  these  last  directions 
is  prepared  by  dissolving  100  gms.  of  molybdic  acid  in  250  c.c. 
of  water  and  150  c.c.  of  concentrated  ammonia,  stirring  until  all 
is  dissolved,  whereupon  65  c.c.  of  nitric  acid,  sp.  gr.  1.42,  are  added. 
Another  solution  is  prepared  containing  400  c.c.  of  the  concentrated 
nitric  acid  and  1100  c.c.  of  water.  When  the  two  solutions  are 
cold,  the  first  is  poured  slowly  into  the  second  with  constant  stirring 
and  a  few  drops  of  ammonium  phosphate  solution  are  added. 
After  a  little  ammonium  phosphomolybdate  precipitate  has  settled 
out,  the  reagent  is  decanted  off  and  is  ready  for  use.  The  solution 
does  not  keep  very  well,  so  that  the  analysis  should  always  be 
carried  out  with  a  reagent  that  has  not  stood  very  long. 

The  phosphorus  in  iron  and  steel  is  very  conveniently  analyzed 
by  a  volumetric  method.  See  Volumetric  Analysis. 


SEPARATION  OF  PHOSPHORIC  ACID  FROM  THE  METALS.     447 

Determination  of  Phosphoric  Acid  in  Silicates. 

Tn  the  analysis  of  silicates  (see  p.  491)  the  phosphoric  acid  is 
found  in  the  precipitate  produced  by  ammonia  in  the  filtrate 
from  the  silica  together  with  iron  and  aluminium  hydroxides. 
It  is  analyzed  according  to  p.  105. 

Determination  of  Phosphoric  Acid  in  Mineral  Waters. 

The  contents  of  a  5-6  liter  flask  is  acidified  with  hydrochloric 
acid  and  evaporated  to  dryness,  the  residue  is  moistened  with 
concentrated  hydrochloric  acid,  taken  up  with  water,  and  the 
silicic  acid  filtered  off.  The  filtrate  is  precipitated  with  ammonia, 
by  which  means  the  phosphoric  acid  is  usually  completely  thrown 
down  in  the  form  of  phosphate  of  iron,  aluminium,  or  alkaline 
earth.  The  filtered  and  washed  precipitate  is  dissolved  in  nitric 
acid  and  the  phosphoric  acid  present  determined  according  to  one 
of  the  molybdate  methods  (pp.  436-440) . 

Remark. — If  the  mineral  water  does  not  contain  much  iron, 
aluminium,  or  alkaline-earth  metal,  but  is  rich  in  phosphoric  acid 
and  the  alkalies,  the  precipitate  produced  by  ammonia  will  not 
contain  all  of  the  phosphoric  acid.  In  such  a  case  the  hydrochloric 
acid  solution  from  the  silica  is  evaporated  several  times  to  dryness 
with  nitric  acid,  the  residue  is  dissolved  in  as  little  nitric  acid 
as  possible,  and  the  phosphoric  acid  determined  by  one  of  the 
molybdate  methods. 

Recovery  of  Molybdenum  Residues  (H.  Borntrager).* 

In  practice  the  great  majority  of  phosphoric  acid  determina- 
tions are  carried  out  according  to  p.  436.  The  acid  and  am- 
moniacal  filtrates  containing  molybdenum  are  saved,  and  the 
molybdenum  is  recovered  as  follows :  Into  a  large,  wide-mouthed 
flask  250  c.c.  of  strong  ammonia  are  placed  and  the  molyb- 
denum filtrates  are  added  to  this.  Either  immediately  or  after 
standing  some  time  a  crystalline  deposit  of  almost  pure  molybdic 
acid  is  formed.  When  the  flask  is  nearly  full,  the  solution  is 
made  almost  neutral,  the  precipitate  allowed  to  settle,  and  the 


*  Zeit.  f.  anal.  Chem.,  XXXIII  (1894),  p.  341, 


448      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

upper  liquid  containing  only  a  small  amount  of  molybdenum  is 
poured  off.  The  residue  is  poured  upon  a  suction  plate,  washed 
once  with  water  (not  more,  or  the  molybdic  acid  will  dissolve) 
and  sucked  as  dry  as  possible.  The  precipitate  is  dissolved  by 
warming  with  as  little  ammonia  as  possible,  leaving  behind  a 
residue  of  iron  and  aluminium  hydroxides,  magnesia,  and  silicic 
acid.  These  are  filtered  off  and  the  solution  diluted  with  dis- 
tilled water  until  at  17°  C.  it  has  a  specific  gravity  of  1.11  =  14°  Be. 
It  then  contains  150  gms.  of  ammonium  molybdate  in  a  liter.  If 
this  solution  is  diluted  with  four  times  as  much  water,  a  3J  per 
cent,  solution  will  be  obtained. 

Determination  of  Phosphorus  in  Organic  Substances. 

The  substance  is  decomposed  by  the  method  of  Carius.  By 
the  action  of  the  nitric  acid  in  the  closed  tube  the  phosphorus  is 
oxidized  to  phosphoric  acid  and  this  is  determined  as  usual. 

SEPARATION  OF  PHOSPHORIC  ACID  FROM  THE  METALS. 

i.  Separation  from  the  Metals  of  Groups  I  and  II. 

Hydrogen  sulphide  is  conducted  into  the  hydrochloric  acid 
solution,*  by  which  means  all  the  members  of  these  groups  are 
precipitated  as  sulphides  while  the  phosphoric  acid  remains  in 
solution. 

2.  Separation  from  the  Metals  of  Group  III. 

(a)  The  phosphoric  acid  is  first  precipitated  as  ammonium 
phosphomolybdate  according  to  p.  436.  In  order  to  determine 
the  metals,  the  solution  containing  molybdenum,  but  free  from 
phosphoric  acid,  is  evaporated  with  the  addition  of  sulphuric 
acid  to  a  syrupy  consistency,  and  carefully  heated  over  a  free 
flame  until  the  nitric  acid  is  expelled.  After  cooling,  the  residue  is 
moistened  with  hydrochloric  acid  and  taken  up  in  water.  The 
solution  is  placed  in  a  pressure-flask,  saturated  with  hydrogen 
sulphide,  the  flask  stoppered  and  heated  for  some  time  on  the 
water-bath,  when  the  molybdenum  is  precipitated  in  large  flocks. 
After  cooling,  the  pressure-flask  is  slowly  opened  and  the  molyb- 

*  When  silver  is  present  it  is  precipitated  as  silver  chloride,  filtered  off, 
Mid  the  filtrate  treated  with  hydrogen  sulphide. 


SEPARATION  OF  PHOSPHORIC  ACID  FROM  THE  METALS.      449 

denum  sulphide  is  filtered  off.  The  filtrate,  now  free  from  phos- 
phoric acid  and  molybdenum,  is  analyzed  for  the  metals  as 
described  on  pages  82  to  167. 

(6)  The  phosphoric  acid  is  separated  as  before,  the  filtrate 
is  made  slightly  ammoniacal  and  saturated  with  hydrogen  sul- 
phide. After  standing  for  some  time  the  solution  becomes  red- 
dish yellow  in  color,  when  the  precipitate  is  filtered  off.  The 
metals  of  this  group  will  be  found  in  the  precipitate  while  the 
molybdenum  is  in  the  filtrate  in  the  form  of  its  sulpho-salt. 

Remark. — If  nickel  is  present,  some  of  it  will  remain  in  the 
filtrate  with  the  molybdenum  on  account  of  the  solubility  of 
nickel  sulphide  in  ammonium  sulphide,  so  that  method  (a)  will 
then  give  more  accurate  results. 

3.  Separation  of  Phosphoric  Acid  from  Iron,  Cobalt, 
Manganese,  and  Zinc. 

In  case  the  solution  contains  iron  in  the  ferric  form,  it  is  acidi- 
fied with  hydrochloric  acid,  saturated  with  hydrogen  sulphide, 
and  for  each  gram  of  the  mixed  oxides  3  gms.  of  tartaric  acid  are 
added;  the  solution  is  made  slightly  ammoniacal  and  allowed 
to  stand  overnight  in  a  stoppered  flask.  The  precipitate  con- 
tains the  metals  as  sulphides  free  from  phosphoric  acid.  It  is 
filtered,  washed  with  water  containing  ammonium  sulphide,  dis- 
solved in  acids,  and  analyzed  according  to  pp.  150  and  156. 

4.  Separation  from  Chromic  Acid. 

If  the  solution  contains  free  alkali  or  alkali  carbonate  it  is  acidi- 
fied with  nitric  acid,  then  made  slightly  alkaline  with  ammonia 
and  the  phosphoric  acid  precipitated  with  "  magnesia  mixture  "  as 
described  on  page  434. 

5.  Separation  from    Calcium,  Strontium,  Barium,    Magnesium, 
and  the  Alkalies. 

Ammonium  carbonate  is  added  to  the  hydrochloric  acid  solu- 
tion until  a  slight  permanent  turbidity  *  is  produced,  which  is 

*  If  only  alkalies  are  present  there  will  be  no  turbidity,  and  the  ammo- 
nium carbonate  is  added  until  the  solution  is  neutral. 


45°      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

redissolved  by  a  few  drops  of  hydrochloric  acid.  Ferric  chloride 
is  then  added  drop  by  drop  until  the  liquid  above  the  yellowish- 
white  precipitate  of  ferric  phosphate  becomes  distinctly  brown 
in  color.  The  solution  is  diluted  with  water  to  a  volume  of  300 
to  400  c.c.,  boiled  for  one  minute,  filtered  and  washed  with  water 
containing  ammonium  acetate.  In  the  filtrate  are  now  found 
the  alkaline  earths  and  alkalies,  which,  after  expelling  the  am- 
monium salts  by  igniting  the  residue  obtained  after  evaporating  to 
dryness,  is  analyzed  in  the  usual  way  (seepages  43  and  76  ff.). 


THIOSULPHURIC  ACID,  H£2Oa.    Mol.  Wt.  114.16. 
Form:  Barium  Sulphate,  BaS04. 

The  aqueous  solution  of  the  alkali  thiosulphate  is  treated 
with  an  ammoriiacal  solution  of  hydrogen  peroxide,  or  with  am- 
moniacal  per  carbon  ate  solution,  heated  for  some  time  on  the  water- 
bath,  and  then  boiled  to  destroy  the  excess  of  the  reagent.  This 
solution  is  acidified  with  hydrochloric  acid  and  the  sulphuric 
acid  formed  by  the  above  treatment  is  precipitated  as  barium  sul- 
phate. Two  mols.  BaS04  correspond  to  1  mol.  H2S203. 

A  much  better  procedure  for  the  estimation  of  thiosulphuric 
acid  will  be  discussed  under  lodimetry,  Part  II. 

The  remaining  acids  of  this  group,  arsenious,  arsenic,  vanadic, 
and  chromic,  have  been  discussed  under  the  respective  metals,  while 
periodic  acid  is  analyzed  precisely  in  the  same  way  as  iodic  acid. 


DETERMINATION  OF  NITRIC    ACID  AS  NITRON  NITRATE.    45 * 

GROUP  V. 

NITRIC,  CHLORIC,  AND  PERCHLORIC  ACIDS. 

NITRIC  ACID,  HNO3.    Mol.  Wt.  63.02. 

Forms:  Nitron  Nitrate,  C2oH16N4.HNO3,  Nitrogen  Pentoxide,  N2O5; 
Ammonia,  NH3;   Nitric  Oxide,  NO,  and  Volumetrically. 

i.  Determination  of  Nitric  Acid  as  Nitron  Nitrate.* 
The  base  diphenyl-endo-anilo-hydro-triazole,  G^HieN^  or 

-N 


called  "  nitron  "  for  the  sake  of  brevity,  forms  a  fairly  insoluble, 
crystalline  nitrate,  C^HieN^HNOa,  which  can  be  used  for  the 
separation  and  quantitative  estimation  of  this  acid. 

Procedure. — Enough  of  the  substance  is  taken  to  furnish  about 
0.1  gm.  of  nitric  acid,  and  dissolved  in  80-100  c.c.  of  water  with 
the  addition  of  10  drops  of  dilute  sulphuric  acid.  The  solution  is 
heated  nearly  to  boiling  and  treated  with  10-12  c.c.  of  nitron 
acetate  solutionf,  which  is  added  all  at  one  time.  The  beaker 
containing  the  solution  and  precipitate  is  kept  surrounded  by  ice- 
water  for  about  two  hours.  The  precipitate  is  then  transferred  to 
a  Munroe  crucible  and  drained  as  completely  as  possible  from  the 


*  M.  Busch,  Ber.  38,  861  (1905).      A.  Gutbier,  Z.  angew.  Chem.,  1905,  494. 

t  The  reagent  is  prepared  by  dissolving  10  gm.  of  nitron  (which  can  be 
obtained  of  Merck)  in  100  c.c.  of  5  per  cent,  acetic  acid.  The  solution  usually 
has  a  reddish  color,  but  can  be  kept  for  a  long  time  in  a  dark-colored  bottle 
without  its  undergoing  any  change. 


45 2         GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

pale  yellow  mother-liquor.  It  is  washed  with  10  or  12  c.c.  of 
ice-water,  added  in  small  portions,  and  the  precipitate  drained 
well  after  each  washing.  The  precipitate  is  dried  at  110°  to 
constant  weight.  It  contains  16.53  per  cent,  of  NO3. 

Remarks. — The  following  acids  interfere  with  the  determination 
of  nitric  acid  by  the  nitron  method, — hydrobromic,  hydriodic, 
nitrous,  chromic,  chloric,  perchloric  and  the  less  common  thio- 
cyanic,  hydroferrocyanic,  hydroferricyanic,  picric  and  oxalic 
acids.  All  of  the  above  acids  form  salts  with  nitron  which  are  not 
very  soluble;  these  acids  must,  therefore,  be  removed  from  the 
solution  before  precipitating  the  nitric  acid. 

Hydrobromic  acid  is  decomposed  by  adding  chlorine  water 
drop  by  drop  to  the  neutral  solution  and  boiling,  until  the  yellow 
coloration  entirely  disappears. 

Hydriodic  acid  is  removed  by  adding  an  excess  of  potassium 
iodate  to  the  neutral  solution,  and  boiling  until  the  iodine  is  all 
expelled. 

Nitrous  acid  is  removed  by  dropping  finely  powdered  hydrazine 
sulphate  into  the  concentrated  solution  (0.2  gm.  of  substance  in 
5  or  6  c.c.  of  water). 

Chromic  acid  is  reduced  by  hydrazine  sulphate. 

Some  idea  as  to  the  relative  solubilities  of  the  various  salts  of 
nitron  is  obtained  from  the  following  table : 

100  c.c.  of  slightly  acid  water  dissolve  at  ordinary  temperatures 
about 


0.0099  gm.  of  nitron  nitrate,  corresponding  to  0.0017  gm.  HNO3 


0.61 

0.017 

0.19 

0.06 

0.12 

0.008 

0.04 


bromide, 

iodide, 

nitrite, 

chromate, 

chlorate, 

perchlorate, 

thiocyanate, 


0.125 
0.005 
0.022 
0.011 
0.022 
0.002 
0.007 


HBr 

HI 

HNO2 

H2Cr207 

HC103 

HC103 

HCNS 


These  values  are  only  approximate.  The  solubility  of  the 
nitrate  is  given  a  little  too  high  and  that  of  the  other  salts  a  little 
too  low. 


DETERMINATION  OF  NITRIC  ACID.  453 

On  account  of  the  appreciable  solubility  of  the  nitrate,  it  was  to 
be  expected  that  the  results  would  be  a  little  low.  This  is  not  the 
case,  however,  as  Busch  and  Gutbier  have  proved.  It  is  probable 
that  the  precipitate  occludes  a  little  nitron  acetate  and  in  his  way 
the  error  caused  by  amount  left  in  solution  is  compensated. 


2.  Determination  of  Nitric  Acid  as  Nitrogen  Pentoxide.* 

This  method  is  based  upon  the  fact  that  when  an  intimate 
mixture  of  a  dry  nitrate  is  heated  with  an  excess  of  silica,  nitrogen 
pentoxide  is  evolved  and  the  amount  is  determined  by  the  loss  in 
weight. 

2NaNO3  +  SiO2  =  Na2SiO3  +  N205. 


This  method  cannot  be  used  when  there  is  any  other  volatile 
substance  present,  which  is  usually  the  case. 


3.  Determination  of  Nitric  Acid  as  Ammonia. 

The  usual  method  for  the  determination  of  nitric  acid  is  to 
reduce  it  in  alkaline  solution  to  ammonia  by  means  of  aluminium, 
zinc,  or,  best,  Devarda's  alloy  (cf.  Vol.  I,  page  6) : 

NaNO3  +  8H  =  2H2O  +  XaOH  +  NH3. 


After  the  reduction,  the  solution  is  distilled  into  a  known 
amount  of  acid  and  the  excess  of  the  acid  is  found  by  titration, 
or  the  ammonia  is  determined  as  ammonium  chloroplatinate  or 
as  platinum  (cf.  page  58,  b  and  c). 


*  Reich.  Z.  Chem.,  1,  86  (1862). 


454     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 


Procedure  of  Devarda* 

About  0.5  gm.  of  the  nitrate  is  placed  in  a  600-800-c.c. 
Erlenmeyer  flask  (Fig.  78)  and  dissolved  in  110  c.c.  of  water.  To 
this  solution  5  c.c.  of  alcohol,  50  c.c.  of  caustic  potash  (sp.  gr.  1.3), 
and  2  to  2J  gms.  of  powdered  Devarda's  alloy  are  added.  After 


K 


B 


this  the  flask  is  immediately  connected  with  the  distilla- 
tion apparatus  as  shown  in  the  figure.  The  Peligot  tube,  A, 
of  about  250  c.c.  capacity,  is  constructed  as  proposed  by  F.  Pan- 
nertz.f  Its  left  arm  is  connected  by  a  curved  tube  with  the  mid- 
dle bulb,  so  that  a  spurting  back  of  the  liquid  is  avoided.  The 
delivery-tube  (of  potash  glass)  connecting  the  flask  K  with  the 
tube  A  is  about  1  cm.  in  diameter  and  is  provided  with  a  small 
opening  at  o,  inside  the  flask,  to  prevent  any  of  the  alkaline  solution 
being  carried  over  with  the  ammonia.  Twenty  cubic  centimeters 


*Zeit.  f.  anal.  Chem.,  XXXIII  (1894),  p.  113. 
}lbid.,  XXXIX  (1900),  p.  318. 


DETERMINATION  OF  NITRIC  ACID  AS  AMMONIA.  455 

of  half-normal  sulphuric  acid  are  added  to  the  tube  A  *  and  diluted 
so  that  the  solution  just  reaches  to  each  of  the  bulbs  0:1  the  side, 
while  5  c.c.  of  the  acid  are  placed  in  B,  with  a  few  drops  of  methyl 
orange,  and  diluted  in  the  same  way.  The  tubes  A  and  B  are 
connected  by  means  of  a  T  tube,  of  which  the  upper  end  is  closed 
by  a  pinch-cock  upon  a  piece  of  rubber  tubing,  so  that  a  piece  of  red 
litmus  paper  may  be  introduced  here. 

When  all  is  ready,  the  contents  of  the  flask  K  are  gently  heated 
in  order  to  start  the  reaction,  then  the  flame  is  removed  and  the 
reaction  allowed  to  proceed  by  itself.  After  an  hour  this  will  be 
shown  to  be  complete  by  the  cessation  of  the  hydrogen  evolution. 
The  liquid  in  K  is  then  slowly  heated  to  boiling,  and  kept  at  this 
temperature  until  about  half  of  the  liquid  has  distilled  over  into  A ; 
this  requires  about  half  an  hour.  During  the  last  ten  minutes 
a  slow  current  of  air  is  passed  through  the  tube  r. 

If  the  distillation  has  been  correctly  performed,  all  of  the 
ammonia  will  now  be  found  in  A ;  no  trace  should  reach  B,  and  the 
red  litmus  paper  in  the  T-tube  should  show  no  tinge  of  blue. 

When  the  distillation  is  finished,  the  pinch-cock  at  r  is  opened 
and  the  flame  removed.  A  little  methyl  orange  is  added  to  A  where- 
by the  liquid  is  colored  red,  the  contents  of  B  are  poured  in,  the 
latter  tube  is  washed  with  water  that  is  added  to  A,  and  the  excess 
of  the  sulphuric  acid  is  titrated  with  half-normal  caustic  potash 
solution  until  the  solution  is  changed  to  yellow.  The  amount  of 
nitric  acid  is  computed  as  follows: 

The  tubes  originally  contained  25  c.c.  of  half-normal  acid, 
and  t  c.c.  of  half-normal  caustic  potash  solution  were  used  up 
in  the  titration;  consequently  the  ammonia  formed  from  0.5  gm. 
of  the  nitrate  was  neutralized  by  25  —  t  c.c.  of  half -normal  sul- 
phuric acid. 

Since  1  mol.f  of  HNOs  (63.02  gms.)  on  being  reduced  yields 
1  mol.  of  XH3,  and  one  liter  of  half-normal  sulphuric  acid  con- 
tains enough  sulphuric  acid  to  neutralize  \  mol.  of  NH3,  it  is  evi- 

63  02 
dent  that  1  c.c.  of  the  acid  is  equivalent  to       n    =0.03151  gm. 


*  The  tube  A  has  a  capacity  of  about  250  cubic  centimeters. 
f  Ostwald  has  proposed  that  the  molecular  weight  in  grams  of  a  sub- 
stance be  designated  by  the  word  "mol." 


456       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

of  nitric  acid  so  that  25- 1  c.c.  =  (25-0X0.03151  gm.  of  HNO3  01 
(25  —  0-03101  gm.  NO3,  and  the  per  cent,  of  N03  present  is 

0.5:  (25-0 -0.03101=  100:3 
x  =  6.202-  (25 -0=per   cent.  NO3. 

Determination  of  Nitric  Acid  as  Nitric  Oxide. 

Method  of  Schlosing  and  Grandeau,  modified  by  Tiemann  and 

Schulze* 

Principle. — If  a  nitrate  is  heated  with  ferrous  chloride  and 
hydrochloric  acid,  the  nitric  acid  is  reduced  to  nitric  oxide: 

NaNO3+  3FeCl2+  4HC1  =  NaCl+  3FeCl3+  2H2O+  NO. 

From  the  volume  of  the  nitric  oxide  its  weight  is  calculated. 

The  method  of  Schlosing  in  its  original  formf  was  not  much 
used  on  account  of  the  apparatus  required;  but  after  being  modi- 
fied by  Grandeau  J  it  has  become  one  of  the  best  methods  for  the 
determination  of  nitric  acid. 

The  apparatus  necessary  is  shown  in  Fig.  79  and  consists  of 
a  150-c  c.  flask  K  fitted  with  a  double-bored  rubber  stopper. 
Through  one  of  the  holes  is  passed  the  tube  b,  which  reaches  into 
the  flask  just  to  the  lower  surface  of  the  stopper;  through  the 
other  hole  passes  the  tube  a,  §  ending  in  a  restriction  about  1  mm. 
wide  and  reaching  1J  cm.  below  the  stopper.  The  tube  b  is  con- 
nected by  means  of  a  piece  of  rubber  tubing  5  cm.  long,  which  is 
wired  on  to  the  tube,  and  is  provided  with  a  pinch-cock,  with  a 
second  tube  whose  lower  end  reaches  up  into  the  measuring-tube 
and  is  covered  with  rubber  tubing  as  is  shown  in  the  figure.  In 
the  same  way  the  tube  a  is  connected  with  a  straight  tube. 

Solutions  required — 1.  A  nitrate  solution  of  known  strength, 
prepared  by  dissolving  in  one  liter  of  water  2. 0222  gms.  of  recrystal- 
lized  potassium  nitrate  that  has  been  dried  at  160°  C.  Fifty  c.c. 

*  Zeit.  f.  anal.  Chem.,  IX  (1870),  p.  401,  and  Berichte,  VI  (1873),  p.  1041. 
f  Annales  de  chim.  et  de  phys.,  [3],   40  (1853),  479. 
J  Grandeau," Analyse  chimique  appliquee  a  I'agriculture. 
§  Grandeau  used  a  separatory  funnel  instead  of  the  tube  a;    the  lattei 
was  proposed  by  Tiemann  and  Schulze. 


DETERMINATION  OF  NITRIC  AUD  AS  NITRIC  OXIDE.       457 

of  this  solution  evolve  at  0°  C.  and  760  mm.  pressure  22.41    c.c. 
of  NO. 

2.  A  ferrous  chloride  solution  obtained  by  dissolving  20  gm. 
of  iron  (nails)  in  100  c.c.  of  concentrated  hydrochloric  acid. 

3.  Hydrochloric  acid,  of  specific  gravity  1.1. 

Procedure. — First  of  all,   10  c.c.  of  water  are  poured  into  K 
and  its  upper  level  is  marked  on  the  outside  of  the  flask  by  means  of 


FIG.  79. 

a  colored  pencil,  then  40  c.c.  more  are  added  and  its  position  is 
also  marked. 

The  water  is  now  poured  out  and  exactly  50  c.c.  of  the  standard 
nitrate  solution  is  added  to  K,  the  stopper  fitted  with  the  two 
tubes  is  placed  in  the  flask,  and  the  pinch-cocks  In!  and  h"  are 
opened.  The  contents  of  the  flask  are  heated  to  boiling  with  a 
free  flame  (a  wire  gauze  is  not  used)  until  finally  no  more  bubbles 
of  air  escape  from  the  lower  end  of  6  into  the  bath  containing 
boiled  water.  To  make  sure  that  the  air  is  all  expelled  from 
the  apparatus,  the  rubber  tubing  at  h'  is  pinched  with  the  thumb 
and  finger,  when,  if  no  air  is  present,  the  liquid  will  quickly  rise 
in  b.  exerting  a  noticeable  pressure.  The  pinch-cock  In!  is  then 
closed  and  the  boiling  is  continued  until  the  50  c.c.  has  been  re- 


458      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

duced  to  a  volume  of  10  c.c.;when  the  flame  is  removed  and  the 
pinch-cock  h"  is  immediately  closed.  The  lower  end  of  a,  which 
dips  into  distilled  water,  is  immediately  filled  with  the  latter  up 
to  the  pinch-cock.  The  vapors  in  the  flask  condense,  forming  a 
vacuum,  as  shown  by  the  closing  together  of  the  rubber  tubing 
at  h'  and  h". 

30  c.c.  of  the  ferrous  chloride  solution  are  poured  into  a  beaker 
and  the  upper  level  is  marked  on  the  outside  with  a  colored  pencil, 
then  20  c.c.  more  are  added  and  the  position  in  the  beaker  is  again 
marked.  The  lower  end  of  the  tube  a  is  placed  in  the  ferrous 
chloride  solution  so  that  it  reaches  below  the  lower  mark  on  the 
beaker,  and,  by  opening  h",  20  c.c.  of  the  solution  are  allowed 
to  pass  into  the  flask  K.  The  beaker  containing  the  ferrous  chloride 
is  then  replaced  by  one  containing  boiled  water.  The  tube  a 
should  not  extend  vertically  into  the  water,  but  should  be  inclined 
as  much  as  possible.  The  specifically  heavier  ferrous  chloride 
solution  in  the  tube  passes  into  the  water,  while  the  latter  takes 
its  place.  When  the  lower  end  of  a  has  become  filled  with  pure 
water  in  this  way,  it  is  dipped  into  a  beaker  containing  hydrochloric 
acid  (sp.  gr.  1.1)  and  about  20  c.c.  of  the  acid  are  allowed  to  flow 
into  K,  and  finally  3-4  c.c.  of  water  are  added  to  replace  the  acid 
in  a.  A  50-c.c.  measuring-cylinder  is  now  filled  with  boiled  water, 
placed  over  the  lower  end  of  b  as  shown  in  the  figure,  and  the  con- 
tents of  the  flask  K  are  heated  fifteen  minutes  on  the  water-bath,* 
then  heated  to  boiling  with  a  free  flame.  As  soon  as  the  com- 
pressed rubber  tubing  begins  to  expand  hf  is  opened,  but  the  rubber 
tubing  is  at  the  same  time  pinched  between  the  thumb  and  finger. 
As  soon  as  the  liquid  no  longer  rises  in  6,  the  hand  is  removed  from 
the  rubber  tubing  and  the  nitric  oxide  begins  to  collect  slowly  in 
the  measuring-tube.  After  half  of  the  liquid  has  evaporated 
there  is  no  further  evolution  of  nitric  oxide  to  be  noticed,  although 
the  brown  color  of  the  solution  shows  that  the  gas  has  not  been 
completely  expelled.  In  order  to  accomplish  this,  the  flame  is  re- 
moved, hf  is  closed,  and  the  liquid  in  K  allowed  to  cool.  By  means 
of  the  vacuum  thus  produced  the  remainder  of  the  nitric  oxide 
is  expelled  from  the  solution  and  the  boiling  is  once  more  repeated, 
with  the  same  precautions  as  before,  until  the  lower  mark  is 
*  The  heating  on  the  water-bath  is  necessary,  as  otherwise  a  little  nitric 
will  distil  over  and  not  be  reduced.  A.  Wegelin,  Inaug.  Dissert.  Zurich,  1907. 


DETERMINATION  OF  NITRIC  ACID  AS  NITRIC  OXIDE.        459 

reached.  The  flame  is  removed,  h'  is  closed,  and  the  measuring- 
tube  containing  the  nitric  oxide  is  placed  in  a  cylinder  containing 
pure  water  at  the  temperature  of  the  room.  To  prevent  the  tube 
containing  the  gas  from  sinking,  its  upper  end  is  encased  in  a  large 
cork  so  that  it  floats  on  the  water.  After  standing  fifteen  to 
twenty  minutes  the  tube  is  raised  by  means  of  the  cork  until 
the  level  of  the  liquid  within  stands  at  the  same  height  as  that 
in  the  cylinder  without,  and  the  volume  of  the  gas  is  read.  At 
the  same  time  the  temperature  of  the  water  is  taken  and  the  barom- 
eter reading  is  noted. 

The  volume  thus  obtained  is  reduced  to  0°  C.  and  760  mm. 
pressure.  If  the  temperature  was  t°,  the  barometer  reading  B 
millimeters,  and  w  the  tension  of  aqueous  vapor  at  t°,  then  the 
reduced  volume  is 

V  ^^-w)273  * 
°~    760(273+0" 

Now  50  c.c.  of  the  standard  potassium  nitrate  solution  con- 
tain 0.1011  gm.  of  KXO3  corresponding  to  0.06201  gm.  of  XO3> 
so  that  the  volume  VQ  of  the  nitric  oxide  corresponds  to  0.06201 
gm.  N03. 

The  same  procedure  is  now  followed  with  50  c.c.  of  the  solu- 
tion of  the  unknown  nitrate,  which  should  be  prepared  so  that 
the  amount  of  nitric  oxide  evolved  will  be  about  the  same  as  that 
from  50  c.c.  of  the  standard  solution.  If  at  t'°  C.  and  B^  mm. 
pressure  the  volume  V  of  nitric  oxide  is  obtained,  and  wl  is  the 
tension  of  aqueous  vapor  at  t°,  then  the  reduced  volume  of  the 
nitrogen  will  be  as  before 


0 


760(273+0  * 
The  following  proportion  now  holds: 
70  :  0.0620  1=V'Q:  x 

7*0.06201 
x=  _<>      -  =  gm.  XO3  in  50  c.c.  of  solution. 

Vo 

*  Three  or  four  experiments   are  performed  with  the  standard  solution, 
and  the  mean  value  is  used. 


460     GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

Remark. — It  is  not  permissible  to  compute  directly  the  weight 
of  NO3  which  corresponds  to  the  volume  of  nitric  oxide  obtained, 
for  some  nitric  oxide  always  remains  in  the  flask,  so  that  low 
values  would  result.  This  error  is  eliminated  by  the  above  pro- 
cedure. 

L.  L.  de  Koninck  *  has  devised  an  apparatus  which  prevents 
the  liquid  from  sucking  back  into  the  decomposition-flask  and 
at  the  same  time  permits  the  carrying  out  of  a  number  of  deter- 
minations one  after  the  other  without  cleaning  the  apparatus 
or  boiling  it  free  from  air  in  the  meantime. 

Determination  of  Nitric  Acid  in  a  Drinking-water. 

From  100  to  300  c.c.  of  the  water  are  evaporated  to  40-50  c.c. 
in  a  porcelain  dish,  a  few  drops  of  methyl  orange  are  added,  fol- 
lowed by  dilute  hydrochloric  acid,  free  from  nitrate,  until  the  solu- 
tion is  pink  in  color.  Sodium  carbonate  solution  is  now7  added 
until  the  liquid  is  barely  alkaline  (it  becomes  yellow)  and  the  con- 
tents of  the  flask  are  washed  into  the  decomposition-flask  K, 
Fig.  79,  and  analyzed  as  described  on  page  457  with  the 
difference  that,  instead  of  collecting  the  gas  over  water,  a  10  per 
cent,  solution  of  sodium  hydroxide  is  used,  to  make  sure  that  the 
carbonic  acid  which  is  set  free  is  completely  absorbed. 

After  the  experiment  has  been  performed  with  the  water  to 
be  analyzed,  it  is  repeated  with  an  amount  of  the  standard  solu- 
tion sufficient  to  evolve  about  the  same  quantity  of  nitric  oxide. 
The  analysis  is  then  computed  as  before. 

Remark. — In  drinking-water  the  neutralization  of  the  evap- 
orated sample  is  not  absolutely  necessary,  except  in  the  case  of 
alkaline  mineral  waters ;  in  that  case  the  introduction  of  the  hydro- 
chloric acid  would  otherwise  cause  such  a  violent  evolution  of 
carbon  dioxide  that  the  flask  might  crack. 

CHLORIC  ACID,  HC1O,.    Mol.  Wt.  84.47. 

Forms:  Silver  Chloride,  AgCl,  besides  volumetric  and  gasometric 

methods. 

In  order  to  determine  chloric  acid  as  silver  chloride  it  must 
previously  be  reduced  to  chloride  by  means  of  ferrous  sulphate  or 
zinc. 

*  Z.  anal.  Chem.,  33,  300  (1894).  See  also  Liechti  and  Hitter,  ibid.,  42 
205,  (1903). 


CHLORIC  ACID.  461 

Reduction  by  means  of  Ferrous  Sulphate. 

About  0.3  gm.  of  the  salt  is  dissolved  in  100  e.c.  of  water, 
treated  with  50  c.c.  of  a  10  per  cent,  solution  of  crystallized  ferrous 
sulphate,  heated  with  constant  stirring  till  it  begins  to  boil,  and 
kept  at  this  temperature  for  fifteen  minutes.  After  cooling, 
nitric  acid  is  added  until  the  deposited  basic  ferric  salt  is  dis- 
solved, when  the  chloride  is  precipitated  by  means  of  silver  nitrate 
and  weighed  after  the  usual  treatment. 

One  grain  of  silver  chloride  corresponds  to  0.8550  gin.  KC103. 

Reduction  with  Zinc. 

Although  chlorates  are  reduced  in  neutral  solution  by  means 
of  zinc  or  Devarda's  alloy,  it  is  not  advisable  to  effect  the  reduc- 
tion in  this  way  for  quantitative  purposes.  The  same  end  is 
reached  more  expeditiously  by  adding  zinc-dust  to  the  acetic 
acid  solution.  The  dilute  chlorate  solution  is  treated  with  acetic 
acid  until  it  reacts  distinctly  acid,  an  excess  of  powdered  zinc  is 
added,  and  the  solution  boiled  for  one  hour.  After  cooling,  nitric 
acid  is  added  in  sufficient  quantity  to  dissolve  all  of  the  excess  of 
zinc,  after  which  the  solution  is  filtered  if  necessary  and  the  chloride 
precipitated  and  determined  as  silver  chloride. 

Remark. — Both  methods  afford  exact  results,  but  the  former 
is  to  be  preferred,  fo_r  it  is  accomplished  in  less  time. 

Chlorates  are  not  quantitatively  decomposed  into  chlorides 
by  ignition  in  open  vessels  or  in  a  current  of  carbon  dioxide.  Some 
chlorine  and  a  little  alkali  is  always  lost,  so  that  even  when  the 
residue  is  evaporated  with  kydrochloric  acid,  too  low  results  are 
obtained.  L.  Blangey,  working  in  the  author's  laboratory,  ob- 
tained results  which  were  from  0.3  to  1.1  per  cent,  below  the  theo- 
retical value. 

According  to  the  two  following  methods,  the  decomposition 
of  alkali  chlorate  into  chloride  is  quantitative. 

(a)  By  Evaporation  with  Hydrochloric  Acid. 

The  chlorate  contained  in  a  weighed  porcelain  crucible  is  covered 
with  hydrochloric  acid  (1:3).  A  watch-glass  is  placed  upon  the 
crucible,  and  the  contents  of  the  latter  are  heated  on  the  water- 


462      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

bath  until  the  evolution  of  chlorine  ceases.  The  liquid  on  the  lower 
surface  of  the  watch-glass  is  then  washed  into  the  crucible,  and 
its  contents  are  evaporated  to  dryness  on  the  water-bath.  The 
cover  is  placed  upon  it  and  it  is  then  gently  ignited  over  a  free 
flame  until  the  decrepitation  ceases.  After  cooling  in  a  desic- 
cator, the  crucible  is  again  weighed.  In  this  way  L.  Blangey 
obtained,  as  a  mean  of  four  experiments,  100.02  per  cent,  of  the 
theoretical  value. 

(b)  By  Ignition  with  Ammonium  Chloride. 

The  alkali  chlorate  is  mixed  in  a  porcelain  crucible  with  three 
times  as  much  pure  ammonium  chloride,  covered  with  a  watch- 
glass,  and  heated  over  a  free  flame,  kept  in  constant  motion,  until 
the  ammonium  chloride  is  completely  removed.  The  crucible 
is  then  weighed.  As  a  mean  of  two  experiments,  L.  Blangey 
obtained  100.06  per  cent,  of  the  theoretical  value. 

PERCHLORIC  ACID,  HC1O4.    Mol.  Wt.  100.47.' 

Form:  Silver  Chloride,  AgCl. 

Perchlorates  cannot  be  reduced  to  chloride  by  means  of  ferrous 
sulphate,  zinc,  or  by  repeated  evaporation  with  concentrated 
hydrochloric  acid.*  On  ignition,  some  chlorine  and  alkali  chloride 
(are  lost,  so  that  an  error  amounting  to  as  much  as  1  per  cent,  may 
be  expected.  On. the  other  hand,  Winteler  has  shown  that  per- 
chlorates  may  be  changed  to  chlorides  by  heating  with  concen- 
trated nitric  acid  and  silver  nitrate  in  a  closed  tube  (see  Carius' 
method  for  determining  chlorine  in  organic  substances,  page  325) , 
while  L.  Blangey  found  that  ignition  with  ammonium  chloride 
would  accomplish  the  same  result. 

Decomposition    of   Perchlorates   by   Ignition   with   Ammonium 

Chloride. 

By  twice  igniting  an  intimate  mixture  of  0.5  gm.  potassium 
perchlorate  with  1 J  to  2  gms.  of  ammonium  chloride  f  in  a  platinum 

*  On  evaporating  with  hydrochloric  acid  there  is  a  loss  without  any  evo- 
lution .of  chlorine;  it  must  be  due  to  the  volatilization  of  small  amounts  of 
perchloric  acid. 

f  When  2  gms.  of  NH4C1  are  used,  one  and  one-Lalf  to  two  hours  are  nec- 
essary. 


PERCHLORIC,   CHLORIC,  AND  HYDROCHLORIC  ACIDS.       463 

crucible  covered  with  a  watch-glass,  the  former  is  completely 
changed  to  chloride.  Care  should  be  taken  not  to  melt  the  residual 
chloride,  for  in  that  case  the  platinum  is  attacked,  although  the 
accuracy  of  the  results  is  not  affected.  Blangey  obtained  in  two 
experiments  100.06  and  100.08  per  cent,  of  the  theoretical  values. 
It  is  worth  mentioning  that  complete  decomposition  could 
not  be  effected  by  igniting  three  times  in  a  porcelain  *  crucible; 
the  platinum  evidently  plays  the  part  of  a  catalyser,  as  was  proved 
by  the  following  experiment:  0.4767  gm.  of  KC1O4  was  mixed 
in  a  porcelain  crucible  with  1J  gm.  of  NH4C1,  and  1  c.c.  of  hydro- 
chlorplatinic  acid  (containing  0.0918  gm.  Pt)  was  added.  After 
evaporating  to  dryness  on  the  water-bath,  the  ammonium  chloride 
was  completely  expelled  and  the  residue  was  ignited  twice  more 
with  the  same  amount  of  the  latter.  The  residue  of  potassium 
chloride  then  weighed  0.2572  gm.,  corresponding  to  100.24  per 
cent,  of  the  theoretical  amount. f 

Determination  of  Perchloric  Together  with  Chloric  Acid. 

In  one  portion  the  chlorate  is  reduced,  as  described  on  page  461, 
with  ferrous  sulphate,  and  the  chloride  formed  determined  as 
silver  chloride.  A  second  portion  is  ignited  in  an  old  platinum 
crucible  (or  in  one  of  porcelain)  with  the  addition  of  1  c.c.  of  hydro- 
chlorplatinic  acid  and  three  times  as  much  ammonium  chloride 
(as  described  above).  In  this  way  the  total  amount  of  chlorine 
is  obtained  and  from  these  data  the  amount  of  each  acid  can  be 
calculated. 

Determination  of  Perchloric,  Chloric,  and  Hydrochloric  Acids 
in  the  Presence  of  One  Another. 

The  three  acids  are  assumed  to  be  present  in  the  form  of  their 
alkali  salts. 

*Thus  on  igniting  0.4395  gm.  KC1O4  with  2  gms.  NH4C1  a  residue  of 
0.3205  gm.  was  obtained  instead  of  one  weighing  0.2365  gm. 

f  There  is  often  a  slight  deposit  of  alkali  chloride  upon  the  cover-glass. 
To  determine  this,  the  glass  together  with  the  deposit  is  weighed,  then  the 
glass  is  washed,  dried,  and  again  weighed;  the  difference  between  the  «wo 
weights  represents  the  amount  of  alkali  chloride.  This  rarely  amounts  to 
more  than  a  fraction  of  a  milligram,  and  if  the  ignition  was  performed  with 
care,  there  will  be  no  deposit  at  all  upon  the  glass. 


464     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOID*. 

In  one  portion  the  chloride-chlorine  is  determined  by  precipita- 
tion with  silver  nitrate.  In  a  second  sample  the  chlorate  and 
chloride-chlorine  are  determined  after  the  former  has  been  reduced 
to  chloride  by  means  of  ferrous  sulphate.  The  total  amount 
of  chlorine  present  is  determined  in  a  third  portion  after  ignition 
with  ammonium  chloride. 


GROUP  VI. 

SULPHURIC,  HYDROFLUORIC,  AND    HYDROFLUOSILICTC  ACIDS. 

SULPHURIC  ACID,  H,SO4.    Mol.  Wt.  98.09. 
Form:  Barium  Sulphate,  BaS04. 

Theoretically  the  gravimetric  determination  of  sulphuric  acid 
is  extremely  simple,  it  being  only  necessary  to  precipitate  with 
barium  chloride,  filter  and  weigh  the  barium  sulphate.  Prac- 
tically, however,  it  is  a  process  connected  with  many  difficulties. 

According  to  the  manner  of  precipitating  barium  sulphate, 
the  composition  of  the  precipitate  varies  in  such  a  way  that  some- 
times the  results  are  too  high  and  sometimes  too  low. 

Errors  which  may  Occur  in  the  Precipitation  of  Barium  Sulphate.* 
/.  In  the  Precipitation  of  Barium  Chloride  with  Pure  Sulphuric  Acid. 

If  a  dilute,  slightly  acid  solution  of  barium  chloride  is  treated 
at  the  boiling  temperature  with  an  excess  of  dilute  sulphuric 
acid,  the  precipitate  contains  all  of  the  barium  except  a  very  small, 
negligible  amount.'  If,  however,  the  precipitate  is  weighed,  the 
result  is  invariably  too  low;  and  this  is  true  even  when  the  solution 
is  evaporated  to  dryness  in  order  to  recover  the  last  traces  of 
barium.  The  precipitate  always  contains  barium  chloride  in  a 
form  which  cannot  be  removed  by  washing.  A  mixture,  there- 
fore, of  barium  sulphate  and  barium  chloride  is  weighed,  and  as 
the  molecular  weight  of  the  latter  is  less  than  that  of  the  former, 
the  result  must  be  too  low.  In  order  to  obtain  accurate  results  the 
chlorine  combined  with  barium  in  the  precipitate  must  be  replaced 

*  See  the  interesting  article  by  M.  J.  van't  Kruys  on  the  determination  of 
sulphuric  acid  in  the  presence  of  various  salts  which  affect  the  result.  Z.  anal, 
Chem.,  1910  ,393. 


SULPHURIC  ACID.  465. 

by  S04,*  and  this  is  easily  accomplished  by  moistening  the  precip- 
itate with  concentrated  sulphuric  acid,  and  heating  until  the 
excess  of  the  latter  is  removed  by  volatilization. 

Xot  only  is  barium  chloride  carried  down  with  barium  sulphate, 
but  all  barium  salts  as  well,  especially  the  chlorate  and  nitrate. 
These  are,  however,  readily  changed  to  sulphate  by  the  above 
treatment  with  concentrated  sulphuric  acid.  It  is  immaterial  in 
the  estimation  of  barium  how  the  precipitation  is  effected;  whether 
the  sulphuric  acid  is  added  quickly,  or  drop  by  drop,  the  results 
are  always  the  same. 

II.  In  the  Precipitation  of  Pure  Sulphuric  acid  with  Barium  Chloride. 

This  is  the  reverse  process,  but  in  this  case  it  is  not  a  matter 
of  indifference  whether  the  barium  chloride  is  added  slowly,  drop 
by  drop,  or  rapidly  all  at  one  time.  In  the  first  instance,  the 
results  are  very  near  the  truth  without  applying  any  correction 
whatsoever;  in  the  latter  instance,  too  high  results  are  obtained, 
because  by  the  rapid  addition  of  the  reagent  much  more  barium 
chloride  is  carried  down  with  the  precipitate  than  when  the 
reagent  is  added  very  slowly. 

In  order  to  obtain  the  true  weight  of  barium  sulphate,  it  is 
necessary  to  make  a  deduction  for  the  amount  of  barium  chloride 
contained  in  the  precipitate  and  to  add  the  weight  of  barium 
sulphate  remaining  in  solution. 

The  chlorine  contained  in  the  precipitate  can  be  determined  in 
several  different  ways. 

1.  The  precipitate  is  fused  with  four  times  as  much  pure 
sodium  carbonate,  the  melt  extracted  with  hot  water,  the  solution 
filtered,  acidified  with  nitric  acid,  and  the  chlorine  precipitated 
with  silver  nitrate  which  is  filtered  off  and  weighed.    (Cf.  p.  320.) 

2.  Still  more  accurate  is  the  process  of  Hulett  and  Duschack  * 
The  greater  part  of  the  ignited  precipitate  of  barium  sulphate  is 
placed  in  a  U-tube  of  which  one  arm  is  drawn  out  into  a  thin, 
right-angled,  gas  delivery  tube.     Concentrated  sulphuric  acid  is 
added  to  the  precipitate  and  the  mixture  is  heated  by  placing  the 
U-tube  in  hot  water.     The  barium  sulphate  dissolves  readily  in 

*  Z.  anorg.  Chem.,  40,  196  (1904). 


465       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

the  hot  concentrated  sulphuric  acid  and  the  barium  chloride 
present  is  decomposed.  In  order  to  determine  the  amount  of 
hydrochloric  acid  set  free,  a  slow  stream  of  air,  which  has  pre- 
viously passed  through  caustic  potash  solution,  is  led  through  the 
tube,  which  has  the  drawn-out  end  of  the  latter  dipping  into  a 
stout  test-tube  containing  0.01  N.  silver  nitrate  solution.  After 
two  or  two  and  one-half  hours  all  of  the  hydrochloric  acid  will  have 
been  expelled  from  the  sulphuric  acid. 

The  decomposition  apparatus  is  then  removed,  the  gas  delivery 
tube  washed  out  with  a  little  water,  and  the  silver  remaining  in 
solution  is  determined  volumetrically  (cf .  pp.  702-05) . 

For  the  determination  of  the  dissolved  barium  sulphate  the 
filtrate  from  the  first  precipitation  is  evaporated  to  dry  ness,* 
the  residue  moistened  with  a  few  drops  of  concentrated  hydro- 
chloric acid,  taken  up  with  water  and  the  slight  precipitate  of 
barium  sulphate  filtered  off  and  weighed. 

Calculation  of  the  true  weight  of  Barium  Sulphate. — If  the  weight 
of  the  first  precipitate  of  crude  barium  sulphate  is  a,  the  weight 
of  the  barium  chloride  contained  in  this  precipitate,  as  deter- 
mined by  titration  of  the  amount  of  chlorine,  is  6,  and  the  amount 
of  barium  sulphate  in  solution  is  c,  then  a  —  b  +  c  represents  the 
weight  of  pure  barium  sulphate. 

Experience  has  shown,  however,  that  when  pure  sulphuric 
acid  is  precipitated  by  means  of  dilute  barium  chloride  solution 
added  drop  by  drop,  the  errors  b  and  c  are  approximately  equal 
and  counterbalance  one  another  so  that  the  weight  a  is  very  close 
to  that  of  the  pure  barium  sulphate. 

///.  In  the  Precipitation  of  Sulphates  with  Barium  Chloride. 

Here  the  relations  are  far  more  complicated  than  in  the  pre- 
cipitation of  pure  sulphuric  acid,  partly  because  the  barium 
sulphate  is  much  more -soluble  in  salt  solutions  than  in  water 
containing  a  little  acid,  and  partly  because  of  the  tendency  of 
barium  sulphate  to  occlude  not  only  barium  chloride  but  many 
other  salts  as  well.  Solutions  of  chromium  sulphate  are  either 

*  During  all  such  work  care  should  be  taken  to  prevent  sulphuric  acid 
contamination  from  the  air  in  the  laboratory.  The  evaporation  should 
therefore  take  place  on  the  steam  bath  or  steam  table. 


SULPHURIC  ACID.  467 

violet  or  green.  From  the  boiling-hot  green  solution  only  one- 
third  of  the  sulphuric  acid  is  precipitated,  the  remainder  probably 
being  present  in  the  form  of  a  complex  chromium  sulphate  cation;  * 
on  cooling  there  is  a  tendency  for  the  green  solution  to  become 
violet  and  after  some  time  all  of  the  sulphuric  acid  is  precipitated. 
The  precipitation  of  barium  sulphate  in  the  presence  of  ferric  iron 
has  been  much  studied.  In  the  boiling  hot  solution  all  of  the 
sulphuric  acid  is  not  precipitated  and  considerable  iron  is  thrown 
down  with  the  barium  sulphate  and  furthermore  the  precipitate 
then  loses  SOs  on  ignition.  Since  ferric  oxide  weighs  much  less 
than  an  equivalent  weight  of  barium  sulphate  sometimes  the  results 
are  as  much  as  10  per  cent,  too  low.  On  the  other  hand,  Kiister 
and  Thiel,f  were  able  to  get  satisfactory  results  (1)  by  precipitating 
the  sulphuric  acid  from  such  a  solution  in  the  cold,  or  (2)  by 
slowly  adding  the  ferric  chloride  and  sulphuric  acid  solution  to  the 
hot  solution  of  barium  chloride,  or  (3)  by  precipitating  the  iron 
by  an  excess  of  ammonia,  heating,  and  adding  barium  chloride  to 
the  solution  without  filtering  off  the  ferric  hydroxide,  and  finally 
dissolving  the  latter  in  dilute  hydrochloric  acid. 

Most  chemists,  however,  deem  it  advisable  to  remove  trivalent 
metals  before  attempting  to  determine  the  sulphuric  acid.  This 
is  accomplished  in  the  case  of  ferric  iron  by  adding  a  liberal 
excess  of  ammonia  to  the  dilute  slightly  acid  solution  which  is  at  a 
temperature  of  about  70°.  If  from  5-7  c.c.  of  concentrated 
ammonia  (sp.  gr.  0.90)  is  added  in  excess  of  the  amount  required 
for  neutralization,^  the  precipitate  is  not  likely  to  contain  any 
basic  ferric  sulphate.  If,  on  the  other  hand,  the  solution  is  barely 
neutralized  with  ammonia,  the  precipitate  produced  will  invariably 
contain  some  sulphate. 

The  bivalent  metals  are  occluded  to  a  much  less  extent,  so 
that  it  is  not,  as  a  rule,  necessary  to  remove  them.  On  the  other 
hand,  in  the  presence  of  considerable  amounts  of  bivalent  metal 
with  relatively  small  amounts  of  sulphuric  acid,  the  error  arising 
from  occlusion  is  likely  to  be  large,  so  that  it  is  better  to  remove 
the  bivalent  metals  in  all  such  cases. 

*  Recoura,  Comptes  rendus,  113,  857;   114,  477. 

t  Z.  Anorg.  Chem.,  22,  424. 

j  Pattinson,  J.  Soc.  Chem.  Ind.,  24,  7. 


468       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

In  the  presence  of  nitric  or  chloric  acid  the  barium  sulphate 
precipitate  will  contain  considerable  quantities  of  barium  chlorate ' 
and  nitrate  which  it  is^  impossible  to  remove  by  washing  with  hot 
water.  These  acids,  therefore,  must  be  decomposed  by  evapora- 
tion with  hydrochloric  acid  before  attempting  to  precipitate  the 
sulphuric  acid. 

In  ordinary  chemical  practice  it  is  usually  a  question  of  deter- 
mining sulphuric  acid  in  a  solution  containing  considerable 
amounts  of  ammonium  or  alkali  chloride, -•ammonium  or  alkali 
sulphate,  and  some  free  hydrochloric  acid.  Now  ammonium  and 
alkali  sulphates  are  also  occluded  by  barium  sulphate,  and  the 
amount  of  occulsion  increases  as  the  solution  is  more  concentrated 
with  respect  to  these  substances.  For  this  reason  it  is  evident 
that  barium  sulphate  should  always  be  precipitated  in  a  very 
dilute  solution.  Furthermore,  a  small  amount  of  free  hydrochloric 
acid  is  indispensable,  but  larger  amounts  have  a  solvent  effect  upon 
the  precipitate. 

For  an  amount  of  sulphuric  acid  corresponding  to  between 
1  and  2  gms.  of  barium  sulphate,  the  precipitation  should  take 
place  in  a  volume  of  between  350  and  400  c.c.  and  in  the  presence 
of  hydrochloric  acid  amounting  to  1  c.c.  of  sp.  gr.  1.17. 

If  a  neutral  solution  is  at  hand,  it  is  diluted  to  a  volume  of 
350  c.c.  and  1  c.c.  of  concentrated  hydrochloric  acid  is  added. 

An  alkaline  solution  is  carefully  neutralized  with  hydrochloric 
acid,  using  methyl  orange  as  indicator,  1  c.c.  of  concentrated 
hydrochloric  acid  is  added  in  excess,  and  the  solution  is  diluted 
to  350  c.c. 

Finally,  in  the  case  of  an  acid  solution,  it  is  either  evaporated 
to  dry  ness,  the  residue  moistened  with  1  c.c.  of  concentrated 
hydrochloric  acid  and  350  c.c.  of  water  added,  or,  with  methyl 
orange  as  indicator,  the  solution  is  neutralized  with  ammonia, 
treated  with  1  c.c.  of  concentrated  hydrochloric  acid,  and  diluted 
to  350  c.c. 

After  the  solution  has  been  prepared  in  accordance  with  the 
above  directions  it  is  ready  for  the 


SULPHURIC  ACID.  469 

Precipitation  of  Sulphuric  Acid  in  the  Presence  of  Ammonium  or 
Alkali  Salts  according  to  E.  Hintz  and  H.  Weber. 

The  solution  is  heated  to  boiling,  and  then  for  each  gram  of 
barium  sulphate  precipitate  10  c.c.  of  normal  barium  chloride 
solution  are  taken,  diluted  'to  100  c.c.,  the  solution  heated  to 
boiling,  and  added  all  at  one  time  to  the  hot  sulphate  solution 
which  is  being  stirred  continuously.  After  the  solution  has  stood 
for  half  an  hour,  best  in  a  warm  place,  it  is  filtered,  washed  with 
hot  water  and  ignited  (of.  p.  74) .  The  use  of  a  Gooch  or  Munroe 
crucible  is  to  be  recommended. 

Remarks. — In  the  presence  of  ammonium  salts  the  pre- 
cipitation of  the  barium  sulphate  should  not  be  effected,  as  is 
otherwise  desirable,  by  the  cautious  addition  of  the  barium 
chloride,  for,  as  Hintz  and  Weber  have  shown,  this  leads  to  low 
results  whereas  the  occlusion  caused  by  the  rapid  addition  of  the 
barium  chloride  counterbalances  this  error. 

Under  no  circumstances  should  a  precipitate  of  barium 
sulphate  be  heated  over  a  blast  lamp,  for  in  that  case  sulphuric 
anhydride  would  be  evolved  from  the  barium  sulphate  itself. 

To  explain  the  occlusion  of  barium  chloride  by  barium  sulphate, 
Hulett  and  Duschak  *  have  suggested  that  perhaps  the  precipitate 
may  contain  salts  such  as  BaCl.HSO4,  (BaCl)2SO4,  and  Ba(HS04)2 
and  Folin  f  believes  that  such  is  this  case  because  some  of  his 
precipitates  have  lost  SOs  on  ignition  while  others  have  lost 
HC1.  He  also  suggests  the  possibility  of  salts  such  as  Ba(KSO4)2 
being  precipitated. 

Determination  of  Sulphuric  Acid  in  Insoluble  Sulphates. 

Calcium  and  strontium  sulphates  are  decomposed  by  long 
digestion  with  ammonium  carbonate  solution,  but  barium  sulphate 
is  not.  The  latter  is  mixed  with  four  times  as  much  sodium  car- 
bonate, fused  in  a  platinum  crucible,  the  melt  extracted  with 
water,  and  the  barium  carbonate  residue  washed  with  sodium 
carbonate  solution.  After  acidifying  the  nitrate  with  hydro- 

*  Z.  Anorg.  Chem.,  40,  196  (1904). 
f  J.  Biol.  Chem.,  1,  131  (1905). 


470      GRAVIMETRIC   DETERMINATION  OF  THE  METALLOIDS. 

chloric  acid  and  boiling  off  the  carbon  dioxide,  the  sulphuric  acid 
is  precipitated  as  usual. 

Lead  sulphate  is  boiled  with  sodium  carbonate  solution;  after 
cooling,  the  solution  is  saturated  with  carbon  dioxide  and  filtered. 
The  lead  remains  behind  as  carbonate,  while  the  nitrate  contains 
all  of  the  sulphuric  acid. 

For  the  determination  of  sulphuric  acid  in  silicates,  the  finely 
powdered  substance  is  fused  with  six  times  as  much  sodium  car- 
bonate, the  melt  is  extracted  with  water,  the  filtrate  acidified 
with  hydrochloric  acid  and  evaporated  to  dryness  in  order  to 
dehydrate  the  silica.  The  residue  is  moistened  with  a  little 
concentrated  hydrochloric  acid,  taken  up  in  hot  water,  and  the 
silicic  acid  filtered  off;  the  sulphuric  acid  is  determined  in  this 
filtrate. 


Determination  of  Sulphuric  Acid  in  the  Presence  of  Soluble 

Sulphides. 

The  substance  is  placed  in  a  flask,  the  air  replaced  by  carbon 
dioxide,  dilute  hydrochloric  acid  is  added,  and  the  solution  boiled 
while  carbon  dioxide  is  passed  through  it  until  all  of  the  sulphide 
has  been  expelled.  The  sulphuric  acid  is  then  precipitated  from 
the  solution. 

This  determination  is  used  for  the  analysis  of  cements.  In 
this  case,  however,  the  hydrochloric  acid  solution  will  contain 
much  calcium  as  well  as  iron  and  aluminium,  so  that  these  metals 
are  precipitated  by  the  addition  of  ammonia  and  ammonium 
carbonate  and  the  sulphuric  acid  determined  in  the  filtrate. 

If  it  is  desired  to  determine  the  amount  of  sulphide-sulphur, 
the  substance  is  covered  with  bromine  water  until  the  color  of 
the  bromine  is  permanent,  hydrochloric  acid  is  added,  and  the 
solution  boiled  to  expel  the  excess  of  the  bromine.  The  iron,  alu- 
minium, and  calcium  are  precipitated  by  ammonia  and  ammonium 
carbonate,  and  the  total  sulphur  is  determined  in  the  filtrate.  The 
difference  between  the  two  results  represents  the  amount  of  sul- 
phur present  as  sulphide.  For  the  volumetric  determination 
of  sulphuric  acid  consult  Part  II. 


HYDROFLUORIC  ACID.  471 

HYDROFLUORIC  ACID,  HF.    Mol.  Wt.  20.01. 

Forms:  Calcium  Fluoride,  CaF2;  Silicon  Fluoride,    SiF4,  besides 
volumetric  and  gasometric  methods. 

i.  Determination  as  Calcium  Fluoride. 

If  the  solution  contains  free  hydrofluoric  acid  or  an  acid  fluoride, 
sodium  carbonate  is  added  until  the  reaction  is  alkaline  and  from 
one-fourth  to  one-fifth  as  much  more  in  excess.*  To  solutions  of  neu- 
tral fluorides  about  1  c.c.  of  double-normal  sodium  carbonate  solution 
is  added.  The  alkaline  solution  is  heated  to  boiling,  precipitated 
by  means  of  an  excess  of  calcium  chloride  solution,  filtered,  and 
thoroughly  washed  with  hot  water.  The  precipitate  consisting  of  the 
fluoride  and  carbonate  of  calcium  is  dried,  as  much  of  it  as  possible 
is  transferred  to  a  platinum  crucible,  the  ash  of  the  filter  is  added, 
and  the  contents  of  the  crucible  are  ignited.f  After  cooling,  the 
mass  is  covered  with  an  excess  of  dilute  acetic  acid,  by  which  the 
lime  is  changed  to  the  soluble  acetate,  while  the  fluoride  is  unaf- 
fected. Aft?r  evaporating  to  dryness  on  the  water-bath,  the  mass 
is  taken  up  in  water,  filtered,  washed,  and  dried. %  After  transfer- 
ring as  much  of  the  dried  precipitate  to  the  crucible  as  possible,  the 
tilter-paper  is  burned,  its  ash  added,  and  after  ignition  the  crucible 
is  again  weighed.  To  confirm  the  result  the  substance  is  treated 
with  a  little  concentrated  sulphuric  acid  (added  cautiously),  and 
after  evaporating  off  the  excess  of  the  latter  and  once  more  igniting, 
the  contents  of  the  crucible  are  weighed  as  calcium  sulphate. 

1  gm.  CaF2  yields  1.7436  gms.  CaSO4. 

Remark. — The  results  are  usually  a  little  low  on  account  of 
the  solubility  of  calcium  fluoride;  100  c.c.  water  dissolves  0.0016 
gm.,  and  100  c.c.  1.5  X.  acetic  acid  dissolves  0.011  gm.  CaF2  at 
the  temperature  of  the  water  bath. 

*  By  the  addition  of  the  excess  of  sodium  carbonate  the  precipitate  of 
calcium  fluoride  will  contain  calcium  carbonate,  and  presence  of  the  latter 
renders  the  precipitate  easy  to  filter.  A  pure  precipitate  of  calcium  fluoride 
is  so  slimy  that  the  pores  of  the  filter  become  so  clogged  that  it  is  almost 
impossible  to  complete  the  filtration. 

f  The  ignition  makes  the  CaF2  denser  and  hence  easier  to  filter. 

%  The  calcium  fluoride  is  not  volatilized  in  an  open  platinum  crucible 
heated  over  a  Bunsen  burner.  Heated  over  the  blast,  there  is  appreciable 
volatilization. 


472        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

Example:  Determination  of  Fluorine  in  Calcium  Fluoride. — 
As  was  stated  in  Vol.  I,  page  391,*  calcium  fluoride  is  not  com- 
pletely decomposed  by  fusing  with  sodium  carbonate;  but  if  the 
fluoride  is  mixed  with  2^  times  as  much  silicic  acid  and  then  fused 
with  6  times  as  much  sodium-potassium  carbonate,  the  greater 
part  of  the  silicic  acid  and  all  of  the  fluorine  will  be  changed  to  sol- 
uble alkali  salts,  while  the  calcium  will  be  left  as  insoluble  calcium 
carbonate.  The  mixture  must  be  heated  gradually,  as  otherwise 
the  evolution  of  carbon  dioxide  may  cause  the  contents  of  the 
crucible  to  run  over  its  sides.  The  thin  liquid  fusion  soon  changes 
to  a  thick  paste  or  only  sinters  somewhat.  On  raising  the  tempera- 
ture, it  is  almost  impossible  to  further  melt  this  mass,  and  it  is 
not  necessary.  In  fact  too  high  a  temperature  is  to  be  avoided  on 
account  of  the  danger  of  losing  some  alkali  fluoride  by  volatilization. 
The  reaction  is  complete  when  there  is  no  further  evolution  of  car- 
bon dioxide.  After  cooling,  the  melt  is  treated  with  water,  the 
insoluble  residue  is  filtered  off  and  thoroughly  washed.  The 
alkaline  solution  containing  all  the  fluorine  and  considerable 
silicic  acid  is  freed  from  the  latter  by  the  addition  of  considerable 
ammonium  carbonate  f  (about  4  gms.  of  the  solid  salt) .  The  liquid 
is  heated  for  some  time  at  about  40°  0.,  allowed  to  stand  over- 
night, and  in  the  morning  the  voluminous  precipitate  is  filtered 
off  and  washed  with  ammonium  carbonate  water.  The  filtrate 
now  contains  only  a  small  amount  of  silicic  acid.  It  is  evaporated 
almost  to  dryness  on  the  water-bath,  {  diluted  with  a  little  water 
and  a  few  drops  of  phenolphthale'in  are  added.  The  liquid  is  colored 
pink  by  the  indicator  and  enough  hydrochloric  acid  is  now  added 
to  make  it  colorless.  The  solution  is  heated  to  boiling,  and  this 
causes  the  reappearance  of  the  pink  color.  After  cooling  the  color 
is  again  discharged  with  hydrochloric  acid,  and  this  operation 
is  repeated  until  finally  the  addition  of  1-1^  c.c.  of  double- 
normal  hydrochloric  acid  is  sufficient  to  effect  the  decolorization. 
It  is  best  to  perform  the  operation  in  a  platinum  dish,  but  if  this 
is  lacking  one  of  porcelain  may  be  used. 

*  Second  edition. 

f  Before  adding  the  ammonium  carbonate,  the  greater  part  of  the  alkali 
carbonate  should  be  neutralized  with  dilute  hydrochloric  acid,  but  care 
should  be  taken  not  to  make  the  solution  acid. 

J  The  liquid  foams  during  the  evaporation  owing  to  the  decomposition 
of  the  excess  of  ammonium  carbonate;  the  evaporating-dish  is  covered  with 
a  watch-glass  until  the  evolution  of  carbon  dioxide  ceases. 


HYDROFLUORIC  ACID.  473 

The  solution  still  contains  traces  of  silicic  acid,  which  are  re- 
moved, as  recommended  by  Berzelius,  as  follows :  The  solution  is 
treated  with  1  or  2  c.c.  of  ammoniacal  zinc  oxide  solution,* 
boiled  until  the  ammonia  is  completely  expelled  and  the  precipi- 
tate of  zinc  silicate  and  oxide  is  filtered  and  washed  with  water. 
An  excess  of  calcium  chloride  is  added  to  the  filtrate  and  the  re- 
sulting precipitate,  consisting  of  calcium  carbonate  and  fluoride,  is 
treated  as  described  on  page  471. 

The  calcium  fluoride  finally  obtained  should  be  tested  for 
fluorine,  for  the  addition  of  calcium  chloride  will  almost  always 
cause  a  precipitation  (cf.  page  471).  which  may  consist  of  calcium 
fluoride  and  phosphate,  or  the  latter  only.  After  weighing  the 
precipitate,  it  is  treated  with  a  few  drops  of  concentrated  sulphuric 
acid  and  covered  with  a  watch-glass  whose  convex  surface  is 
coated  with  a  thin  layer  of  beeswax  with  a  few  lines  scratched 
in  the  latter.  The  crucible  is  allowed  to  stand  this  way  for 
twelve  hours  at  the  ordinary  temperature.  A  little  water  is  then 
poured  upon  the  watch-glass  and  the  crucible  is  heated  over  a  tiny 
flame  until  the  vapors  of  sulphuric  acid  begin  to  be  evolved.  If 
fluorine  is  present  there  will  be  a  distinct  etching  of  the  glass 
where  the  wax  coating  was  removed. 

The  weight  of  the  calcium  fluoride  obtained  should  stand  in  the 
same  relation  to  that  of  calcium  sulphate  obtained  after  treatment 
with  concentrated  sulphuric  acid,  as 

CaF2(78.09)  :CaSO4(136.16). 

This  relation  does  not  hold  exactly  in  practice,  for  it  is  almost 
impossible  to  obtain  a  precipitate  of  calcium  fluoride  absolutely 
free  from  silica. 

Remark. — By  this  method  the  fluorine  present  in  all  fluorides 
can  be  determined,  e.g.,  in  topaz,  lepidolite,  cryolite,  etc.  With 
a  silicate  containing  much  silica,  the  addition  of  silicic  acid  is 

*  Moist  zinc  oxide  is  dissolved  in  ammonia  water.  The  oxide  is  best 
prepare  1  by  dissolving  chemically  pure  zinc  in  hydrochloric  acid,  and  pre- 
cipitating the  zinc  with  potassium  hydroxide;  the  precipitate  is  filtered  and 
washed. 


474      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

unnecessary,  and  the  substance  is  at  once  fused  with  4-5  times 
as  much  sodium-potassium  carbonate;  with  silicates  containing 
little  silica,  from  J-l  part  of  silicic  acid  is  added. 

If  the  substance  contains  phosphoric  acid,  the  fluorine  cannot 
be  determined  by  the  above  method,  because  the  calcium  fluoride 
precipitate  is  then  contaminated  with  calcium  phosphate.  It  is 
then  necessary  to  effect  a 


Separation  of  Phosphoric  and  Hydrofluoric  Acids. 

The  following  method  is  that  of  Rose  as  modified  by  the  author 
and  A.  A.  Koch.*  It  is  based  upon  the  fact  that  silver  phosphate 
is  insoluble  in  water  whereas  silver  fluoride  is  soluble. 

Procedure. — The  alkaline  solution  of  the  two  acids  f  is  carefully 
neutralized  with  nitric  acid  and  then  transferred  to  a  250  c.c. 
calibrated  flask.  A  slight  excess  of  silver  nitrate  is  added,  the 
solution  diluted  to  the  mark,  thoroughly  mixed  and  the  precipitate 
allowed  to  settle  completely.  The  solution  is  then  filtered  through 
a  dry  filter,  but  the  first  10  c.c.  of  the  filtrate  are  rejected,  and  the 
rest  allowed  to  run  into  a  dry  beaker.  Of  this  filtrate,  exactly 
200  c.c.  are  transferred  to  a  250-c.c.  flask  again,  the  excess  of  the 
silver  nitrate  precipitated  by  the  addition  of  some  dissolved 
sodium  chloride,  the  solution  made  up  to  the  mark,  well  shaken 
and  the  precipitate  allowed  to  settle  completely.  This  solution  is 
filtered,  using  the  same  precautions  as  before  and  the  fluorine  is 
determined  in  200  c.c.  of  the  filtrate  as  calcium  fluoride  according 
to  the  directions  on  p.  471. 

If  the  weight  of  the  calcium  fluoride  precipitate  is  p,  that  of 
the  original  substance  is  a,  and  x  is  the  per  cent,  of  fluorine  origi- 
nally present,  then 

76.03  p_M 

~  /c  L  • 


*  Z.  anal.  Chem.,  43,  469  (1904). 

t  It  is  usually  a  matter  of  analyzing  an  aqueous  solution  of  a  sodium 
carbonate  fusion. 


DETERMINATION  OF  FLUORINE  AS  SILICON  FLUORIDE.        475 

Determination  as  Silicon  Fluoride. 

This  method,  proposed  by  Fresenius,  depends  upon  the  fact 
that  many  fluorides  are  decomposed  by  the  action  of  concentrated 
sulphuric  acid  and  silica,  while  the  fluorine  escapes  as  silicon 
fluoride,  which  can  be  absorbed  and  weighed. 

Procedure.  —  The  same  reagents  and  a  very  similar  apparatus 
to  that  described  on  p.  477  is  required  for  this  determination, 
except  that  in  place  of  the  Peligot  tubes  (Fig.  80)  two  weighed 
U-tubes  are  used,*  of  which  the  first  is  filled  with  moistened  pieces 
ofTpumice,  and  the  second  has  one  arm  filled  with  soda  lime  and 
the  other  with  calcium  chloride.  The  analysis  is  carried  out  in 
exactly  the  same  way  as  is  described  for  the  Penfield  method 
(see  below)  but  at  the  end  of  the  experiment  the  two  U-tubes  are 
weighed.  The  increase  in  weight  represents  the  amount  of  SiF4, 
and  from  this  the  amount  of  fluorine  present  is  calculated  as 
follows:*  Assume  that  a  gms.  of  calcium  fluoride  yielded  p  gms. 
of  SiF4.  The  treatment  with  the  concentrated  sulphuric  acid 
caused  the  following  reaction  to  take  place: 

2CaF2+  2H2SO4+  SiO2  =  2CaS04+  2H20+  SiF4, 
consequently  the  following  proportion  holds: 


and  in  per  cent. 


400F    p 
SiF4    'a 
or 


4F 

•  p  =  gms.  fluorine 


=  72.87-  —  =  per  cent,  fluorine. 


*  It  is  best  to  use  U-tubes  for  the   absorption  which  are  provided  with 
ground-glass  stoppers  (see  Fig.  59,  p.  381). 


47<>        GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

Remark. — This  method  is  suitable  for  the  determination  of 
fluorine  in  all  fluorides  which  are  decomposed  by  sulphuric  acid. 
The  analysis  can  be  carried  out  in  the  presence  of  phosphates,  but 
if  carbonates  are  present  they  should  be  decomposed  by  ignition 
before  the  treatment  with  sulphuric  acid.  According  to  K. 
Daniel,*  exact  results  are  obtained  only  when  the  decomposition 
of  the  fluoride  takes  place  at  the  temperature  at  which  sulphuric 
acid  boils. 

For  the  determination  of  fluorine  in  topaz  and  micas,  the 
method  is  not  suitable. 

Determination  of  Fluorine  as  Hydrofluosilicic  Acid,  according 
to  S.  L.  Penfield.f 

Modified  by  Treadwell  and  Koch.% 


Principle.  —  Penfield  expels  the  fluorine  as  silicon  fluoride  in 
exactly  the  same  way  as  in  the  method  of  Fresenius  (page  475), 
but  the  gas  is  absorbed  in  50  per  cent,  alcoholic  potassium  chloride 
solution.  By  contact  with  water  the  silicon  fluoride  is  decomposed 
into  hydrofluosilicic  and  silicic  acids.  The  former  unites  with  the 
potassium  chloride,  forming  potassium  silicofluoride,  insoluble  in 
50  per  cent,  alcohol: 

H2SiF6+  2KC1  =  K2Si  F.+  2HC1, 

and  sets  free  an  equivalent  amount  of  hydrochloric  acid;  the 
latter  is  titrated  with  N/5  sodium  hydroxide  solution,  with 
cochineal  is  an  indicator.  For  the  calculation  the  following  pro- 
portion holds: 


1000  c.c.  N/5  HCl=tV  mo1-  CaF2=|F. 
1  c.c.  N/5  NaOH  =  0.0234  gm.  CaF2  or  0.0114  gm.  F. 


*Z.  anorg.  Chem.,  38,  257  (1904). 

t  Chem.  News,  39,  p.  179;  also  Am.  Chem.  Jour.,  1,  p.  27 

v  7..  anal.  Chem.,  43,  469  (1904). 


DETERMINATION  OF  FLUORINE  AS  HYDROFLUOS1LIC1C  ACID.   477 

Requirements. — 1.  Pure  Quartz  Powder.  Pieces  of  pure  rock 
crystal  are  placed  in  a  platinum  crucible,  heated  strongly  over  the 
blast  lamp,  and  then  thrown  into  cold  water.  After  this  treatment 
it  is  very  easy  to  reduce  the  quartz  to  a  fine  powder  by  grinding  in 
an  agate  mortar.  The  powder  is  ignited,  and  while  still  warm  is 
transferred  to  a  flask,  fitted  with  ground-glass  stopper.  The  open 
flask  and  its  contents  are  allowed  to  cool  in  a  desiccator,  after 
which  the  flask  is  stoppered  and  set  aside. 

'2.  Sea  Sand.  The  purest  sea  sand  is  treated  with  boiling, 
concentrated  sulphuric  acid,  washed,  dried,  ignited,  and  cooled 
in  a  desiccator. 

3.  Anhydrous  Sulphuric  Acid.  Chemically  pure  concentrated 
sulphuric  acid  is  heated  in  a  porcelain  crucible  until  it  has  fumed 
strongly  for  some  time  and  then  allowed  to  cool  in  a  desiccator 
over  phosphorus  pent  oxide. 

Procedure. — The  weighed  sample  of  the  fluoride  is  intimately 
mixed  in  an  agate  mortar,  which  is  placed  upon  black  glazed  paper, 
with  1.5-2  gms.  of  the  quartz  powder  and  then  transferred  through 
the  cylindrical  arm  A  of  the  perfectly  dry  decomposition  apparatus 
to  the  pear-shaped  compartment  B  shown  in  Fig.  80.  Then 
1.5-2  gms.  of  the  sea  sand  are  added,  and  mixed  with  the  rest  of 
the  material  by  shaking  the  apparatus,  which  is  then  connected 
with  the  dry  U-tube  containing  glass  beads.  The  two  Peligot 
tubes  P  and  PI  each  contain  between  10  and  15  c.c.  of  alcohol 
which  is  saturated  with  potassium  chloride.  When  the  apparatus 
is  all  connected  as  shown  in  the  drawing,  a  dry  current  of  air,+ 
free  from  carbon  dioxide,  is  allowed  to  enter  at  h,  and  pass  through 
the  apparatus  at  the  rate  of  2  or  3  bubbles  per  second.  Then 
without  stopping  the  air  current,  about  20  c.c.  of  anhydrous  sul- 
phuric acid  is  allowed  to  enter  the  decomposition  apparatus 
through  the  funnel  T.  By  introducing  the  sulphuric  acid  in  this 
way,  while  maintaining  the  air  current,  the  sulphuric  acid  and  the 

*  This  tube  serves  to  keep  back  any  sulphuric  acid  that  is  carried  over 
mechanically. 

t  The  air  is  passed  through  caustic  potash  solution,  then  over  calcium 
chloride,  and  finally  through  concentrated  sulphuric  acid  before  entering 
the  apparatus. 


478        GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

greater  part  of  the  silica  and  fluoride  mixture  is  made  to  pass 
directly  into  the  compartment  B.  After  adding  the  sulphuric  acid, 
the  decomposition  vessel  is  placed  in  a  paraffin  bath  which  is 
slowly  heated  to  a  temperature  of  130°  to  140°  C.  The  evolution 
of  silicon  tetrafluoride  at  once  begins  to  take  place,  as  is  evident 
from  the  formation  of  foam.  The  passage  of  the  air  and  heating 
of  the  bath  is  allowed  to  continue  for  five  hours,  at  the  end  of 
which  time  the  flame  under  the  bath  is  turned  down  and  air  is 
passed  through  the  apparatus  for  half  an  hour  longer  at  the  rate 


FIG.  80. 


of  3  to  4  bubbles  per  second.  During  the  heating  the  apparatus 
should  be  frequently  shaken  in  order  that  the  sulphuric  acid  is 
brought  into  contact  with  all  portions  of  the  solid  mixture.  It  is 
not  necessary,  however,  with  this  arrangement  of  the  apparatus, 
to  shake  as  frequently  as  in  the  forms  of  apparatus  described  by 
Penfield  and  by  Fresenius,  because  the  air  in  its  passage  through 
the  narrow  connecting  tube  between  A  and  B  of  the  decomposition 
apparatus  serves  of  itself  to  effect  a  good  mixing.  In  order  to 
accomplish  this  end,  however,  it  is  necessary  to  construct  the 
apparatus  exactly  as  shown  in  Fig.  80;  the  connecting  tube 


DETERMINATION  OF  FLUORINE  IN  HYDROFLUOSILICIC  ACID.  479 

between  >4.  and  B  must  be  so  narrow  that  it  is  completely  filled 
with  the  bubbles  of  air  passing,  and  furthermore  the  parts  marked 
c  e  b  must  form  an  inclined  plane  upon  which  the  substance  can 
readily  pass  back  and  forth.  If  there  is  a  hollow  in  the  apparatus 
at  c  e  b,  in  which  some  of  the  substance  can  collect,  the  sulphuric 
acid  may  not  come  in  contact  with  some  of  the  fluoride  so  that  the 
decomposition  will  be  incomplete.  Similarly  it  is  necessary  to 
guard  against  making  the  connecting  tube  c  e  too  narrow,  as  other- 
wise the  air  will  not  pass  in  a  uniform  stream,  but  in  spurts,  so 
that  in  spite  of  the  long  tube  D  some  of  the  sulphuric  acid  fumes 
are  likely  to  reach  the  Peligot  tubes  and  thereby  give  rise  to  high 
results. 

If  not  more  than  0.1  gm.  of  the  fluoride  was  present,  the  action 
is  over  at  the  end  of  five  and  one -half  hours,  and  this  is  evident,  as 
Daniel  *  was  the  first  to  discover,  from  the  fact  that  the  foaming 
in  the  apparatus  ceases;  the  hydrochloric  acid  which  has  been  set 
free  in  the  Peligot  tubes  can  now  be  titrated.  To  this  end  a  few 
drops  of  fresh  cochineal  f  solution  are  added  to  each  tube  and  the 
contents  are  titrated  with  fifth-normal  potassium  hydroxide 
solution  with  frequent  shaking,  until  the  indicator  changes  from 
yellow  to  red.  This  is,  however,  by  no  means  the  correct  end- 
point,  because  as  Penfield  observed,  the  gelatinous  silicic  acid 
encloses  very  appreciable  amounts  of  hydrochloric  acid.  The 
silicic  acid,  therefore,  must  be  thoroughly  worked  over  with  a 
stirring  rod  and  the  addition  of  the  alkali  continued  until  the 
color  change  is  permanent. 

The  results  obtained  by  this  method,  using  0.1  gm.  of  substance, 
appear  to  be  about  0.4  per  cent,  too  high.  The  method  can  be 
used  in  the  presence  of  phosphoric  acid,  but  carbonates  are  first 
removed  by  ignition  before  the  treatment  with  sulphuric 
acid. 


*  Z.  anorg.  Chem.,  88,  257  (1904). 

f  Instead  of  cochineal,  methyl  orange  may  be  used,  although  it  is  neces- 
sary then  to  add  an  equal  volume  of  alcohol  before  titrating  the  hydro- 
chloric acid. 


480       GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 


Determination  of  Fluorine  in  Mineral  Waters. 

From  1  to  10  liters  of  the  water  (according  to  the  amount  of 
salts  present)  are  evaporated  in  a  large  platinum  or  porcelain 
dish  to  a  small  volume,  with  the  addition  of  enough  sodium 
carbonate  to  keep  the  solution  slightly  alkaline.  Then  an  excess 
of  calcium  chloride  is  added,  the  liquid  boiled,  and  the  precipitate 
filtered  and  washed  with  hot  water  until  free  from  chlorides.  The 
precipitate  is  dried,  transferred  as  completely  as  possible  to  a 
platinum  dish,  and  the  ash  of  the  filter  added  to  the  main  precipitate 
which  is  then  gently  ignited.  This  residue  contains  all  of  the 
fluorine  as  calcium  fluoride;  besides  considerable  calcium  (possible 
strontium)  and  magnesium  carbonates;  iron,  aluminium  and 
manganese  oxides ;  often  barium  sulphate ;  and  almost  invariably 
some  calcium  phosphate.  It  is  treated  with  an  excess  of  dilute 
acetic  acid,  allowed  to  stand  for  some  time  with  frequent  stirring, 
and  then  evaporated  to  dryness  on  the  water  bath.  This  residue 
is  treated  with  water,  filtered,  and  washed  with  hot  water.  As 
much  of  it  as  possible  is  transferred  to  a  platinum  crucible,  the 
ash  of  the  filter  added,  and  the  contents  of  the  crucible  gently 
ignited.  From  J— 2  gms.  of  ignited  quartz  powder  are  then 
intimately  mixed  with  the  residue  in  an  agate  mortar.  The 
mixture  is  transferred  to  the  decomposition  vessel  A,  Fig.  80, 
and  treated  with  concentrated  sulphuric  acid  exactly  as  described 
on  p.  477  by  the  method  of  Penfield.  As  only  very  little  fluorine 
is  present  in  this  case,  two  small  U-tubes  are  used  instead  of  the 
large  Peligot  tubes  shown  in  Fig.  80. 

Remark. — The  formation  of  a  precipitate  in  the  first  U-tube 
at  the  place  marked  a  a  in  Fig.  80  indicates  the  presence  of  fluorine. 
It  is  well  to  confirm  it  by  the  etching-test.  After  carrying  out  the 
titration  of  the  hydrochloric  acid  set  free,  the  contents  of  the  U-tube 
are  transferred  to  a  platinum  dish,  a  few  drops  of  double  normal 
sodium  carbonate  added,  and  the  solution  evaporated  to  dryness. 
Ammoniacal  zinc  oxide  is  then  added  (cf.  p.  473),  the  liquid  again 
removed  by  evaporation,  the  residue  taken  up  in  water,  and  the 


DETERMINATION  OF  FLUORINE.  481 

zinc  oxide  and  silicate  filtered  off.  The  filtrate  is  treated  with 
calcium  chloride  as  described  on  p.  473  and  the  etching  test 
applied. 

In  the  former  editions  of  this  book  it  was  recommended  to 
determine  the  fluorine  in  mineral  waters  by  evaporating  a  very 
much  larger  volume  of  the  water  nearly  to  dryness,  filtering,  and 
then  examining  the  insoluble  residue  alone  for  fluorine.  In  this 
way  the  fluorine  content  of  many  mineral  waters  was  entirely 
overlooked. 


Gas-Volumetric  Determination  of  Fluorine  according  to  Hempel 

and  Oettel. 

See  Part  III,  Gas  Analysis. 


Separation  of  Fluorine. 
(a)  From  the  Metals. 

For  the  determination  of  the  metals  present,  the  fluorine  usually 
can  be  removed  by  heating  with  concentrated  sulphuric  acid; 
in  the  case  of  many  silicates  containing  fluorine,  however,.  e.g.r 
topaz,  lepidolite.  and  other  micas,  this  treatment  will  not  accom- 
plish the  desired  result.  In  such  cases  the  mineral  is  fused  with  4  to 
6  times  as  much  sodium-potassium  carbonate,  the  melt  is  extracted 
with  water,  the  silica  and  aluminium  precipitated  from  the  solu- 
tion obtained  by  means  of  ammonium  carbonate  (see  page  472). 
and  these  two  substances  determined  in  the  residue,  while  the  filtrate 
is  used  for  the  fluorine  analysis.  The  metals  and  the  remainder  of 
the  silicic  acid  are  determined  in  the  residue  obtained  on  extracting 
the  melt  with  water  (cf.  p.  493).  The  estimation  of  the  alkalies 
must  be  undertaken  in  a  separate  portion  of  the  substance  (pp. 
497-501). 


482        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 


(6)  Separation  of  Fluorine  from  the  Acids. 

1.   Determination  of  Hydrochloric  and  Hydrofluoric  Acids  in  the 
Presence  of  One  Another. 

In  the  case  of  soluble  alkali  salts,  the  fluorine  is  first  pre- 
cipitated from  the  solution  by  means  of  a  little  sodium  car- 
bonate and  an  excess  of  calcium  nitrate  solution,  as  described  on 
p.  471.  The  filtrate  is  acidified  with  nitric  acid  and  the  chlorine 
determined  by  precipitation  with  silver  nitrate,  according  to 
p.  320. 

It  is  simpler  to  treat  the  solution  containing  hydrochloric  and 
hydrofluoric  acids  in  a  platinum  evaporating  dish  with  nitric  acid 
and  silver  nitrate.  Silver  chloride  is  alone  precipitated  and  can 
be  filtered  off,  using  a  funnel  of  hard  rubber,  or  a  glass  one  coated 
over  with  wax.  The  precipitate  is  washed  and  weighed  as 
described  on  p.  320.  When  phosphoric  acid  also  is  present,  this 
is  precipitated  with  the  hydrochloric  acid  by  the  addition  of 
silver  nitrate  to  the  slightly  alkaline  solution,  the  precipitate 
is  filtered  off,  washed  with  as  little  cold  water  as  possible,  and 
the  precipitate  treated  with  dilute  nitric  acid.  By  this  means 
the  silver  phosphate  goes  into  solution,  while  the  silver  chloride 
is  unaffected.  In  order  to  determine  the  amount  of  phosphoric 
acid  present,  the  silver  is  removed  from  the  solution  by  the 
addition  of  hydrochloric  acid,  and  the  phosphoric  acid  is 
precipitated  in  the  filtrate  by  addition  of  magnesia  mixture  and 
ammonia  (cf.  p.  434). 

In  the  filtrate  from  the  silver  phosphate  and  silver  chloride 
precipitate,  the  excess  of  silver  nitrate  is  removed  by  the  addi- 
tion of  sodium  chloride  and  the  fluorine  is  determined  as  calcium 
fluoride. 

In  the  case  of  an  insoluble  compound  containing  chlorine 
and  fluorine,  the  melt  obtained  after  fusing  with  sodium- 
potassium  carbonate  is  extracted  with  water,  the  silica  is  removed 
with  ammonium  carbonate  and  zinc-ammonium  hydroxide  as 


HYDROFLUOSILICIC  ACID.  483 

described  on  pp.  472-3,  and  the  chlorine  and  fluorine  determined 
as  above. 

In  a  majority  of  cases  it  is  more  convenient  to  determine  the 
two  acids  in  separate  portions  of  the  substance. 

2.  Determination  of  Boric  and  Hydrofluoric  Acids. 

The  solution  containing  the  alkali  salts  of  these  two  acids  is 
precipitated  at  the  boiling  temperature  by  means  of  an  excess  of 
calcium  chloride  ;  the  precipitate  is  filtered  off  and  washed  with 
hot  water. 

The  precipitate,  consisting  of  calcium  carbonate,  calcium 
fluoride,  and  some  calcium  borate,  is  gently  ignited,  treated  with 
dilute  acetic  acid,  evaporated  to  dryness,  and  more  acetic  acid  and 
water  are  added.  By  this  means  the  calcium  acetate  and  cal- 
cium borate  go  into  solution,  while  the  calcium  fluoride  is  left 
behind  and  is  determined  as  described  on  p.  471.  For  the  boric 
acid  determination  a  second  portion  of  the  solution  is  taken, 
made  barely  acid  with  acetic  acid,  and  treated  with  a  slight 
excess  of  calcium  acetate  solution  in  order  to  precipitate  the 
fluorine.  The  solution,  together  with  the  calcium  fluoride,  is 
placed  in  the  Gooch  retort  and  subjected  to  distillation  as 
described  on  p.  428. 


HYDROFLUOSILICIC  ACID,  H2SiF6.    Mol.  Wt.  144.32. 

Forms:  Calcium  Fluoride,  CaF2;   Potassium  Silicofluoride; 
or  volumetrically. 

i.  Determination  as  Calcium  Fluoride. 

Principle.  —  Alkali    fluosilicates    are    decomposed    on    heating 
with  sodium  carbonate  solution  into  fluoride  and  silicic  acid: 


If  a  solution  is  to  be  analyzed  containing  free  hydrofluosilicic 
acid  or  its  sodium  salt,  it  is  treated  with  sodium  carbonate  solu- 


484      GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

tion  until  the  reaction  is  alkaline,  a  considerable  amount  of  ammo- 
nium carbonate  is  added,  the  solution  heated  to  about  40°  C., 
and,  after  standing  twelve  hours,  the  precipitated  silicic  acid  is 
filtered  off. 

The  solution  now  contains  all  the  fluorine  as  sodium  fluoride, 
in  the  presence  of  small  amounts  of  silicic  acid,  which  are  pre- 
cipitated by  the  addition  of  zinc-ammonia  hydroxide  (see  p.  473). 
In  the  filtrate  the  fluorine  is  determined  as  calcium  fluoride,  as 
described  on  p.  471. 

An  insoluble  fluosilicate  is  fused  with  four  times  as  much 
sodium-potassium  carbonate,  the  melt  extracted  with  water,  and 
the  solution  subjected  to  the  above  treatment. 

2.  Determination  as  Potassium  Silicofluoride. 

This  analysis  is  only  applicable  for  the  determination  of  free 
hydrofluosilicic  acid  in  aqueous  solution. 

Procedure. — The  solution  is  treated  with  potassium  chloride 
and  an  equal  volume  of  absolute  alcohol.  The  barely-visible 
potassium  silicofluoride  is  filtered  through  a  tared  filter  which 
has  been  dried  at  100°  C.  After  washing  with  50  per  cent,  alcohol 
the  precipitate  is  dried  at  100°  C.  and  weighed  as  K2SiF6. 

The  volumetric  determination  of  hydrofluosilicic  acid  will  be 
discussed  in  Part  II. 

Analysis  of  Salts  of  Hydrofluosilicic  Acid. 

For  the  determination  of  the  metal  present,  the  salt  is  treated 
with  concentrated  sulphuric  acid  in  a  platinum  dish  and  heated  until 
dense  fumes  of  sulphuric  anhydride  are  given  off;  silicon  fluoride 
and  hydrofluoric  acid  volatilize,  while  the  metals  are  left  behind 
as  sulphates  (cf.  Vol.  I). 

Determination  of  Water  Present  in  Fluosilicates,  (Rose-Jannasch)* 

The  water  cannot  be  determined  by  ignition,  because  all  fluo- 
silicates,  even  topaz,  evolve  silicon  fluoride  when  subjected  to 
this  treatment  (cf.  Vol.  I,  p.  355).  If,  as  proposed  by  Rose,  the 

*  Rose-Finkener:  Lehrbuch  der  analyt.  Ch.,  Bd.  II;  and  Jannasch,  Prak- 
tischer  Leitfaden  der  Gewichtsanalyse,  Leipzig,  1897,  p.  243. 


SILICIC  ACID.  485 

substance  is  fused  with  six  or  eight  times  as  much  lead  oxide,  all 
the  water  is  evolved,  while  the  fluorine  remains  behind : 

R2SiF6 + 3PbO  =  2RF + 2PbF2 + PbSiO3. 

The  analysis  is  best  performed  according  to  the  directions  of 
Jannasch:  A  bulb  with  a  capacity  of  about  25  c.c.  is  blown  near 
one  end  of  a  tube  of  difficultly  fusible  glass  which  is  26  cm.  long 
and  1  cm.  wide.  Near  the  middle  of  the  longer  side  of  the  tube 
is  placed,  between  asbestos  plugs,  a  layer  3  to  5  cm.  long  of  pulver- 
ized, anhydrous  lead  oxide,  and  this  end  of  the  tube  is  connected 
with  two  weighed  calcium  chloride  tubes.  The  substance  is  placed 
in  the  bulb,  after  which  six  or  eight  times  as  much  lead  oxide  is 
added  and  mixed  with  the  substance  by  carefully  revolving  the  tube. 
A  dry  current  of  air  is  now  conducted  through  the  apparatus  and 
the  contents  of  the  bulb  are  slowly  melted.  All  of  the  water  and 
often  some  of  the  fluorine  is  thereby  expelled,  and  the  latter  is 
absorbed  by  the  layer  of  lead  oxide.  At  the  end  of  the  operation 
this  layer  is  cautiously  heated  with  a  moving  flame  until  no  more 
water  condenses  in  the  cooler  part  of  the  tube.  When  all  of  the 
water  has  been  driven  over  into  the  calcium  chloride  tubes  the 
latter  are  weighed  with  the  customary  precautions. 


GROUP  VH. 

SILICIC  ACID  (ALSO  TITANIC,  ZIRCONIC,  TANTALIC,  AND 
NIOBIC  ACIDS). 

SILICIC  ACID,  H2SiO3.    Mol.  Wt.  78.32. 
Form:  Silicon  Dioxide,  Si02. 

Two  cases  must  be  considered: 

(a)  The  silicate  is  decomposed  by  acids. 

(6)  The  silicate  is  not  decomposed  by  acids. 

(a)  Silicates  Decomposed  by  Acids. 

These  are  treated  with  hydrochloric  acid  in  a  porcelain  dish 
and  evaporated  upon  the  water-bath  with  frequent  stirring  until 
the  residue  is  obtained  in  the  form  of  a  dry  powder.  In  many 
cases  the  decomposition  is  shown  to  be  complete  by  the  fact  that 


486     GR/tyiMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

no  gritty  particles  can  be  felt  with  the  stirring- rod  on  the  bottom 
of  the  dish.  If,  however,  the  substance  contained  quartz  or  some 
silicate  that  is  not  decomposed  by  hydrochloric  acid,  this  is  not 
the  case  and  the  procedure  described  on  p.  507  may  be  followed. 
The  dry  powder  is  moistened  with  concentrated  hydrochloric 
acid  and  the  covered  dish  is  allowed  to  stand  for  10  or  at  the  most 
20  minutes  at  the  ordinary  temperature,  in  order  that  basic  salts 
and  oxides  formed  during  the  evaporation  and  drying  may  be 
once  more  changed  to  chlorides.  Then  100  c.c.  of  water  are 
added,  it  is  heated  to  boiling,  and  after  the  silicic  acid  has  been 
allowed  to  settle,  the  clear  liquid  is  decanted  through  a  filter  sup- 
ported upon  a  platinum  cone  placed  in  the  apex  of  the  funnel.  The 
residue  is  washed  3  or  4  times  with  hot  water  by  decantation,  then 
transferred  to  the  filter  and  washed  with  hot  water  until  free  from 
chloride.*  The  precipitate  is  then  dried  by  means  of  suction, 
placed  in  a  platinum  crucible,  and  set  aside  for  the  time  being. 
The  separation  of  the  silicic  acid  is  now  by  no  means  quantitative ; 
as  much  as  5  per  cent,  of  the  total  amount  may  remain  in  the 
filtrate.  In  order  to  remove  this,  the  solution  is  once  more  evap- 
orated to  dryness  on  the  water-bath,  kept  at  this  temperature 
for  one  or  two  hours  (or  more),  moistened  with  a  few  cubic  cen- 
timeters of  concentrated  hydrochloric  acid,  and  allowed  to  stand 
not  more  than  fifteen  minutes. t  Hot  water  is  then  added,  the 
residue  is  filtered  through  a  new  and  correspondingly  small  filter, 
and  washed  with  hot  water.  The  amount  of  silicic  acid  now 
remaining  in  the  filtrate  amounts  to  not  more  than  0.15  per  cent, 
of  the  total  amount,  and  for  most  purposes  can  be  neglected.  It 
can  be  removed,  however,  by  a  third  evaporation  to  dryness. 
The  filters  containing  the  silica  are  ignited  wet  in  a  platinum 
crucible  and  finally  over  the  blast-lamp,  and  weighed.!  The 
silica  obtained  is  only  slightly  hygroscopic. 

*  If  the  precipitate  is  not  perfectly  white,  but  somewhat  brownish  owing 
to  the  presence  of  a  basic  ferric  salt,  concentrated  hydrochloric  acid  is  allowed 
to  run  around  the  upper  edge  of  the  filter  and  is  immediately  washed  down 
through  the  funnel  by  means  of  a  stream  of  hot  water.  This  is  repeated 
until  the  nitrate  comes  through  perfectly  colorless. 

f  By  being  kept  in  contact  with  the  acid  for  too  long  a  time  some  silicb 
acid  will  go  into  solution. 

J  With  regard  to  the  temperature  at  which  silica  is  completely  dehydrated, 


SILICIC  ACID.  487 

Testing  the  Purity  of  the  Silica. 

The  silica  thus  obtained  is  never  absolutely  pure,  except  in 
the  analysis  of  a  water-glass.  Its  purity  must  always  be  tested. 
For  this  purpose  it  is  covered  with  2  or  3  c.c.  of  water,*  a  drop  of 
concentrated  sulphuric  acid  is  added,  and  3  to  5  c.c.  of  pure  hydro- 
fluoric acid  (distilled  from  a  platinum  retort).  The  crucible  is  then 
placed  in  a  platinum-lined  cone  (Fig.  16,  p.  31)  on  the  water-bath 
and  evaporated  under  a  good  hood  until  no  more  vapors  are 
expelled.  The  excess  of  sulphuric  acid  is  then  removed  by  heat- 
ing over  a  free  flame.  The  temperature  is  raised  and  the  crucible 
is  finally  heated  over  a  blast-lamp,  after  which  it  is  again  weighed. 
This  process  is  repeated  until  the  contents  of  the  crucible  (usually 
A12O3  and  Fe203)  are  at  a  constant  weight,  and  this  amount  is 
deducted  from  the  weight  of  impure  silica. 

Remark. — In  order  to  make  the  separation  of  silicic  acid  more 
nearly  quantitative  it  has  been  proposed  to  heat  the  residue 
obtained  by  evaporation  at  110°-120°  C.f  This  hastens  the 
dehydration,  but  the  temperature  of  120°  should  not  be 
exceeded  on  account  of  the  danger  of  the  silicic  acid  being 
contaminated  with  basic  salts,  and  because  of  the  tendency  for 

there  is  a  difference  of  opinion.  Lunge  and  Millberg  (Zeit.  f.  angew.  Chem., 
(1897),  p.  425)  state  that  the  temperature  of  the  Bunsen  burner  is  sufficient, 
but  they  operated  with  silica  obtained  by  the  hydrolysis  of  silicon  tetra- 
chloride,  in  order  to  obtain  a  product  absolutely  free  from  alkalies.  Hille- 
brand  (Am.  Chem.  Soc.,  XXIV  (1902),  p.  362)  confirmed  the  results 
of  Lunge  and  Millberg  with  regard  to  the  ignition  of  a  silica  obtained 
in  this  way,  but  positively  asserts  that  silicic  acid  when  obtained  by  the  decom- 
position of  an  alkali  silicate  with  acid  must  be  ignited  over  the  blast-lamp 
in  order  to  dehydrate  it  completely.  The  results  of  Hillebrand  have  been 
confirmed  in  the  author's  laboratory  by  A.  Schroter.  Jordis  and  Kanter 
(Z.  anorg.  Chem.,  35,  20  (1903)  )  state  that  hydrated  silica  forms  a  small 
amount  of  a  chlorine  compound  with  hydrochloric  acid.  This  compound 
is  decomposed  on  heating  only  with  great  difficulty,  unless  it  is  treated  with 
water  and  evaporated  to  dryness  several  times.  This  treatment  is  recom- 
mended by  Jordis  and  Kanter  in  all  accurate  silica  determinations. 

*  If  the  water  is  not  added,  the  mass  will  effervesce  so  strongly  that 
there  is  danger  of  losing  some  of  the  impure  silica. 

t  James  P.  Gilbert,  Tech.   Quarterly,  III,  p.  61,   and    Zeit.    fur    anaL 
Chem.,  XXIX  (1S90),  688. 


488        GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

it  to  recombine  with  these.*  It  is,  therefore,  not  advisable  to 
attempt  to  dehydrate  the  silica  at  a  temperature  higher  than  that 
of  the  water-bath. 

(6)  Silicates  Not  Decomposed  by  Acids. 
These  must  be  fused;  this  can  be  effected  by 

(a)  The  Sodium  Carbonate  Method. 

One  gram  of  the  very  finely  powdered  substance  is  placed  in 
a  spacious  platinum  crucible  together  with  4  to  6  parts  of  calcined 
sodium  carbonate  (or  a  mixture  of  equal  parts  sodium  and  potas- 
sium carbonates)  and  fused.  The  powdered  silicate  should  be 
intimately  mixed  with  the  flux  and  a  little  sodium  carbonate 
sprinkled  on  top,  the  crucible  covered  and  heated  for  some  time 
over  a  small  flame  in  order  to  drive  out  any  moisture  present. 
The  temperature  is  raised  gradually  until  finally  the  highest  heat 
of  a  good  Teclu  burner  is  obtained;  or,  lacking  the  latter,  a  blast- 
lamp  should  be  used.  As  soon  as  the  mass  melts  quietly  and 
there  is  no  further  evolution  of  carbon  dioxide,  the  decompo- 
sition is  complete.  The  crucible  is  seized  with  a  pair  of  crucible 
tongs  having  platinum  points  and  placed  in  cold  water,  but  so 
that  the  water  does  not  enter  the  crucible.  By  means  of  this 
rapid  cooling  the  melt  is  usually  detached  from  the  sides  of  the 
crucible  and  can  be  removed  by  simply  turning  the  crucible  up- 
side down  and  gently  tapping  its  sides,  f  The  melt  is  received 
in  a  good-sized  beaker,  covered  with  water,  a  sufficient  quantity 
of  strong  hydrochloric  acid  is  added,  and  the  beaker  covered  with 
a  watch-glass.  A  lively  evolution  of  carbon  dioxide  at  once  takes 

*  When  considerable  magnesium  was  present,  more  silica  was  found  in 
the  filtrate  after  igniting  at  280°  than  when  dried  on  the  water-bath.  This 
is  due  to  the  fact  that  magnesia  formed  by  hydrolysis  reunites  with  the  silica 
to  form  magnesium  silicate,  and  the  latter  is  decomposed  by  hydrochloric 
acid  with  the  formation  of  soluble  silicic  acid. 

t  A  better  method  of  removing  the  melt  from  the  crucible  is  recom- 
mended by  Hillebrand:  The  crucible  is  seized  with  the  tongs  while  its  con- 
tents are  still  liquid  and  a  circular  motion  is  imparted  to  the  latter.  By 
this  means  the  melt  solidifies  on  the  sides  and  bottom  of  the  crucible  in  thin 
layers  which  can  usually  be  removed  from  the  crucible,  and  the  decomposi- 
tion by  acids  is  greatly  facilitated.  For  another  method,  see  Talbot's 
Quantitative  Chemical  Analysis,  p.  33. 


SILICATES  NOT  DECOMPOSED  BY  ACIDS.  489 

place,  but  in  proportion  as  silicic  acid  separates  out,  the  inner 
part  of  the  cake  gradually  becomes  coated  with  a  film  of  silicic 
acid  which  protects  it  from  the  further  action  of  the  acid.  Con- 
sequently it  is  necessary  to  break  up  the  cake  from  time  to  time 
by  means  of  a  glass  rod  until  finally  there  is  no  further  evolution 
of  a  gas  and  no  more  hard  lumps  remain.  When  manganese 
is  present  the  melt  is  colored  green  and  the  solution  is  pink.  The 
latter  is  heated  until  this  pink  color  disappears  and  is  then 
transferred  to  a  platinum  dish  (or  lacking  this,  one  of  porcelain 
may  be  used).  The  small  amount  of  the  melt  adhering  to  the 
sides  of  the  crucible  is  transferred  to  the  contents  of  the  dish  by 
means  of  water  and  hydrochloric  acid.  The  solution  is  then 
analyzed  as  described  on  page  485. 

Remark. — If  the  fusion  cannot  be  removed  from  the  crucible, 
it  is  placed,  together  with  its  cover,  in  the  beaker  and  treated  as 
above. 

In  this  case,  if  the  melt  was  very  green-colored,  it  should  not 
be  decomposed  with  hydrochloric  acid,  but  with  nitric  acid,  for 
the  chlorine  evolved  by  the  action  of  the  hydrochloric  acid  upon 
the  manganate  would  attack  the  platinum. 

Substances  containing  considerable  fluorine  cannot  be  treated 
as  above,  for  silicon  fluoride  will  be  lost  by  volatilization.  In 
this  case  it  is  necessary  to  use  the  old  method  of  Berzelius.  The 
melt  from  the  sodium  carbonate  fusion  is  extracted  with  water, 
as  in  the  determination  of  fluorine  (p.  472),  and  the  greater  part 
of  the  silica  removed  by  means  of  ammonium  carbonate.  The 
precipitate  is  filtered  off,  ignited,  and  weighed. 

The  silicic  acid  remaining  in  the  filtrate  is  precipitated  by 
means  of  ammoniacal  zinc  hydroxide.  The  precipitate  thus  ob- 
tained, consisting  of  zinc  oxide  and  zinc  silicate,  is  decomposed 
with  hydrochloric  acid  and  the  silica  obtained  by  evaporation 
on  the  water-bath  as  usual.  As  a  rule,  the  insoluble  part  of  the 
melt  contains  silicic  acid,  and  this  must  also  be  removed  by  evapo- 
ration with  hydrochloric  acid.  All  three  silica  precipitates  are 
ignited  together  and  the  purity  of  the  silica  tested. 

Besides  the  sodium  carbonate  method  for  the  analysis  of 
silicates  not  decomposable  with  acids  a  great  number  of  other 
methods  have  been  proposed,  but  of  these  only  the  following  will 
be  mentioned  here. 


490     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

(ft)  Lead  Oxide  Method  of  Jannasch* 

This  analysis  is  interesting  because  it  permits  of  an  exact 
determination  of  the  alkalies  and  of  silicic  acid  in  the  same  sample. 

Inasmuch  as  commercial  lead  oxide  (litharge)  is  not  free  from 
impurities,  it  is  prepared  for  the  analysis  by  the  ignition  of  pure 
lead  carbonate. 

The  lead  carbonate  is  prepared  by  adding  the  theoretical 
amount  of  ammonium  carbonate  to  a  boiling  solution  of  lead 
acetate.  The  precipitate  is  washed  several  times  by  decantation 
with  hot  water,  then  transferred  to  a  hardened  filter,  and  com- 
pletely washed,  using  suction.  The  mass  is  finally  carefully  re- 
moved from  the  filter-paper  and  dried  on  the  water-bath. 

Procedure. — For  each  gram  of  the  silicate  10-12  gms.  of  lead 
carbonate  are  used.  First  of  all  a  little  lead  carbonate  is  placed 
in  the  crucible,  then  the  very  finely  powdered  substance,  and  after 
mixing  thoroughly  with  a  platinum  spatula  the  covered  crucible 
is  heated  for  fifteen  to  twenty  minutes  over  a  flame  which  is 
not  more  than  3-4  cm.  high,  by  which  means  the  greater  part 
of  the  carbon  dioxide  is  expelled.  The  contents  of  the  crucible 
are  then  more  strongly  heated  until  fusion  is  effected,  taking  care 
that  the  flame  used  is  strictly  non-luminous;  the  lower  third  of 
the  crucible,  and  no  more,  may  be  heated  to  redness. 

After  fusing  for  ten  to  fifteen  minutes  the  decomposition  is 
complete,  and  the  covered  crucible  is  quickly  touched  into  cold 
water,  but  so  that  its  contents  remain  dry.  The  melt  is  placed  in 
a  platinum  dish,  covered  with  hot  water  and  a  sufficient  quantity 
of  concentrated  nitric  acid  and  evaporated  on  the  water- bath, 
breaking  up  the  melt  with  a  stirring-rod  as  much  as  possible. 
When  the  cake  is  completely  disintegrated,  as  is  shown  by  there 
remaining  no  more  hard  yellow  pieces  and  only  slightly  colored 
flocks  of  silicic  acid  floating  in  the  liquid,  the  latter  is  evaporated 
on  the  water-bath  until  a  dry  powder  is  obtained ;  this  is  moist- 
ened with  concentrated  nitric  acid  and  once  more  evaporated 

*Gaston  Bong,  Zeit.  fur  anal.  Chem  ,  XV11I  (1879),  p.  270,  first  pro- 
posed that  silicates  be  decomposed  by  fusion  with  red  lead  (Pb.,O4),  but 
Jannasch  in  his  Praktisches  Leitfaden  der  Gewichtsanalyse  hag  greatly 
improved  the  method. 


ANALYSIS  OF  SILICATES— ORTHOCLASE.  491 

to  complete  dryness.  The  dry  residue  is  moistened  with  20  c.c.  of 
concentrated  nitric  acid,  and  allowed  to  stand  fifteen  minutes;  100 
c.c.  of  water  are  added,  and  the  liquid  is  heated  for  twenty 
minutes  on  the  water-bath.  The  residue  of  silicic  acid  is  filtered 
off,  washed  first  with  hot  water  containing  nitric  acid,  then  with 
pure  water,  and  weighed  after  the  usual  ignition. 

Remark. — In  the  analysis  of  minerals  containing  fluorine,  e.g. 
topaz,  Jannasch  finds  that  the  results  obtained  are  about  0.5-1 
per  cent,  lower  than  when  the  Berzelius  method  is  used.  In  such 
a  case  this  method  of  decomposition  is  used  only  for  the  deter- 
mination of  the  metals  and  of  the  alkalies,  after  introduction  of 
hydrogen  sulphide  and  removal  of  the  lead. 

ANALYSIS  OF  SILICATES. 
Orthoclase. 

Constituents :  silicic  acid  (63-70  per  cent.) ;  aluminium  oxide 
( 16-20  per  cent.) ;  ferric  oxide  (0.3  per  cent.) ;  potassium  oxide 
(8-16  per  cent.);  sodium  oxide  (1-6  per  cent.);  and  often  small 
amounts  of  calcium  oxide,  magnesium  oxide,  and  in  rare  cases 
barium  and  ferrous  oxides. 

Preparation  of  the  Substance  for  Analysis. 

The  substance  is  placed  upon  a  thick  steel  plate  within  a  steel 
ring  (about  2  cm.  high  and  6  cm.  in  diameter)  and  broken  into 
small  pieces  by  means  of  a  hardened  steel  hammer ;  the  pieces  are 
then  reduced  to  a  coarse  powder.  The  latter  is  placed  in  an 
agate  mortar  in  small  portions  and  ground  as  fine  as  possible 
and  preserved  in  a  glass-stoppered  bottle.  In  this  way  from  5-6 
gms.  of  powder  are  obtained. 

By  this  means,  as  proposed  by  Hillebrand,  there  is  less  danger 
of  contaminating  the  substance  with  small  particles  of  iron  than 
when  a  so-called  steel  mortar  is  used,  especially  after  the  latter 
has  been  worn  rough  on  its  inner  surface.  Further,  the  practice 
of  passing  the  powder  through  bolting-cloth  is  to  be  avoided  when 
possible,  as  in  this  way  the  substance  becomes  contaminated  with 


492      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

fibres  of  cloth  and  too  large  an  amount  of  ferrous  iron  will  be 
found. 


Weighing  the  Substance. 

•  It  is  customary  to  dry  the  powder  before  weighing  at  100- 
110°  C.  until  a  constant  weight  is  obtained.  If  there  is  danger  of 
losing  combined  water  by  this  procedure,  it  has  been  recommended 
to  dry  the  powder  in  a  vacuum  over  concentrated  sulphuric  acid. 
The  practice  of  drying  the  substance  in  either  of  the  above  ways  ' 
is,  however,  to  be  discountenanced.  It  is  far  better  to  use  the  air- 
dried  substance  for  the  analysis,  and  to  determine  the  moisture  in  a 
separate  sample.  This  is  more  accurate,  because  the  dry  silicate 
powder  is  hygroscopic,  so  that  a  portion  weighed  out  to-day  is 
likely  to  contain  a  different  amount  of  moisture  than  one  taken 
to-morrow,  and  this  is  not  the  case  when  the  air-dried  powder  is 
taken  for  the  analysis.  Further,  as  Hillebrand  has  conclusively 
shown,  chemically  combined  water  is  not  only  likely  to  be  ex- 
pelled by  heating  at  100°  C.,  but  also  by  drying  in  a  vacuum 
over  sulphuric  acid.  This  is  particularly  true  of  the  zeolites.  In 
the  case  of  orthoclase,  however,  only  about  0.1  per  cent,  of  moist- 
ure is  present,  so  that  in  this  particular  case  accurate  results  will 
be  obtained  by  either  method. 

For  the  analysis  two  portions  must  be  taken,  each  amounting 
to  about  1  gm.  in  weight.     The  first  serves  for  the  determination 
of  SiO2,  AlA-f  Fe2O3,  CaO,  and  MgO;  the  second  for  that  of  the 
alkalies. 
> 

Determination  of  Silica,  Aluminium,  etc. 

About  1  gm.  of  the  air-dried  substance  is  placed  in  a  spacious 
platinum  crucible,  dried  for  one  hour  at  105°-7°  C.,  cooled  in  a 
desiccator,  and  weighed.  The  difference  in  weight  represents  the 
amount  of  hygroscopic  moisture. 

The  dry  substance  is  mixed  with  4  to  5  grns.  of  calcined  sodium 
carbonate  by  means  of  a  platinum  spatula,  and  the  silicic  acid  is 


DETERMINATION  OF  ALUMINIUM  AND  FERRIC  OXIDES.      493 

determined  exactly  as  described  on  p.  488.*  The  silica  obtained 
is  treated  with  sulphuric  and  hydrofluoric  acids,  as  described  on 
p.  487,  and  the  residue  of  A12O3  in  the  crucible  is  placed  at  one 
side  for  the  present. 


Determination  of  Aluminium  and  Ferric  Oxides. 

The  nitrate  from  the  silicic  acid  contains,  besides  the  chlorides 
of  aluminium,  iron,  calcium,  and  magnesium,  weighable  amounts 
of  platinum,  partly  coming  from  the  crucible  in  which  the  fusion 
was  made,  and  partly  from  the  action  of  the  ferric  chloride  and 
hydrochloric  acid  upon  the  platinum  dish  in  which  the  evapora- 
tion took  place  (cf.  p.  110,  foot-note). 

To  remove  the  platinum,  the  solution  is  heated  to  boiling  and 
hydrogen  sulphide  is  passed  into  it.  The  mixture  of  platinum 
sulphide  and  sulphur  is  filtered  off  and  the  solution  is  boiled  to 
expel  the  excess  of  hydrogen  sulphide.  The  iron  is  then  completely 
oxidized  back  to  the  ferric  state  by  the  addition  of  bromine  water 
and  boiling  until  the  excess  of  the  latter  is  expelled.  After  this 
about  10  c.c.  of  double-normal  ammonium  chloride  solution  are 
added  and  the  boiling-hot  solution  is  precipitated  by  the  addition  of 
a  slight  excess  of  ammonia,  free  from  carbonate  (cf .  p.  149,  Remark). 

*  Formerly  a  single  evaporation  of  the.  melt  with  hydrochloric  acid  was 
made,  and  it  was  assumed  that  the  silica  remaining  in  solution  was  quan- 
titatively precipitated  with  the  iron  and  aluminium  by  the  addition  of  ammo- 
nia. After  obtaining  the  weight  of  the  ignited  ammonia  precipitate  it  was 
fused  with  potassium  pyrosulphate  and  the  melt  taken  up  in  the  dilute  sul- 
phuric acid;  the  residual  silica  was  filtered  off  and  weighed.  The  filtrate 
wras  analyzed  as  above  described.  Hillebrand  has  recently  shown  that  this 
procedure  is  inaccurate.  In  the  first  place,  the  silica  remaining  in  solution 
is  not  completely  thrown  down  with  the  iron  and  aluminium  precipitate, 
and  in  the  second  place  the  silicic  acid  is  not  absolutely  insoluble  in  dilute 
sulphuric  acid.  Hillebrand  found  that  from  a  solution  containing  0.20 
gm.  A12O3  and  0.0101  gm.  SiO2  as  much  as  0.0007  gm.  SiO2  could  be  detected1, 
in  the  filtrate  from  the  ammonia  precipitate.  From  the  potassium  pyro- 
sulphate melt  he  succeeded  in  obtaining,  according  to  the  old  method,  only 
0.0033  gm.  SiO,,  while  he  obtained,  by  evaporating  the  solution  until  fumes 
of  sulphuric  acid  came  off  and  subsequently  diluting  with  water,  as  much, 
as  0.0060  gm.  SiO2,  or  about  twice  as  much  as  was  at  first  insoluble. 


494      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

The  precipitate  is  allowed  to  settle,  after  which  it  is  filtered,  and 
washed  twice  by  decantation  with  hot  water.  It  is  then  dissolved 
by  running  hot  dilute  hydrochloric  acid  through  the  filter  into  the 
beaker  containing  the  greater  part  of  the  precipitate.  The  precipi- 
tation with  ammonia  is  repeated  as  before,  and  after  filtering  and 
washing  by  decantation,  the  precipitate  is  transferred  to  the  filter 
and  washed  until  free  from  chloride  with  water  containing  am- 
monium nitrate.  The  precipitate  is  allowed  to  drain  as  completely 
as  possible,  and  is  ignited  wet  in  the  crucible  containing  the  residue 
obtained  from  the  treatment  of  the  impure  silica  with  sulphuric 
and  hydrofluoric  acids.  After  igniting  strongly  over  a  good  Teclu 
burner  (or  the  blast-lamp)  the  crucible  is  weighed;  its  contents 
represents  the  sum  of  the  aluminium  and  ferric  oxides. 

For  the  determination  of  the  ferric  oxide,  the  mixed  oxides 
are  fused  with  potassium  pyrosulphate  as  described  on  p.  109. 
The  decomposition  is  complete  after  two  to  four  hours.  The  melt 
is  dissolved  in  water  containing  a  little  sulphuric  acid  and  the 
iron  is  determined,  after  previous  reduction  with  hydrogen  sulphide, 
by  titration  with  potassium  permanganate  (cf.  p.  99).  If  the 
weight  of  the  Fe2O3  is  deducted  from  the  weight  of  Fe2O3-f  A12O3, 
the  weight  of  A12O3  is  obtained.* 

Determination  of  Calcium. 

The  combined  filtrates  from  the  ammonia  precipitate  are 
evaporated  to  a  small  volume,  heated  to  boiling,  and  precipitated 
by  means  of  a  boiling  solution  of  ammonium  oxalate.  After 
standing  twelve  hours  the  calcium  oxalate  is  filtered  off,  and  with 
small  amounts  of  calcium  this  precipitate  is  ignited  wet  in  a  platinum 
crucible  and  weighed.  If,  however,  considerable  calcium  is  present, f 

*  The  amount  of  iron  and  aluminium  can  be  determined  more  quickly, 
though  less  accurately,  as  follows:  The  moist  ammonia  precipitate  is  dis- 
solved in  hot  dilute  sulphuric  acid  and  diluted  to  a  volume  of  exactly  250 
c.c  After  thoroughly  mixing,  100  c.c.  are  removed  by  means  of  a  pipette 
into  a  beaker  and  a  second  portion  of  the  same  volume  is  placed  in  a  200- 
c.c.  flask.  Iii  the  first  portion  the  sum  of  Fe2O3  +  Al2O3  is  determined  by 
precipitating  with  ammonia,  filtering,  igniting,  and  weighing;  in  the  other 
portion  the  iron  is  reduced  by  hydrogen  sulphide  and  then  Crated  with 
permanganate . 

t  Cf.  pp.  76-78. 


DETERMINATION  OF  MAGNESIUM.  495 

the  moist  precipitate  is  redissolved  in  hydrochloric  acid,  and  again 
precipitated  by  the  addition  of  ammonia  and  a  little  more  am- 
monium oxalate.  The  precipitate  is  ignited  strongly,  and  weighed 
as  CaO.  (Gf.  p.  70.) 

Testing  of  the  Calcium  Oxide  Precipitate  for  Barium. 

Although  it  is  usually  unnecessary  to  make  either  a  qualitative 
or  quantitative  test  for  barium  in  a  sample  of  orthoclase,  yet  it 
is  likely  to  be  present  in  traces  so  that  it  may  be  well  to  show 
how  this  can  be  done.  As  far  as  the  author  knows  strontium 
has  never  been  found  in  orthoclase.  On  account  of  the  solubility 
of  barium  oxalate  in  a  solution  of  ammonium  oxalate,  the  barium 
will  rarely  be  found  hi  the  calcium  precipitate  when  a  double  pre- 
cipitation is  made,  except  when  it  is  present  to  an  extent  of  more 
than  3  or  4  mgms.* 

To  test  the  calcium  precipitate  for  barium,  it  is  dissolved  in 
nitric  acid,  evaporated  to  dryness,  and  heated  for  some  time  at  140° 
C.  The  calcium  nitrate  is  dissolved  out  by  ether-alcohol  (p.  79,  a), 
and  any  residue  remaining  behind  is  tested  in  the  spectroscope  for 
barium.  If  an  appreciable  amount  of  the  latter  is  found,  the 
calcium  must  be  determined  in  the  ether-alcohol  extract.  It  is 
carefully  evaporated  to  dryness,  the  residue  dissolved  in  a  little 
water  and  precipitated  as  before  by  the  addition  of  ammonium 
oxalate.  After  standing  twelve  hours  the  precipitate  is  filtered 
off,  washed,  ignited,  and  weighed.  If  no  barium  is  found  with 
the  lime,  it  is  by  no  means  safe  to  conclude  that  the  former  is 
absent;  it  can  very  well  have  gone  into  the  filtrate  from  the 
double  precipitation  of  calcium.  This  amount  will  be  precipitated 
with  the  magnesium  as  barium  phosphate  unless  it  is  removed  as 
indicated  below. 

For  the  quantitative  determination  of  barium  a  separate 
portion  of  the  substance  is  taken  (see  below). 

Determination  of  Magnesium. 

The  combined  filtrates  from  the  calcium  oxalate  are  evapo- 
rated to  dryness,  ignited  in  a  porcelain  dish,  and  the  residue  dis- 

*  W.  F.  HiUebrand,  Journ.  Am.  Chem.  Soc.,  16  (1894),  p.  83. 


496       GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

solved  in  water  to  which  a  few  drops  of  hydrochloric  acid  have 
been  added.  The  carbonaceous  residue  is  filtered  off,  a  drop  of 
sulphuric  acid  added,  and  the  solution  is  allowed  to  stand  twelve 
hours  to  see  if  any  precipitate  of  barium  sulphate  will  form.  In 
the  latter  case,  the  precipitate  is  filtered  off  and  tested  for  barium 
according  to  Vol.  I,  p.  62;  in  the  filtrate  from  the  barium  sulphate 
the  magnesium  is  determined  as  described  on  page  65. 

Determination  of  Barium. 

If  the  qualitative  tests  have  shown  the  presence  of  barium, 
a  larger  sample  of  the  substance  is  weighed  out  (about  2  gms.) 
moistened  in  a  platinum  dish  with  10  c.c.  of  sulphuric  acid  (1:4) 
and  5  c.c.  of  hydrofluoric  acid  are  added.  The  liquid  is  evapo- 
rated on  the  water-bath,  with  frequent  stirring,  until  the  mineral 
is  completely  decomposed,  which  is  recognized  by  there  no  longer 
being  any  sandy  particles  perceptible  on  stirring  with  a  platinum 
spatula.  Frequently  a  further  addition  of  hydrofluoric  acid  is 
necessary.  When  the  decomposition  is  complete,  the  greater 
part  of  the  sulphuric  acid  is  removed  by  heating  the  contents  of 
the  dish  in  an  air-bath.  After  cooling,  the  residue  is  taken  up  in 
water,  and  the  barium  sulphate  is  filtered  off,  and  ignited  wet 
in  a  platinum  crucible.  The  precipitate  thus  obtained  always 
contains  small  amounts  of  calcium  sulphate  which  must  be  elim- 
inated. To  accomplish  this,  the  residue  in  the  crucible  is  dis- 
solved in  a  little  hot  concentrated  sulphuric  acid,  and  after  cool- 
ing the  solution  is  diluted  with  cold  water.  The  barium  sulphate 
is  now  completely  free  from  calcium;  it  is  filtered  off,  ignited, 
and  weighed. 

Determination  of  the  Alkalies. 

(a)  Method  of  J.  Laurence  Smith* 

Principle. — The  substance  is  heated  with  a  mixture  of  1  part 
ammonium  chloride  and  8  parts  calcium  carbonate.  By  this 
means  the  alkalies  are  obtained  in  the  form  of  chlorides,  while  the 
remaining  metals  are  for  the  most  part  left  behind  as  oxides, 

*  Am.  Jour.  Science  [2],  50,  p.  269,  and  Ann.  d.  Chem.  u.  Phann..  159, 
p.  82  (1871). 


DETERMINATION  OF  THE  ALKALIES.  497 

and  the  silica  is  changed  to  calcium  silicate,  as  represented  by  the 
following  equation: 

2KAlSi,O.+  6CaCO3+  2NH4C1= 

=  6CaSiO3+  6CO2+  A12O3+  2KC1+  2NH3+  H2O . 

The  alkali  chlorides  together  with  calcium  chloride  can  be 
removed  from  the  sintered  mass  by  leaching  with  water,  while 
the  other  constituents  remain  undissolved. 

Preparation. — The  ammonium  chloride  necessary  for  the  de- 
termination is  prepared  by  subliming  the  commercial  salt;  the 
calcium  carbonate  by  dissolving  the  purest  calcite  obtainable  in 
hydrochloric  acid  and  precipitating  with  ammonia  and  ammonium 
carbonate.  This  last  operation  is  performed  in  a  large  porcelain 
dish.  After  the  precipitate  has  settled,  the  clear  solution  is  poured 
off  and  the  precipitate  is  washed  by  decantation  until  free  from 
chlorides.  The  product  thus  obtained  contains  traces  of  alkalies, 
but  the  amount  present  is  determined  once  for  all  by  a  blank 
test  and  a  corresponding  deduction  made  from  the  results  of  the 
analysis;  it  is  usually  sodium  chloride  and  amounts  to  0.0012- 
0.0016  gm.  for  8  gms.  calcium  carbonate.  The  decomposition 
was  performed  by  Smith  in  a  finger-shaped  crucible  about  8  cm. 
long  and  with  a  diameter  of  about  2  cm.  at  the  top  and  1J  cm.  at 
the  bottom.  Such  a  crucible  is  suitable  for  the  decomposition 
of  about  0.5  gm.  of  the  mineral.  A  larger  quantity  can  be  analyzed 
in  a  somewhat  wider  crucible. 

Filling  the  Crucible. — About  0.5  gm.  of  the  mineral  is  mixed 
with  an  equal  quantity  of  sublimed  ammonium  chloride  by  trit- 
uration  in  an  agate  mortar,  then  3  gms.  of  calcium  carbonate 
are  added  and  intimately  mixed  with  the  former.  The  mixture 
is  transferred  to  a  platinum  crucible  with  the  help  of  a  piece  of 
glazed  paper,  and  the  mortar  is  rinsed  with  one  gram  of  calcium 
carbonate,  which  is  added  to  the  contents  of  the  crucible. 

The  Ignition. — The  covered  crucible  is  placed  in  a  slightly 
inclined  position  and  gradually  heated  over  a  small  flame  until 
no  more  ammonia  is  evolved  *  (this  should  take  about  fifteen 

*  During  this  part  of  the  operation  the  heat  should  be  kept  so  low  that 
ammonium  chloride  does  not  escape.  The  latter  is  dissociated  into  ammonia 
and  hydrochloric  acid  by  the  heat,  and  the  acid  unites  with  the  calcium 
carbonate  to  form  calcium  chloride.— (Translator.] 


498      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

minutes),  then  the  temperature  is  raised  until  finally  the  lower 
three-fourths  (and  no  more)  of  the  crucible  are  brought  to  a 
dull  red  heat,  and  this  temperature  is  maintained  for  50-60 
minutes.  The  crucible  is  then  allowed  to  cool  and  the  sintered 
cake  usually  can  be  removed  by  gently  tapping  the  inverted  cru- 
cible. Should  this  not  be  the  case,  it  is  digested  a  few  minutes 
with  water,  which  serves  to  soften  the  cake  so  that  it  can  be  readily 
washed  into  a  large  porcelain,  or,  better,  platinum  dish.  The 
covered  dish  is  heated  with  50-75  c.c.  of  water  for  half  an  hour, 
replacing  the  water  lost  by  evaporation,  and  the  large  particles 
are  reduced  to  a  fine  powder  by  rubbing  with  a  pestle  in  the  dish. 
The  clear  solution  is  decanted  through  a  filter  and  the  residue  is 
washed  four  times  by  decantation,  then  transferred  to  the  filter 
and  washed  with  hot  water  until  a  few  cubic  centimeters  of  -the 
washings  give  only  a  slight  turbidity  with  silver  nitrate.  To 
make  sure  that  the  decomposition  of  the  mineral  has  been  com- 
plete, the  residue  is  treated  with  hydrochloric  acid;  it  should 
dissolve  completely,  leaving  no  trace  of  undecomposed  mineral. 
Precipitation  of  the  Calcium. — The  aqueous  solution  is  treated 
with  ammonia  and  ammonium  carbonate,  heated  and  filtered. 
As  this  precipitate  contains  small  amounts  of  alkali,  it  is  redis- 
solved  in  hydrochloric  acid  and  the  precipitation  with  ammonia 
and  ammonium  carbonate  is  repeated.  The  combined  filtrates 
are  evaporated  to  dryness  in  a  porcelain  or  platinum  dish,  and  the 
ammonium  salts  are  removed  by  careful  ignition  over  a  moving 
flame.*  After  cooling,  the  residue  is  dissolved  in  a  little  water  and 
the  last  traces  of  calcium  are  removed  by  the  addition  of  ammonia 
and  ammonium  oxalate.  After  standing  twelve  hours,  the  cal- 
cium oxalate  is  filtered  off  and  the  filtrate  is  received  in  a  weighed 
platinum  dish,  evaporated  to  dryness,  and  gently  ignited.  After 
cooling  the  mass  is  moistened  with  hydrochloric  acid  in  order  to 
transform  any  carbonate  into  chloride,  the  evaporation  and  igni- 
tion is  repeated,  and  the  weight  of  the  contents  of  the  dish  is  deter- 
mined; this  represents  the  amount  of  alkali  chloride  present.  To 
determine  potassium,  the  residue  is  dissolved  in  water,  and  the 


*  Before  igniting,  it  is  well  to  heat  the  contents  of  the  dish  in  a  drying-oven  at 
110°.    By  this  means  there  is  no  danger  of  loss  by  decrepitation.— {Translator.] 


DETERMINATION  OF    THE  ALKALIES.  499 

potassium    is    precipitated  as  potassium  chloroplatinate,  as  de- 
scribed on  p.  44.     The  sodium  is  determined  by  difference. 

(b)    The  Hydrofluoric  Acid  Method  of  Berzelius. 

About  0.5  gm.  of  the  mineral  is  weighed  into  a  platinum  dish, 
2  c.c.  of  water  and  0.5  c.c.  of  concentrated  sulphuric  acid  are 
added,  and  mixed  with  the  substance  by  means  of  a  platinum 
spatula;  after  cooling  about  5  c.c.  of  pure,  concentrated  hydro- 
fluoric acid,  which  has  been  distilled  from  a  platinum  retort  with 
the  addition  of  a  little  potassium  permanganate,  are  added.* 
The  liquid  is  evaporated  on  the  water-bath,  frequently  stirring 
with  the  platinum  spatula,  until  no  more  hydrofluoric  acid  is 
expelled  and  no  more  hard  particles  can  be  felt  at  the  bottom 
of  the  dish. 

The  dish  is  heated  in  an  air-bath  until  the  greater  part  of  the 
sulphuric  acid  is  removed;  this  is  necessary  to  make  sure  that 
the  hydrofluoric  acid  is  completely  expelled.  It  is  not  advisable, 
however,  to  remove  all  of  the  sulphuric  acid,  on  account  of  the 
danger  of  forming  insoluble  basic  salts.  The  mass  is  allowed  to 
cool,  covered  with  200  c.c.  of  water,  and  digested  until  all  of  the 
residue  has  gone  into  solution. f  The  sulphates  are  now  transformed 
to  chlorides  by  precipitation  with  as  slight  an  excess  of  barium 
chloride  as  possible;  and  then,  without  stopping  to  filter  off  the 
barium  sulphate,  the  aluminium,  calcium,  and  excess  of  barium 
are  precipitated  by  the  addition  of  ammonia  and  ammonium 
carbonate.  The  precipitate  is  allowed  to  settle,  washed  four 
times  by  decantation,  then  transferred  to  the  filter  and  washed 
free  from  chloride.  The  filtrate  is  evaporated  to  dry  ness,  and  the 
ammonium  salts  removed  by  gentle  ignition.  A  few  drops  of 
hydrochloric  acid  are  added,  and  the  magnesium  is  removed  by 
adding  barium  hydroxide  solution  until  slightly  alkaline,  boiling 
and  filtering.  The  filtrate  is  treated  with  ammonia  and  am- 
monium carbonate,  boiled,  and  the  precipitated  barium  carbonate 
filtered  off.  This  filtrate  is  again  evaporated  to  dry  ness,  the 
ammonium  salts  are  expelled,  the  residue  is  dissolved  in  a 

*The  permanganate  serves  to  destroy  organic  matter  that  is  likely 
to  be  present  in  commercial  hydrofluoric  acid. 

f  If  barium  was  present,  it  is  left  behind  as  the  sulphate. 


500       GR/tyiMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

little  water,  and  a  little  more  barium  carbonate  is  pre- 
cipitated by  the  addition  of  ammonia  and  ammonium  car- 
bonate. This  treatment  is  repeated  until  finally  the  addition 
of  ammonia  and  ammonium  carbonate  produces  no  further  pre- 
cipitation. The  last  filtrate  is  evaporated  to  dryness,  gently 
ignited,  moistened  with  hydrochloric  acid,  again  evaporated, 
ignited  and  weighed;  this  represents  the  weight  of  the  alkali 
chlorides  together  with  a  small  amount  of  magnesium  chloride. 
The  chlorides  are  dissolved  in  a  little  water,  and  the  potassium 
precipitated  as  potassium  chlorplatinate  (p.  44).  If  the  cor- 
responding amount  of  potassium  chloride  is  deducted  from  the 
first  weight,  the  amount  of  sodium  chloride  plus  the  small  amount 
of  magnesium  chloride  will  be  obtained.  In  order  to  determine 
the  latter,  the  alcoholic  filtrate  from  the  potassium  chlorplatinate 
precipitate  is  evaporated  to  dryness  on  the  water-bath  (the  water 
in  the  bath  must  not  boil),  and  the  residue  is  dissolved  in  a  little 
water  and  washed  into  a  small  flask.  The  latter  is  now  fitted 
with  a  rubber  stopper  containing  two  holes,  and  through  these,  two 
right-angled  pieces  of  glass  tubing  are  introduced,  one  reaching 
to  the  bottom  of  the  stopper  and  the  other  until  it  almost 
touches  the  liquid  in  the  flask.  The  solution  is  now  heated  to 
boiling  so  that  steam  escapes  from  both  of  the  tubes.  After  boil- 
ing two  minutes  we  can  assume  that  the  air  is  completely  ex- 
pelled from  the  flask;  the  short  tube  is  connected  with  a  hydrogen 
generator  and  a  rapid  current  of  hydrogen  is  conducted  through 
the  apparatus,  while  at  the  same  time  the  flame  is  removed  from 
beneath  the  flask  and  the  long  tube  is  closed  by  means  of  a  piece 
of  rubber  tubing  containing  a  glass  rod.  The  liquid  is  allowed 
to  cool  completely,  and  the  air-space  above  will  be  entirely  filled 
with  hydrogen.  As  the  hydrogen  is  absorbed  by  the  liquid,  the 
sodium  and  magnesium  chlorplatinates  are  reduced  to  chloride 
with  the  deposition  of  metallic  platinum,  which  floats  on  the  liquid 
in  the  form  of  dendrites  : 


l,,  -f  2H2=  4HC1  +  2NaCl  +  Pt 
MgPtCl6  +  2H2=  4HC1  +  MgCl2  +  Pt 

The   flask  is   placed   in   a   lukewarm  water-bath,    frequently 


ANALYSIS  OF  LEPIDOLITE.  501 

shaken,  and  the  hydrogen  is  allowed  to  act  upon  the  solution  until 
the  reduction  is  shown  to  be  complete  by  the  liquid  becoming  per- 
fectly colorless.  The  connection  with  the  hydrogen  generator 
is  now  broken  and  a  rapid  current  of  carbon  dioxide  is  conducted 
through  the  solution  for  two  minutes  through  the  longer  tube  in 
order  to  remove  the  hydrogen.  This  is  necessary,  as  otherwise 
on  opening  the  flask  there  is  likely  to  be  an  explosion  between  the 
hydrogen  and  oxygen,  owing  to  the  catalytic  action  of  the  plati- 
num. The  platinum  is  filtered  off,  the  filtrate  concentrated,  and 
the  magnesium  precipitated  by  the  addition  of  ammonia  and 
sodium  phosphate.  After  standing  twelve  hours,  the  magnesium 
ammonium  phosphate  is  filtered  off  and  the  magnesium  deter- 
mined as  magnesium  pyrophosphate.  The  corresponding  weight 
of  MgCl2  is  deducted  from  the  weight  of  NaCl-f- MgCl2,  and  in  this 
way  the  amount  of  NaCl  is  determined. 

Remark. — This  method  is  in  very  general  use,  and  the  results 
obtained  agree  closely  with  those  by  the  J.  Lawrence  Smith  method. 
Many  silicates,  such  as  the  feldspars,  are  readily  decomposed  by 
the  action  of  sulphuric  and  hydrofluoric  acids ;  *  others,  such  as 
certain  specimens  of  tourmaline,  only  with  difficulty.  According 
to  Jannasch  the  members  of  the  andalusite  group  are  not  com- 
pletely decomposed  by  hydrofluoric  acid,  but  this  can  be  effected 
by  strongly  igniting  with  ammonium  fluoride.  For  this  purpose 
the  ignited  mineral  is  placed  in  a  platinum  dish,  covered  with 
10  c.c.  of  ammonia,  evaporated  to  dryness,  diluted  with  water, 
strongly  acidified  with  concentrated  hydrofluoric  acid,  and  again 
evaporated  to  dryness.  The  dish  is  placed  in  a  nickel  beaker  and 
ignited  quite  strongly,  until  finally  the  excess  of  ammonium  fluoride 
is  driven  off.  The  residue  is  now  treated  with  sulphuric  acid  (1:2) 
in  order  to  decompose  salts  of  hydrofluosilicic  acid,  evaporated 
on  the  water-bath  as  far  as  possible,  and  then  the  greater  part  of 
the  sulphuric  acid  is  removed.  From  this  point  the  procedure 
is  the  same  as  in  the  regular  Berzelius  method. 

The  Smith  method  is  always  applicable  and  has  the  advan- 
tage that  the  magnesium  is  practically  completely  removed  at 
the  start. 

*  Many  silicates  can  be  decomposed  by  evaporation  with  hydrofluoric 
and  hydrochloric  acids.  F.  Hinder,  Z.  anal.  Chem.,  1906,  332. 


502       GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

Analysis  of  Lepidolite. 

Lepidolite  is  a  member  of  the  mica  group  and  contains  lithium 
and  fluorine  with  the  following  composition: 

Si309Al2(Li,K,Na)2(F,OH)2 

Si02 = 40  to  45  per  ct. ;  A12O3  =  19  to  38  per  ct. ;  MnO = 0  to  5  per  ct. ; 
MgO  =  0  to  0.5  per  ct. ;  K2O = 4  to  11  per  ct. ;  Li2O  =  1  to  6  per  ct.  ; 
Na2O  =  0  to  2  per  ct.;  F=l  to  10  per  ct.;  H2O  =  1  to  3  per  ct. 

Besides  the  above,  calcium,  iron,  phosphoric  acid,  and  chlorine 
are  frequently  found,  and  in  rare  cases  small  amounts  of  caesium 
and  rubidium  are  present. 

The  determination  of  the  silicic  acid,  aluminium,  iron,  man- 
ganese, and  magnesium  is  effected  as  in  the  case  of  the  orthoclase 
analysis,  except  that  in  this  case  the  manganese  must  be  separated 
from  the  iron  and  aluminium  as  described  on  p.  149  or  152. 

Determination  of  the  Alkalies. — The  weight  of  NaCl+  KC1+  LiCl 
is  determined  by  one  of  the  methods  given  under  the  analysis  of 
orthoclase,  and  the  potassium  weighed  as  potassium  chloroplatinate. 
The  platinum  is  then  removed  by  the  treatment  with  hydrogen, 
or  the  solution  is  heated  to  boiling  and  the  platinum  is  precipitated 
as  the  sulphide  by  the  introduction  of  hydrogen  sulphide.  The 
filtrate  free  from  platinum  is  evaporated  to  dryness  and  the  lithium 
separated  from  the  sodium  as  described  on  p.  53  or  p.  55. 

Determination  of  Fluorine. — This  determination  is  the  same  as 
in  the  case  of  analysis  of  fluorine  in  calcium  fluoride  (p.  471), 
except  that  it  is  unnecessary  to  add  any  silica,  for  the  mineral 
itself  already  contains  a  sufficient  quantity. 

Determination  of  Water. — This  is  effected  by  the  method  of 
Rose-Jannasch  (p.  484). 

Determination  of  Ferrous  Iron  in  Silicates  and  Rocks. 

The  very  finely  powdered,  but  not  bolted,  mineral  contained 
in  a  platinum  dish  is  covered  with  5  to  10  c.c.  of  dilute  sulphuric 
acid  (1:4)  and  placed  upon  the  little  triangle  (a)  Fig.  81,  made  of 
glass  or,  better,  platinum.  This  is  placed  in  the  lead  vessel  C  and 
the  latter  rests  in  a  paraffin  bath  (B).  After  the  cover  is  placed  upon 


DETERMINATION  OF  FERROUS  IRON. 


5°3 


C,  a  rapid  current  of  carbon  dioxide  is  passed  through  A,  whereby 
the  air  within  the  apparatus  will  be  replaced  in  about  three  minutes. 
The  cover  is  quickly  removed,  and  5  to  10  c.c.  of  concentrated  hydro- 
fluoric acid  are  added.  The  cover  is  immediately  replaced,  and  the 
current  of  carbon  dioxide  continued,  while  the  contents  of  the  dish 
are  repeatedly  stirred  during  the  whole  operation  by  means  of  a 
platinum  spatula  or  piece  of  coarse  wire  introduced  through  the  other 
hole  in  the  cover.*  At  the  same  time  the  paraffin  bath  is  heated 
to  100°  C.  and  kept  at  this  temperature  for  about  an  hour.  As 


FIG.  81. 

soon  as  no  more  gritty  particles  are  to  be  felt,  the  temperature  of 
the  bath  is  raised  to  about  120°  C.  in  order  to  remove  the  large 
excess  of  hydrofluoric  acid.  This  requires  about  another  hour. 
The  dish  is  then  allowed  to  cool  in  the  carbon  dioxide  atmosphere 
and  its  contents  are  finally  washed  into  400  c.c.  of  cold  distilled 
water,  10  c.c.  of  concentrated  sulphuric  acid  are  added,  and 
the  solution  is  titrated  with  a  potassium  permanganate  solution 
of  known  strength  until  a  pink  color  is  obtained  which  is 
permanent  for  several  seconds.  This  end-point  is  fugitive  in 
proportion  to  the  amount  of  hydrofluoric  acid  remaining  in  the 
solution. 

Remark. — The  above  method  has  been  used  in  the  author's 
laboratory  with  success  for  several  years.  It  is  a  modification 
of  Cooke's  t  method  in  which  the  decomposition  with  hydrofluoric 

*  In  Fig.  81,  this  second  opening  is  incorrectly  shown.     It  should  really 
be  in  the  middle  of  the  cover  directly  over  th:  ovaporating-dish. 
t  J.  P.  Cooke,  Am.  J.  Science  [2],  XLIV,  p.  347  (1867). 


504       GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

acid  took  place  under  a  glass  funnel  upon  the  water-bath.  In 
this  case  a  large  amount  of  hydrofluoric  acid  remains  in  solution 
and  it  is  difficult  to  obtain  a  sharp  end-point. 

Another  method  for  the  determination  of  the  amount  of  ferrous 
iron  present  in  insoluble  silicates  is  that  of  Mitscherlich.  The 
silicate  is  decomposed  in  a  closed  tube  with  sulphuric  acid 
(8  H2SO4: 1  H20)  under  pressure,  and  the  resulting  solution  titrated 
with  potassium  permanganate.  This  method  usually  gives  good 
results  in  the  case  of  a  silicate  analysis,  but  it  is  worthless  for  the 
analysis  of  rocks  containing  pyrite  or  other  sulphides,  which  on 
treatment  with  sulphuric  acid  are  decomposed  with  evolution  of 
SO2.*  The  latter  serves  to  reduce  iron  that  was  originally  present 
in  the  ferric  form,  so  that  a  too  high  result  will  be  obtained. 

Determination  of  Small  Amounts  of  Titanium  in  Rocks. 

The  colorimetric  method  of  A.  Weller  is  best  suited  for  this 
purpose,  and  is  to  be  preferred  over  all  gravimetric  methods. 

Procedure. — The  silicic  acid  is  removed  exactly  as  in  the  analysis 
of  orthoclase  (p.  491)  and  in  the  filtrate  the  iron,  aluminium, 
titanium,  zirconium  (chromium  and  vanadium)  are  separated 
from  the  manganese,  magnesium,  and  calcium,  by  the  acetate 
method.  The  precipitate  thus  obtained  still  contains  traces  of 
manganese,  so  that  it  is  dissolved  in  dilute  hydrochloric  acid  and 
reprecipitated  by  ammonia.  The  precipitate  is  ignited  in  the 
same  crucible  in  which  the  residue  from  the  impure  silica  is  con- 
tained (small  amounts  of  titanium  are  likely  to  be  in  this  residue) 
fused  with  potassium  pyrosulphate,  and  the  melt  dissolved  in  water 
containing  sulphuric  acid.  Any  insoluble  silicic  acid  is  filtered  off 
and  the  titanium  determined  in  the  filtrate  as  described  on  p.  100 
by  treatment  with  hydrogen  peroxide. 

Remark. — In  rock  analysis  it  is  convenient  to  determine  the 
titanium  after  the  determination  of  the  total  iron.  For  this 
purpose  the  solution  of  the  potassium  pyrosulphate  melt  is  satu- 
rated with  hydrogen  sulphide  in  order  to  precipitate  the  platinum 

*L.  L.  de  Koninck,  Zeit.  fur  anorg.  Chem.,  26  (1901).  125,  and  Hille- 
brand  and  Stokes,  J.  Am.  Chern.  Soc.,  XXII  (1900),  p.  625.  See  also  Stokes, 
Am.  J.  Sci.,  Dec.,  1901. 


ZIRCONIUM  AND  SULPHUR  IN  ROCKS.  5°5 

and  reduce  the  iron,  and  the  filtrate  from  the  platinum  sulphide 
is  titrated  with  potassium  permanganate  after  expelling  the  excess 
of  hydrogen  sulphide,  as  described  on  p.  109.  The  solution  is 
afterwards  concentrated  to  about  80  c.c.,  and  the  titanium  de- 
termined as  above. 

Of  the  gravimetric  methods,  that  of  Gooch  is  best  suited  (p.  116), 
but  even  this  fails  in  the  presence  of  zirconium  (Hillebrand),  so 
that  it  is  in  all  cases  better  to  employ  the  colorimetric  method. 

If  it  is  desired  to  analyze  a  rock  for  titanium  alone,  about  one 
gram  should  be  treated  with  hydrofluoric  and  sulphuric  acids 
(see  p.  -199),  the  greater  part  of  the  sulphuric  acid  removed  by 
volatilization,  in  order  to  make  sure  that  the  hydrofluoric  acid 
is  expelled,  and  the  residue  taken  up  in  water.  From  this  solu- 
tion the  titanium  is  determined  as  above. 

Determination  of  Zirconium  and  Sulphur  in  Rocks. 
W.  F.  Hillebrand.* 

About  2  gms.  of  the  substance  are  fused  with  5  or  6  times  as 
much  sodium  carbonate  (free  from  sulphur)  and  0.5  gm.  potassium 
nitrate  in  a  large  platinum  crucible.  The  crucible  should  be 
placed  through  a  hole  in  a  piece  of  asbestos  and  held  in  an  in- 
clined position  so  that  none  of  the  sulphur  from  the  flame  can 
come  in  contact  with  the  contents  of  the  crucible.  The  melt  is 
taken  up  in  water,  a  few  drops  of  alcohol  are  added  in  order  to 
reduce  any  manganate  to  manganous  salt,  the  solution  is  filtered, 
and  the  precipitate  washed  with  dilute  soda  solution.  The  filtrate 
contains  all  the  sulphur  in  the  presence  of  sodium  silicate,  f  while 
the  residue  contains  all  the  barium  and  zirconium  together  with 
the  remaining  oxides  which  wrere  present  in  the  rock. 

(a)  Treatment  of  the  Filtrate. 

This  should  amount  to  100-250  c.c.  in  volume;  it  is  acidified 
with  hydrochloric  acid,  heated  to  boiling,  and  precipitated  with 
hot  barium  chloride  solution.  After  standing  twelve  hours  the 
barium  sulphate  is  filtered  off  and  weighed. 

*  Bulletin  of  the  U.  S.  Geolog.  Survey  (1900),  p.  73. 

t  Besides  sulphuric  and  silicic  acids  the  filtrate  may  contain  chromic 
(yellow  color),  vanadic,  molybdic,  phosphoric,  arsenic,  and  tungstic  acids. 


506      GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

According  to  Hillebrand  it  is  not  necessary  to  evaporate  the 
solution  to  remove  the  silicic  acid  before  precipitating  the  sul- 
phuric acid;  for  from  a  dilute  solution  silicic  acid  is  never  precipi- 
tated with  the  barium  sulphate. 

(b)  Treatment  of  the  Residue. 

The  residue  is  washed  by  means  of  a  stream  of  dilute  sulphuric 
acid  (1:20)  into  an  evaporating-dish,  and,  after  digesting  for  some 
time,  it  is  filtered  through  the  original  filter.  The  filtrate  con- 
tains aluminium,  iron,  and  the  greater  part  of  the  zirconium. 
The  residue  contains  the  rest  of  the  zirconium  together  with  barium 
sulphate  and  some  silicic  acid;  after  being  washed,  it  is  ignited 
in  a  platinum  crucible  and  freed  from  silica  by  evaporation  with 
sulphuric  and  hydrofluoric  acids.  The  residue  in  the  crucible 
is  then  taken  up  in  hot  dilute  sulphuric  acid  and  filtered.  The 
insoluble  portion  can  be  used  for  the  determination  of  barium 
(see  below). 

The  two  sulphuric  acid  filtrates,  containing  at  the  most  only 
1  per  cent,  of  this  acid,  are  treated  with  hydrogen  peroxide 
and  a  few  drops  of  disodium  phosphate.  Aluminium  and  iron 
are  not  precipitated  on  account  of  the  acid  present,  and  only 
traces  of  titanium  are  thrown  down,  while  all  of  the  zirconium 
is  precipitated  as  phosphate,  after  standing  24  to  48  hours. 

If  the  yellow  color  of  the  solution  should  fade  away,  a  little 
more  hydrogen  peroxide  is  added;  the  precipitate  is  filtered  off, 
and,  even  when  it  is  small  in  amount,  it  is  purified  from  the  titanium 
as  follows:  The  filter,  together  with  the  precipitate,  is  ignited, 
fused  with  a  little  sodium  carbonate,  the  melt  extracted  with 
water  and  filtered.  This  residue  is  likewise  ignited,  but  it  is  now 
fused  with  potassium  pyrosulphate,  and  the  fusion  dissolved  in  hot 
water  containing  a  few  drops  of  dilute  sulphuric  acid.  The  solution 
is  poured  into  a  small  Erlenmeyer  flask  of  about  20  c.c.  capacity,  a 
few  drops  of  4  per  cent,  hydrogen  peroxide  and  a  few  drops  of  sodi- 
um phosphate  solution  are  added,  and  after  standing  1  or  2  days  the 
precipitate  is  filtered  off.  The  latter  is  now  free  from  titanium  in 
nearly  every  case,  and  after  ignition  it  is  weighed  as  zirconium  phos- 
phate. Although  zirconium  phosphate  theoretically  contains  51.8 
per  cent.  Zr02,  there  will  be  no  appreciable  error  introduced  if  it  is 


SEPARATION  OF  SOLUBLE  FROM  INSOLUBLE  SILICIC  ACID.   S°7 

assumed  that  one-half  the  weight  of  the  precipitate  represents  the 
amount  of  this  oxide  present. 

Determination  of  Barium. 

The  above-mentioned  precipitate  containing  all  the  barium  as 
sulphate,  in  the  presence  of  calcium  and  perhaps  strontium,  always 
contains  a  little  silicic  acid.  In  order  to  remove  the  latter,  it  is 
heated  with  hydrofluoric  and  sulphuric  acids  and  the  residue  is 
fused  with  sodium  carbonate.  The  melt  is  treated  with  water  and 
the  carbonates  of  barium  and  calcium  are  filtered  off,  washed,  and 
then  dissolved  in  hot  dilute  hydrochloric  acid.  From  this  solution 
the  barium  is  precipitated  by  the  addition  of  a  slight  excess  of 
sulphuric  acid  and  ignited  wet  in  a  platinum  crucible.  The  pre- 
cipitate thus  obtained  contains  a  small  amount  of  calcium  sulphate, 
which  must  be  eliminated.  For  this  purpose  the  residue  is  dis- 
solved in  the  crucible  by  hot  concentrated  sulphuric  acid,  and  after 
cooling  the  solution  is  poured  into  water.  In  this  way  a  precipitate 
of  barium  sulphate  free  from  calcium  is  obtained.  It  is  ignited 
and  weighed. 

Separation  of  Soluble  from  Insoluble  Silicic  Acid:  Lunge  and 

Millberg.* 

Frequently  a  mixture  of  silicates  is  to  be  analyzed  which  is 
partly  decomposed  on  treatment  with  acids,  with  the  separation 
of  gelatinous  silicic  acid,  and  partly  unaffected.  The  silicic  acid 
deposited  from  solution  by  the  addition  of  acids  is  soluble  in  5 
per  cent,  sodium  carbonate  solution,  while  quartz  and  feldspar 
are  not  appreciably  attacked  by  the  latter  (cf.  Vol.  I.  pp.  356,  357). 

If  it  is  desired  to  separate  the  deposited  silicic  acid  from  the 
unattached  silicate  (usually  feldspar  and  quartz),  the  substance  is 
treated  with  acid  (hydrochloric  or  nitric)  and  evaporated  on  the 
water-bath  until  a  dry  powder  is  obtained.  This  is  moistened  with 
acid,  diluted,  boiled,  and  filtered.  After  washing,  the  residue  is 
digested  with  5  per  cent,  sodium  carbonate  solution  on  the 
water-bath,  in  a  porcelain  dish  for  fifteen  minutes.  It  is  then 
filtered,  washed  first  with  soda  solution  and  finally  with  water. 

*  Zeitschr.  f.  angew.  Chemie,  1897,  pp.  393  and  425. 


5°8:     GRAVIMETRIC  DETERMINATION  OF  THE  METALLOIDS. 

If  a  turbid  filtrate  should  be  obtained,  a  little  alcohol  is  added  to 
the  wash  water,  after  which  the  filtrate  will  at  once  run  through 
clear. 

The  alkaline  filtrate  contains  the  soluble  silicic  acid;  this  can 
be  determined  by  acidifying  and  evaporating  to  dry  ness.  The 
residue  from  the  sodium  carbonate  treatment,  consisting  of  quartz 
and  feldspar,  is  weighed.  In  order  to  determine  the  quartz,  the 
mixture  is  acted  upon  by  sulphuric  and  hydrofluoric  acids,  the 
excess  of  the  latter  is  removed  by  heating  with  sulphuric  acid,  and 
the  cold  residue  is  dissolved  in  water,  precipitated  with  ammonia, 
and  the  alumina  weighed.  If  this  weight  is  multiplied  by  5.41, 
the  corresponding  amount  of  feldspar  is  obtained,  and  if  this  is 
deducted  from  the  weight  of  the  quartz  +  feldspar,  the  weight  of 
the  quartz  will  be  found. 

Determination  of  Soluble  Silicic  Acid  in  Clay. 

Clay  contains  besides  alumina,  sand  (quartz  +  breccia)  and 
small  amounts  of  calcium  and  magnesium  carbonates. 

About  2  gms.  of  the  substance,  after  having  been  dried  at  120°, 
and  being  in  the  form  of  a  not-too-fine  powder,  are  moistened  with 
water,  and  a  mixture  of  100  c.c.  water  and  50  c.c.  concentrated  sul- 
phuric acid  *  is  added.  The  porcelain  dish  is  covered  with  a  watch- 
glass  and  heated  over  a  free  flame  until  dense  fumes  of  sulphuric 
acid  vapors  are  evolved.  The  contents  of  the  dish  are  allowed 
to  cool,  150  c.c.  of  water  and  3  c.c.  of  concentrated  hydrochloric 
acid  are  added,  the  solution  boiled  for  fifteen  minutes,  filtered, 
washed  completely,  and  the  mixture  of  soluble  silicic  acid,  quartz, 
and  insoluble  silicate  is  treated  as  above. 

Remark. — It  was  formerly  the  custom  to  separate  the  soluble 
silica  from  the  insoluble  silica  by  boiling  with  potassium  hydroxide 
solution.  According  to  the  experiments  of  Lunge  and  Millberg, 
however,  this  is  not  permissible  because  quartz  is  perceptibly 
soluble  in  caustic  potash  solution.  If,  on  the  other  hand,  the 
substance  is  obtained  in  a  very  finely-divided  condition,  even 
sodium  carbonate  solution  cannot  be  used  for  the  same  reason. 

*  Alexander  Subech,  Die  chem.  Industrie  1902.  p.  17. 


ANALYSIS   OF  CHROMITE.  509 

Analysis  of  Chromite. 

Although  chromite  (chrome  iron  ore)  is  not  a  silicate,  it  is 
insoluble  in  all  acids,  and  can  be  brought  into  solution  by  fusion 
with  alkali  carbonates,  or  borates,  so  that  its  analysis  will  be  dis- 
cussed at  this  place. 

Chromite  contains  18  to  39  per  cent.  FeO,  0  to  18  per  cent.  MgO, 
42  to  64  per  cent.  Cr2O3,  0  to  13  per  cent.  A12O3,  and  0  to  11  per  cent. 
SiO2.  Calcium,  manganese,  and  nickel  are  also  occasionally  present. 

Of  the  finely-powdered  and  bolted  mineral,  0.5  gm.  is  fused 
in  an  inclined,  open  platinum  crucible  with  4  gms.  of  pure  sodium 
carbonate  *  for  two  hours  over  a  good  Teclu  burner.  After  cool- 
ing, the  melt  is  leached  with  water,  acidified  with  hydrochloric 
acid,f  evaporated  in  a  porcelain  dish  until  a  dry  powder  is  obtained, 
moistened  with  hydrochloric  acid,  taken  up  in  water,  and  the 
silica  filtered  off.  The  latter  is  ignited,  weighed,  and  its  purity 
tested  with  hydrofluoric  acid  (p.  487).  The  filtrate  from  the 
silicic  acid  is  precipitated  hot  with  hydrogen  sulphide  and  the 
precipitate  of  platinum  sulphide  and  sulphur  is  filtered  off.  It 
is  then  placed  in  an  Erlenmeyer  flask,  10  c.c.  of  ammonium 
chloride,  enough  ammonia  (free  from  carbonate)  to  make  the 
solution  alkaline,  and  a  little  freshly-prepared  ammonium  sul- 
phide are  added,  after  which  the  flask  is  corked  up  and  allowed 
to  stand  over  night.  In  the  morning  the  precipitate  is  filtered 
off.  washed  twice  with  water  containing  a  little  ammonium  sul- 
phide, then  dissolved  in  hydrochloric  acid,  and  the  precipitation 
by  means  of  ammonium  sulphide  is  repeated.  The  ammonium 
salts  are  removed  from  the  filtrate  and  the  calcium  and  magnesium 
determined  as  described  on  p.  76-8. 

The  ammonium  sulphide  precipitate  is  dissolved  in  dilute 
hydrochloric  acid,  any  residue  of  nickel  or  cobalt  sulphide  is  fil- 


*  Bunsen  fused  the  chromite  vrith  one-third  as  much  SiO2  and  6  to  8 
parts  Xa.,CC);,  and  then  subtracted  the  amount  of  silica  added  from  the 
total  amount  found.  This  makes  the  decomposition  take  place  more  read- 
ily, but  the  author  prefers  not  to  add  the  silica  on  account  of  the  possibility 
of  thereby  introducing  an  error. 

t  If  a  dark  residue  of  undecomposed  mineral  should  remain,  it  is  filtered 
off  and  again  fused  with  sodium  carbonate. 


510     GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

tered  off  and  dried.  This  residue  is  then  ignited  first  in  air, 
then  in  a  current  of  hydrogen,  and  finally  weighed  as  metal.  It 
is  not  worth  while  to  attempt  the  separation  of  the  nickel  from, 
the  cobalt  on  account  of  the  small  amount  present.  The  filtrate 
from  the  sulphides  of  nickel  and  cobalt  is  freed  from  hydrogen 
sulphide  by  boiling,  the  iron  present  is  oxidized  by  evaporating 
with  potassium  chlorate  and  hydrochloric  acid,  and  the  iron, 
chromium,  and  aluminium  are  separated  from  the  manganese  by 
means  of  the  barium  carbonate  method  (p.  149)  and  from  one 
another  as  described  on  p.  Iu7  et  seq.  In  the  filtrate  from  the 
barium  carbonate  precipitate,  the  manganese  is  separated  from 
the  barium  as  described  on  p.  122,  b,  and  determined  as  sulphide 
or  as  sulphate. 

Remark. — If  it  is  desired  to  determine  the  chromium  alone, 
this  is  best  accomplished  by  means  of  a  volumetric  process  (see 
Part  II). 

Determination  of  Thorium  in  Monazite,  according  to  E.  Benz.* 

Monazite  is  a  phosphate  of  the  rare  earths  [PO4(Ce,La,Di,Th)J, 
It  occurs  in  so-called  "monazite  sand"  mixed  with  quartz,  rutile, 
zircon,  tantalates,  etc.,  and  is  at  present  the  raw  material  used  for 
the  preparation  of  thorium  (used  in  the  Welsbach  mantle).  The 
value  of  a  sample  of  monazite  sand  depends  upon  the  amount  of 
thorium  present,  and  its  determination  is  best  effected  as  follows: 

Of  the  bolted  monazite  sand,  0.5  gm.  is  intimately  mixed  with 
10  gms.  of  potassium  pyrosulphate  in  a  spacious  platinum  cruci- 
ble; the  latter  is  covered  and  slowly  heated  until  its  contents  are 
at  a  gentle  fusion.  This  is  best  accomplished  by  placing  the 
platinum  crucible  within  a  larger  porcelain  one  which  is  provided 
with  an  asbestos  ring.  After  no  more  gas  is  given  off,  the  cruci- 
ble is  gently  ignited  over  the  free  flame,  and,  after  cooling,  its 
contents  are  treated  with  water  and  a  little  hydrochloric  acid 
until  it  is  completely  disintegrated.  After  allowing  the  residue 
to  settle,  it  is  filtered,  treated  with  a  little  concentrated  hydro- 
chloric acid,  diluted  with  water,  and  again  fil  tered.  f  In  the  com- 

*  Zeit.  f.  angew  Chem.  15  (1902),  p.  297. 

t  This  residue  is  free  from  thorium,  and  consists  chiefly  of  silicic  and 
tantalic  acids. 


DETERMINATION  OF  THORIUM  IN  MONAZITE.  511 

bined  filtrates  the  hydrochloric  acid  is  nearly  neutralized  with 
ammonia  (the  formation  of  a  permanent  precipitate  is  to  be 
avoided,  for  it  will  be  difficult  to  redissolve  it),  the  solution  is 
heated  to  boiling,  and  3  to  5  gms.  of  solid  ammonium  oxalate  are 
added  while  the  liquid  is  vigorously  stirred.  The  oxalates  of 
the  rare  earths  are  immediately  deposited  in  the  form  of  a  coarse 
powder.  To  make  sure  that  the  precipitation  is  complete,  a  little 
ammonium  oxalate  solution  is  added.  After  standing  twelve 
hours  the  precipitated  oxalates  are  filtered  off,  washed  once  by 
means  of  water  acidified  with  nitric  acid,  then  transferred  to  a 
porcelain  dish,  and  the  last  portions  of  the  precipitate  are  eventually 
washed  from  the  filter  by  repeated  additions  of  hot,  concentrated 
nitric  acid  and  water ;  the  liquid  is  evaporated  almost  to  dryness. 
Ten  cubic  centimeters  of  concentrated  nitric  acid  (sp.  gr.  1.4)  and  20 
c.c.  of  fuming  nitric  acid  are  then  added,  the  dish  covered  with  a 
watch-glass  ana  heated  on  the  water-bath.  After  a  short  time 
the  nitric  acid  oegins  to  decompose  the  oxalic  acid,  shown  by 
the  lively  evolution  of  gas.  After  no  more  gas  is  given  off,  the 
watch-glass  and  sides  of  the  dish  are  washed  down  and  the 
solution  evaporated  to  dryness.  In  order  to  remove  the  free 
nitric  acid,  a  little  water  is  added  and  the  solution  evaporated 
once  more;  after  this  the  filter  fibres  present  are  removed  by 
filtration.  It  is  now  necessary  to  separate  the  thorium  from 
the  remaining  earths.  This  is  effected  by  precipitating  the  former 
with  hydrogen  peroxide  as  thorium  peroxide.  On  ignition  the 
latter  is  changed  into  ThO2,  in  which  form  it  is  weighed. 

The  precipitation  with  hydrogen  peroxide  takes  place  as 
follows:  The  neutral  solution  of  the  nitrates  is  diluted  with  10 
per  cent,  ammonium  nitrate  solution  to  a  volume  of  100  c.c.. 
heated  to  60-80°  C.,  and  precipitated  by  the  addition  of  20  c.c. 
of  pure  3  per  cent,  hydrogen  peroxide  solution.  The  precipitate, 
which  is  colored  yellow  by  traces  of  cerium  peroxide  (at  the  most 
T\  mg.  of  the  latter  is  present),  is  immediately  filtered,  washed 
with  hot  water  containing  ammonium  nitrate,  ignited  wet  in  a 
platinum  crucible,  and  weighed  as  Th02. 

If  it  is  desired  to  obtain  an  absolutely  pure  thorium  oxide, 
the  moist  precipitate  is  dissolved  in  nitric  acid  and  the  above 
precipitation  with  hydrogen  peroxide  is  repeated.  By  this  method 


512      GRAVIMETRIC  DETERMINATION  OF   THE  METALLOIDS. 

E.  Benz  obtained  in  the  analysis  of  a  South  American  monazite 
sand  the  following  results:  4.72,  4.58,  4.50  per  cent.  ThO2. 

Remark. — The  above  process  for  the  determination  of  thorium 
in  monazite  is  quicker  and  more  accurate  than  either  that  of 
Glaser  *  or  that  of  Hintz  and  Weber, f  so  that  it  is  to  be  recom- 
mended for  both  technical  and  scientific  purposes. 

The  determination  of  thorium  oxide  in  thorite  is  carried  out 
in  the  same  way  with  the  difference  that  instead  of  fusing  the 
mineral  with  sodium  fluoride  and  potassium  pyrosulphate.  it  is 
decomposed  by  treatment  with  hydrochloric  acid,  and  the  silica 
removed  as  usual.  The  filtrate  from  the  silica  is  analyzed 
as  above.! 

For  the 

Analysis  of  Incandescent  Mantles 

consult  the  work  of  T.  B.  Stillman,  Chem.  Zeit.,  1906,  60. 

Determination  of  Water  in  Silicates. 

If  the  mineral  on  ignition  loses  nothing  but  water,  the  amount 
of  the  latter  can  be  determined  by  the  loss  in  weight.  In  the 
great  majority  of  cases,  however,  other  constituents  (e.g.  C02.  S02, 
Cl.  F,  etc.)  are  lost  and  the  substance  may  undergo  an  oxidation 
(FeO  is  changed  to  Fe2O3.  PbS  to  PbSO4,  etc.).  In  such  cases 
the  procedure  recommended  by  Jannasch  can  be  used  to  advan- 
tage. The  substance  is  heated  with  lead  oxide,  the  water  vapor 
conducted  over  a  heated  mixture  of  lead  oxide  and  lead  peroxide 
and  absorbed  in  a  weighed  calcium  chloride  tube  (see  p.  484). 

If  the  substance  on  ignition  loses  simply  water  and  carbon 
dioxide  the  former  may  be  accurately  determined  by  the  method 
of  Brush  and  Penfield.§  The  substance  is  introduced  by  means 

*  Chem.  Ztg..  1896.  p.  612. 

t  Zeitschr.  fur  anal  Chemie  (1897).  XXXVI.  p.  27. 

%  As  members  of  the  hydrogen  sulphide  group  are  usually  preseat,  it  is 
advisable  to  first  remove  them  and  to  effect  the  precipitation  of  the  rare 
earths  with  ammonium  oxalate  from  the  slightly  acid  filtrate  from  the  hydro- 
gen sulphide  precipitate. 

§Amer.  Journ.  Sci.  [3],  XL VIII  (1894),  p.  31,  and  Zeit.  fur  anorg. 
Chem  ,  VII  (1894),  p.  22 


DETERMINATION  OF    WATER  IN  SILICATES.  513 

of  a  long  funnel  into  a  bulb  blown  on  the  end  of  a  narrow  tube  made 
of  difficultly-fusible  glass,  and  the  tube  is  provided  with  a  second 
bulb  about  2  or  3  cm.  from  the  end  one.  The  open  end  of  the 
tube  is  connected  by  means  of  a  short  piece  of  rubber  tubing  with 
a  short  tube  drawn  out  into  a  capillary,  and  the  substance  is  heated 
in  the  flame  of  a  good  Teclu  burner.  The  water  is  expelled  and 
condenses  in  the  colder  portion  of  the  tube,  and  as  a  precaution, 
the  latter  is  enveloped  in  moist  blotting-paper.  As  soon  as  no  more 
water  can  be  expelled,  the  end  of  the  tube  is  heated  until  it  softens 
and  the  tube  is  drawn  out  between  the  two  bulbs.  The  front  end 
of  the  tube  now  contains  the  water  in  the  presence  of  a  little  carbon 
dioxide,  and  the  latter  must  be  removed.  For  this  purpose  the 
tube  is  inclined  at  an  angle  of  40°,  so  that  the  heavier  carbon  dioxide 
will  run  out  of  it.  The  weight  of  the  tube  slowly  diminishes, 
but  at  the  end  of  about  three  hours  it  becomes  constant,  losing 
about  0.0003  gm.  per  hour,  due  to  the  evaporation  of  water.  If, 
therefore,  the  tube  is  allowed  to  stand  three  hours  before  weighing, 
0.0009  gm.  must  be  added  to  the  weight  of  the  water.  If  the 
substance  contained  a  large  amount  of  carbonate,  the  escaping 
carbon  dioxide  will  carry  aqueous  vapor  with  it,  so  that  a  further 
correction  must  be  made.  One  gram  of  CO2  at  an  average  baro- 
metric pressure  (760  mm.)  and  temperature  (20°  C.)  will  cause 
a  loss  of  0.0096  gm.  water  vapor.  If  the  amount  of  carbon  dioxide 
present  is  known,  it  is,  therefore,  only  necessary  to  multiply  its 
weight  by  0.0096  to  obtain  the  amount  of  water  that  would  other- 
wise escape  the  determination. 


Determination  of  Silicon. 

See  Steel  Analysis,  p.  441. 

Determination  of  Silicon  in  the  Presence  of  Silicic  Acid. 
Cf.  M.  Phillips,  Z.  angew.  Ghem.,  14,  1969  (1905). 


PART  II. 
VOLUMETEIC  ANALYSIS. 


A  GRAVIMETRIC  analysis  is  accomplished  by  adding  to  the  solu- 
tion of  the  substance  to  be  analyzed  a  reagent  of  only  approxi- 
mately-known strength,  separating  one  of  the  products  of  the  reac- 
tion from  the  solution  and  weighing  it.  On  the  other  hand,  a 
volumetric  analysis  is  made  by  causing  the  reaction  to  take 
place  by  means  of  a  measured  amount  of  a  solution  of  accurately- 
known  strength  and  computing  the  amount  of  substance  present 
by  the  volume  of  the  solution  which  reacts  with  it  (cf.  p.  2). 
For  the  latter  sort  of  analysis  accurately-calibrated  measuring 
instruments  are  necessary,  as  will  be  briefly  described. 

Measuring  Instruments. 

1.  Burettes  are  tubes  of  uniform  bore  throughout  the  whole  length  ; 
they  are  divided  into  cubic  centimeters  and  are  closed  at  the  bottom, 
as  shown  in  Fig.  82,  by  means  of  a  glass  stop-cock  or  with  a  piece 
of  rubber  tubing  containing  a  glass  bead  h.     The  latter  form  was 
devised  by  Bunsen  and  is  used  as  follows :    The  tubing  is  seized 
between  the  thumb  and  forefinger  at  the  place  where  the  glass 
bead  is,  and  by  means  of  a  gentle  pressure  a  canal  is  formed  at 
one  side  of  the  bead  through  which  the  liquid  will  run  out.     Instead 
of  the  glass  bead  an  ordinary  pinch-cock  is  frequently  used. 

Besides  the  above  forms  of  burettes,  a  great  many  others  are 
in  use,  but  it  is  unnecessary  to  describe  them  here. 

2.  Pipettes. — A  distinction  must  be  made  between  a  "full", 
pipette  and  a   "measuring"   one.    A  full  pipette  has  only  one 

514 


MEASURING  INSTRUMENTS.  515 

mark  upon  it,  and  serves  for  measuring  off  a  definite  amount  of 
liquid.  They  are  constructed  in  different  forms;  usually  they 
consist  of  a  glass  tube  with  a  cylindrical  widening  at  the  middle. 
The  lower  end  i«  drawn  out,  leaving  an  opening  about  J-l  mm. 


FIG.  82. 

wide.  Pipettes  of  this  nature  are  constructed  which  will  hold 
respectively  1,  2,  5,  10,  20,  25,  50,  100,  and  200  c.c. 

Measuring  pipettes  are  burette-shaped  tubes  graduated  into> 
cubic  centimeters  and  drawn  out  at  the  lower  end  as  before.  They 
serve  to  measure  out  any  desired  amount  of  liquid  and  are  obtained 
with  a  total  capacity  of  1,  2,  5,  10,  20,  25,  and  50  c.c. 

3.  Measuring-flasks  are  flat-bottomed  flasks  with  narrow 
necks  provided  with  a  mark,  so  that  when  they  are  filled  to  this 


51 6  YOLU METRIC  ANALYSIS. 

point  they  will  contain  respectively  50,  100,  200,  250,  300,  500, 
1000,  and  2000  c.c.  They  serve  for  the  preparation  of  standard 
solutions  and  for  the  dilution  of  liquids  to  a  definite  volume. 

4.  Measuring-cylinders  are  graduated  into  cubic  centimeters 
and  are  used  only  for  rough  measurements.  , 

It  is  clear  that  accurate  results  can  be  obtained  by  a  volu- 
metric analysis  only  when  the  instruments  used  are  accurately 
calibrated.  It  should  never  be  taken  for  granted  that  a  pur- 
chased instrument  is  correct,  but  it  should  always  be  carefully 
tested.  In  the  case  of  measuring-flasks  and  "full"  pipettes,  it 
is  best  for  each  one  to  etch  for  himself  the  position  on  the  flask 
or  tube  up  to  which  they  should  be  filled  with  liquid. 


Normal  Volume  and  Normal  Temperature. 

A  liter,  which  is  the  volume  of  a  kilogram  of  water  at  its 
maximum  density,  is  taken  as  the  normal  volume.  If  it  is  desired 
to  mark  on  the  neck  of  a  liter-flask  the  point  to  which  this  volume 
reaches,  the  position  of  the  mark  depends  upon  the  temperature 
of  the  vessel.  It  is  necessary,  therefore,  to  choose  for  the  vessel 
itself  a  definite  temperature,  the  so-called  normal  temperature. 
At  present  the  temperature  of  +15°  C.  is  almost  universally 
taken  as  the  normal  temperature.  According  to  this,  then,  the 
flask  should  be  marked  at  15°  with  the  volume  occupied  by  a 
kilogram  of  water  at  +4°,  and  as  the  kilogram  is  the  unit  of  mass, 
the  weighing  should  also  take  place  in  a  vacuum. 

This  experimental  impossibility  can  be  avoided  inasmuch  as 
the  weight  of  a  liter  of  water  is  known  accurately  at  temperatures 
other  than  +4°,  also  the  expansion  of  the  glass  with  rise  of 
temperature,  and  the  buoyancy  which  the  weights  and  the  water 
experience  as  a  result  of  weighing  in  the  atmosphere.  The  weights 
which  must  be  placed  upon  the  balance  pan  in  order  to  determine 
the  space  occupied  by  a  true  liter  of  water,  therefore,  depend  upon 
the  temperature  of  the  water  and  of  the  vessel,  as  well  as  the 
density  of  the  air  at  the  time  of  the  experiment.  The  density  of 
the  air  varies  somewhat  from  day  to  day  and  depends  upon  the 
barometric  pressure,  the  temperature,  and  the  amount  of  moisture. 


MEASURING   INSTRUMENTS. 


517 


It  suffices  in  most  cases,  however,  to  assume  the  average  values  of 
these  factors  corresponding  to  the  locality. 

As  regards  the  density  of  water  at  different  temperatures,  this 
is  given  in  the  following  table: 

DENSITY    OF    WATER   AT   DIFFERENT   TEMPERATURES  * 


I 

Density. 

1 

Density 

t 

Density. 

0° 

0.999867 

14° 

0.999271 

28° 

0.996258 

1 

9926 

15 

9126 

29 

0.995969 

2 

9968 

1C 

8969 

30 

5672 

3 

9992 

17 

8801 

31 

5366 

4 

1.000000 

18 

8G21 

32 

5052 

5 

0.999992 

19 

8430 

33 

0.994728 

6 

9968 

20 

8229 

34 

4397 

7 

9929 

21 

8017 

35 

4058 

8 

9876 

22 

0.997795 

36 

0.993711 

9 

9808 

23 

7563 

37 

3356 

10 

9727 

24 

7321 

38 

0.992993 

11 

9032 

25 

7069 

39 

2G22 

12 

9524 

26 

0.996808 

40 

0.992244 

13 

9404 

27 

6538 

*  Thiesen,  Scheel  and  Diesselhorst,  1904. 

With  the  aid  of  this  table  it  is  possible  to  tell  what  the  weight 
of  a  liter  of  water  will  be  at  any  temperature.  It  was  stated  on 
paee  13  that  if  po  is  the  weight  of  a  body  in  a  vacuum  and  p 
that  of  the  same  body  in  the  air,  then 


in  which  expression  ^  denotes  the  density  of  the  air  under  the 
prevailing  conditions,  s  that  of  the  body  and  Si  that  of  the  brass 
weights  at  t°. 

At  J°,  however,  the  volume  occupied  by  the  mass  compared 
with  that  at  15°  is 


where  a  is  the  coefficient  of  cubical  expansion.     The  weight  of 


518  VOLUMETRIC  ANALYSIS. 

water  contained  in  the  mass  at  t°  in  a  vacuum  (disregarding 
quantities  of  the  second  order)  is: 


s 


If,  therefore,  it  is  desired  to  determine  the  volume  of  a  liter 
by  weighing  water  at  17.35°  with  brass  weights,  the  computation 
is  carried  out  as  follows: 

The  density  of  water  at  17.35°  is  given  by  interpolation  in  the 
above  table  as  0.9987  =  s,  the  density  of  the  brass  weights  can 
be  taken  as  8.0  =  si,  the  density  of  the  air  as  0.001214  and  the 
coefficient  of  cubical  expansion  of  glass  as  0.000027,  so  that  by 
inserting  these  values  in  the  above  equation: 


0.9987[1 +0.000027(17.35- 15)1  _09977 
P~  0.001214     0.001214 

0.9987   "        8.0 


or  in  other  words  the  volume  occupied  by  997.7  gms.  of  water 
under  the  above  conditions  represents  one  liter  in  glass 
at  15°. 

Invariably  the  temperature  of  the  laboratory  is  such  that 
somewhat  less  than  1000  gms.  is  used  for  the  calibration  in  true 
cubic  centimeters.  It  is  convenient,  therefore,  to  place  the 
1000-gm.  weight  on  one  side  of  the  balance  together  with  the 
empty  flask,  and  then  place  a  tare  on  the  opposite  side  of  the 
balance.  Then  the  1000-gm.  weight  is  removed  and  in  its  place 
1000  —  p  gms.  are  placed,  after  which  equilibrium  is  restored  by 
filling  the  flask  with  water. 

To  avoid  the  somewhat  tedious  calculation  of  the  value  of 
p,  W.  Schlosser  *  has  calculated  the  following  table  in  which  the 
values  are  given  for  1000  —  p  at  different  temperatures. 

*  Z.  angew.  Chem.,  1903,  960;  Chem.  Ztg.,  1904,  4. 


MEASURING  INSTRUMENTS. 
Correction  Table. 


5*9 


This  table  shows  in  milligrams  how  much  less  than  1000  gin.  is  the  weight 
of  water  which  occupies  a  volume  of  one  liter,  on  the  assumption  that  the 
coefficient  of  cubical  expansion  for  the  glass  is  0.000,027  per  degree  Centi- 
grade, the  normal  temperature  is  15°,  the  barometric  pressure  760  mm.  the 
temperature  of  the  air  15°,  and  the  tension  of  aqueous  vapor  is  normal. 
The  table  reads  from  a  temperature  of  5.0°  to  one  of  30.9°. 


t 

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0.7 

0.8 

0.9 

5° 

1341 

1340 

1339 

1338 

1338 

1338 

1338 

1338 

1338 

1338 

6 

7 
8 
9 

1338 
1350 
1376 
1417 

1339 
1352 
1380 

1421 

1340 
1354 
1384 
1426 

1341 
1356 
1388 
1431 

1342 
1358 
1392 
1436 

1343 
1360 
1396 

1442 

1344 
1363 
1400 
1447 

1345 
1366 
1404 
1452 

1346 
1369 
1408 
1458 

1348 
1372 
1412 
1464 

10 

1471 

1477 

1483 

1489 

1496 

1503 

1510 

1517 

1524 

1531 

11 
12 

13 
14 

1539 
1619 

1713 
1819 

1.547 
1628 
1723 
1830 

1555 
1637 
1733 
1841 

1563 
1646 
1743 
1853 

1571 
1655 
1753 
1865 

1579 
1664 
1764 

1877 

1587 
1673 

1775 
1889 

1595 
1683 
1786 
1901 

1603 
1693 
1797 
1913 

1611 
1703 
1808 
1925 

15 

1937 

1949 

1962 

1975 

1988 

2001 

2014 

2027 

2040 

2053 

16 

17 
18 
19 

2066 
2208 
2360 
2525 

2080 
2223 
2376 
2542 

2094 
2238 

23'.  >2 
2559 

2108 
2253 
2408 
2576 

2122 
2268 
2424 
2593 

2136 
2283 
2440 
2610 

2150 
2298 
2457 
2627 

2164 
2313 
2474 
2645 

2178 
2328 
2491 
2663 

2193 
2344 
2508 
2681 

20 

2699 

2717 

2735 

2753 

2771 

2789 

2807 

2826 

2845 

2864 

21 
22 

23 

24 

2883 
3078 
3283 
3498 

2902 
3098 
3304 
3520 

2921 
3118 
3325 
3542 

2940 
3138 
3346 
3564 

2959 
3158 
3367 
3586 

2978 

3178 
3388 
3609 

2998 
3199 
3410 
3632 

3018 
3220 
3432 
3655 

3038 
3241 
3454 
3678 

3058 
3262 
3476 
3701 

25 

3724 

3747 

3770 

3793 

3816 

3839 

3862 

3886 

3910 

3934 

26 
27 

28 
29 

3958 
4202 
4455 
4716 

3982 
4227 
4481 

4743 

4006 
4252 
4507 
4770 

4030 

4277 
4533 
4797 

4054 
4302 
4559 
4824 

4078 
4327 
4585 
4851 

4102 
4352 
4611 

4878 

4127 
1  4377 
4637 
4905 

4152 
4403 
4663 
4932 

4177 
4429 
4689 
4959 

30 

4987 

5014 

5041 

5069 

5097 

5125 

5153 

5181 

5210 

5239 

If  it  be  desired  to  take  into  consideration  the  deviation  of  the 
temperature  and  barometric  pressure  from  that  assumed  in  the 
above  table,  it  is  sufficient  to  add  (or  subtract)  to  the  figure  given 


520 


yOLU METRIC  ANALYSIS. 


in  the  table  1.4  mgm.  for  each  millimeter  that  the  barometer  reads 
above  (or  below)  760  mm.,  and  to  subtract  (or  add)  4  mgm.  for  each 
degree  that  the  temperature  of  the  air  is  above  (or  below)  15°  C. 

If,  for  example,  the  temperature  of  the  water  is  17.35°,  the 
barometer  reading  720  mm.,  and  the  temperature  of  the  air  23.7°, 
then  the  correction  is  computed  as  follows: 

According  to  the  table  the  value  of  1000  —  p  is  2260  mgm.,. 
this  number,  therefore  should  be  diminished  by 

(760 -720)  1.4  =  56  mgm. 
(23.7-15)0.4  =  35 

91  mgm. 

The  correction  becomes  2260-91  =  2169  mgm.  =  2. 169  gms. 
In  order  to  simplify  the  matter  still  further,  Schlosser  recommends 
preparing  a  special  table  for  localities  where  the  average  barometric 
pressure  is  considerably  less  than  760  mm.  Thus  the  following 
table  applies  to  Zurich  and  can  be  used  for  other  places  where  the 
average  barometric  pressure  is  correspondingly  low. 


CORRECTION    IN    GRAMS    FOR    1000  C.C. 

under  the  assumption  that  the  coefficient  of  cubical  expansion  for  glass  is 
0.000027  per  degree  Centigrade,  the  normal  temperature  of  glass  is  15°,  the 
temperature  of  the  water  between  5°  and  30.5°,  the  barometer  reading  720 
mm.,  the  temperature  of  the  air  15°  and  the  vapor  tension  normal. 


1 

Correction 
in  Grama. 

1 

Correction 
in  Grains. 

t 

Correction 
in  Grams. 

t 

Correction 
in  Grams. 

5.0° 

1.284 

11.5° 

1.522 

18.0° 

2.303 

24.5° 

3.552 

5.5 

1.281 

12.0 

1.562 

18.5 

2.383 

25.0 

3.667 

6.0 

.281 

12.5 

1.607 

19.0 

2.468 

25.5 

3.782 

6.5 

.286 

13.0 

1.656 

19.5 

2.553 

26.0 

3.901 

7.0 

.293 

13.5 

1.707 

20.0 

2.642 

26.5 

4.021 

7.5 

.303 

14.0 

1.762 

20.5 

2.732 

27.0 

4.145 

8.0 

.319 

14.5 

1.820 

21.0 

2.826 

27.5 

4.270 

8.5 

.339 

15.0 

1.880 

21.5 

2.921 

28.0 

4.398 

9.0 

.360 

15.5 

1.944 

22.0 

3.021 

28.5 

4.528 

9.5 

.385 

16.0 

2.009 

22.5 

3.121 

29.0 

4.659 

10.0 

.414 

16.5 

2.079 

23.0 

3.226 

29.5 

4.794 

10.5 

.446 

17.0 

2.151 

23.5 

3.331 

30.0 

4.930 

11.0 

.482 

17.5 

2.226 

24.0 

3.441 

30.5 

5.068 

MEASURING  INSTRUMENTS.  5*1 

To  calibrate  a  500  c.c.  flask  for  the  normal  temperature  of  the 
glass  at  15°  by  means  of  water  at  19.5°  at  Zurich,  the  correction 

2  553 

is  taken  from  the  above  table  and  divided  by  2,  ^—  =  1.276  gms. 

In  most  cases  it  is  not  necessary  to  take  into  consideration 
slight  changes  in  the  barometer  reading  or  in  the  temperature 
of  the  air. 

The  Mohr  Liter. 

Before  the  above  tables  had  been  worked  out  by  Schlosser  it 
was  customary  to  avoid  the  computation  otherwise  necessary  by 
adopting  a  standard  other  than  that  of  the  true  liter,  and  the 
practice  is  still  adhered  to  by  many  chemists.  Thus  for  vol- 
umetric work  the  liter  was  taken  as  the  volume  of  a  kilogram  of 
water  at  either  15°  or  17.5°  C.  as  weighed  in  the  air.  For  all  titra- 
tions  this  standard  is  perfectly  satisfactory,  but  it  is  not  suitable 
for  the  measurement  of  the  volume  of  gases  in  which  it  is  necessary 
to  estimate  the  weight  of  a  gas  from  the  volume,  because  the 
density  of  gases  is  always  referred  to  true  liters. 

A  Mohr  liter  measured  with  water  at  15°  is  1.0019  and  one 
measured  with  water  at  17.5°  is  1.0023  times  the  volume  of  a 
true  liter.  In  other  words  the  former  is  1.9  cm.  and  the  latter 
2.3  cm.  too  large. 

When  in  the  course  of  this  book  a  liter  is  mentioned,  the  true 
liter  is  to  be  understood  in  all  cases. 

Since,  however,  many  instruments  are  still  graduated  at  normal 
temperatures  of  15°,  17.5°  and  20°,  a  table  will  be  given  (see 
p.  522)  which  can  be  used  for  testing  such  apparatus. 

Thus,  to  determine  the  volume  of  a  Mohr  liter  for  the  normal 
temperature  of  15°,  the  liter  flask  and  1  kg.  in  brass  weights 
should  be  counterpoised  against  a  tare.  The  kilogram  weight 
is  then  removed,  the  flask  is  filled  with  water  at  15°  and  the 
position  of  the  meniscus  in  the  neck  of  the  flask  is  marked.  If, 
however,  the  temperature  of  the  water  is  not  15°,  but  say  25.5°, 
then  evidently  the  Mohr  liter  will  weigh,  according  to  the  above 
table,  998.095  gms. 


522 


VOLUMETRIC  ANALYSIS. 


TABLE    FOR    PREPARING    A    MOHR    LITER    AT    THE    NORMAL    TEMPERATURES    OF 
15,   17.5  AND  20°  C.  ACCORDING    TO    W.    SCHLOSSER. 


Temperature 
of  Water. 

Normal  Temperatures. 

15° 

17.5° 

20° 

Grains 

Grams. 

Grams. 

15° 

1000.000 

1000.345 

1000.763 

If 

999.871 

.217 

.634 

17 

.728 

.075 

.491 

18 

.576 

999  .  023 

.339 

19 

.413 

.760 

.175 

20 

.237 

.584 

1000.000 

21 

.053 

.400 

999.816 

22 

998.858 

.204 

.620 

23 

.652 

998  .  999 

.414 

24 

.437 

.783 

.199 

25 

.212 

.558 

998.973 

26 

997.977 

.323 

.739 

27 

.733 

.078 

.494 

28 

.479 

997  .  825 

.240 

29 

.218 

.563 

997.978 

30 

996  .  946 

.292 

.707 

Calibration  of  Measuring-flasks. 

A  flask  is  chosen  with  a  long  neck,  as  cylindrical  as  possible, 
the  diameter  of  which  should  not  exceed  a  certain  value. 


GREATEST    PERMISSIBLE    DIAMETER    OF    THE    NECK 


Contents . .   2000 

Diameter .  .  25 


1000 
18 


500 
15 


250 
15 


200 
12 


100 
12 


50       25  c.c. 
10        6   mm. 


The  flask  is  very  carefully  cleansed,  and  dried,  after  which  it 
is  placed  upon  an  accurate  balance  and  counterpoised  by  a  tare. 
Beside  the  tare  weights  are  placed  corresponding  to  the  volume 
of  the  flask,  and  on  the  opposite  side  of  the  balance  weights 
corresponding  to  the  correction  obtained  from  the  table  on  page 
519  corresponding  to  the  temperature  of  the  water  to  be  used, 
after  which  equilibrium  is  again  established  by  pouring  distilled 
water  into  the  flask.  Care  is  taken  that  no  drops  of  water  are  left 
suspended  from  the  sides  of  the  neck  above  the  water-level;  if  any 
are  present,  they  are  removed  by  touching  with  a  piece  of  filter- 
paper  wrapped  around  the  end  of  a  glass  rod.  An  exact  equi- 
librium is  finally  established  by  adding  or  removing  a  little  water 


MEASURING  INSTRUMENTS.  523 

by  means  of  a  capillary  tube.  The  flask  is  then  placed  upon  a  level 
surface  and  a  piece  of  gummed  paper  with  a  straight  edge  is  fast- 
ened around  the  neck  of  the  flask  so  that  its  upper  edge  is  just 
tangent  to  the  deepest  point  of  the  water  meniscus.  The  flask  is 
now  emptied,  dried,  its  neck  covered  with  a  uniform  layer  of  bees- 
wax, and  allowed  to  cool;  this  usually  requires  about  fifteen 
minutes.  The  flask  is  then  held,  as  is  shown  in  Fig.  83,  against  the 
piece  of  wood  s,  the  blade  of  a  pocket-knife  is  placed  firmly  against 
the  upper  edge  of  the  thick  paper  ring,  and  the  flask  is  revolved 
through  360°  around  its  horizontal  axis;  in  this  way  a  circle  is 
cut  in  the  wax  layer.  By  means  of  a  feather  (Fig.  6,  p.  22)  a 
drop  of  hydrofluoric  acid  is  placed  along  this  circle  while  the 
flask  is  held  hi  the  horizontal  position.  By  turning  the  flask 
around  its  axis,  the  drop  of  hydrofluoric  acid  is  allowed  to  act 
upon  the  glass  where  the  wax  coating  has  been  cut.  At  the  end 
of  two  minutes  the  excess  of  hydrofluoric  acid  is  washed  off,  the 
neck  of  the  flask  dried  by  means  of  filter-paper  and  heated  until 
the  wax  melts,  when  the  latter  can  be  readily  wiped  off.  The  last 
traces  of  wax  are  removed  by  nibbing  with  a  cloth  wet  with 
alcohol.  As  it  is  possible  that  the  etched  circle  will  not  exact1"' 


FIG.  83. 

coincide  with  the  upper  edge  of  the  paper,  the  flask  should  always 

b<?  tested.* 

*  The  attempt  should  not  be  made  to  test  the  correctness  of  the  calibra- 
tion by  filling  the  flask  with  water  which  has  been  brought  to  a  definite 
temperature;  it  is  important,  on  the  other  hand,  that  the  flask  and  water 
should  be  allowed  to  remain  for  some  time  in  the  same  place  in  order  that 
the  temperature  of  the  two  may  be  nearly  the  same. 


524  VOLUMETRIC  ANALYSIS. 

Testing  Calibrated  Flasks. 

The  flask  is  counterbalanced  with  a  tare  and  then  weights 
are  added  to  the  tare  corresponding  to  the  volume  of  the  flask. 
The  flask  is  filled  with  distilled  water  up  to  the  mark  and  equi- 
librium is  restored  by  adding  small  weights. 

Thus  in  testing  the  calibration  of  a  liter  flask,  it  was  filled  three 
times  with  water  at  21.5°  after  it  had  been  counterbalanced  with 
a  tare  when  perfectly  dry.  It  was  found  necessary  to  place 
small  weights  on  the  side  with  the  filled  flask  amounting  to 
2.987,  2.893  and  3.122  gms.;  average  3.001  gms.  If  the  flask  had 
been  perfectly  accurate  it  is  found  from  the  table  on  page  520  that 
the  small  weights  should  have  been  equal  to  2.921  gms.  for  water 
at  21.5°.  The  flask  is,  therefore,  3.001-2.921  =  0.080  cm.  too 
small. 

This  is,  however,  an  unusually  good  agreement.  According 
to  the  Royal  Commission  of  Berlin,  the  allowable  error  in  cal- 
ibrated flasks  is  shown  by  the  following  table : 

PERMISSIBLE    ERROR    FOR   FLASKS    CALIBRATED    FOR    CONTENTS 

(The  allowable  error  for  flasks  calibrated  for  delivery  is  twice  as  large.) 
Contents  . .  2000  1000  500  400  300  250  200  100  50  c.c. 
Error 0.5  0.25  0.14  0.11  0.11  0.08  0.08  0.08  0.05  c.c. 

Liter  flasks  which  are  calibrated  for  contents  are  marked  in 

1 5°  ' 

Germany  1 1.  -^5-  (E)  in  case  they  are  calibrated   in  terms  of  true 

15° 

cubic  centimeters,  and  1 1.  -^  (E),*  in  case  they  are  calibrated  in 

ID 

Mohr  liters.  Flasks  calibrated  for  delivery  are  marked  with  an 
A  instead  of  the  E. 

Calibration  of  Pipettes. 

It  is  best  to  have  pipettes  prepared  by  the  glass-blower  and 
to  etch  them  for  one's  self.  First  of  all,  the  pipette  must  be 
scrupulously  clean;  no  trace  of  fat  should  be  left  on  the  inner  sides 
of  the  tube,  for  it  will  cause  drops  of  moisture  to  adhere  and  escape 

17  5°  20° 

*  Or  — '- —  (E),  or  — -  E,  according  to  the  normal  temperature  chosen. 


CALIBRATION  OF  PIPETTES.  525 

measurement.  The  pipette  is,  therefore,  cleaned  by  placing  it  in 
a  tall  beaker  containing  a  little  soap  solution  and  the  latter  is 
drawn  to  the  top  of  the  pipette  by  sucking  through  a  rubber  tube 
fastened  to  its  upper  end  and  which  is  provided  with  a  pinch-cock. 
The  solution  is  allowed  to  remain  in  the  pipette  for  about  fifteen 
minutes. 

The  alkali  is  then  allowed  to  run  out,  the  pipette  washed  with 
water  and  filled  with  a  warm  solution  of  chromic  acid  in  con- 
centrated sulphuric  acid.*  This  is  allowed  to  remain  from 
five  to  ten  minutes  in  the  pipette  and  is  then  removed,  the  tube 
washed  first  with  water  from  the  tap  and  finally  with  distilled 
water. 

The  pipette  is  now  clean  and  ready  to  be  calibrated.  A  long 
strip  of  paper  is  fastened  upon  the  upper  part  of  the  tube,  the 
lower  end  is  closed  with  the  finger,  and  the  pipette  is  filled  with 
water  which  has  stood  for  some  time  in  the  balance  room,  from 
another  of  the  same  size  or  from  a  burette.  The  position  of  the 
bottom  of  the  mensicus  is  noted  with  a  lead-pencil  upon  the  paper 
which  was  fastened  to  the  side  of  the  pipette.  Assume,  for 
example,  that  it  is  desired  to  calibrate  a  10-c.c.  pipette,  and  that 
the  water  to_be  used  is  at  a  temperature  of  18°.  According  to  the 
table  on  p.  ?^0  one  liter  of  water  at  18°  weighs  in  the  air  exactly 
1000-2.303  =  997.70  gms.,  consequently  10  c.c.  should  weigh 
9.9770  gms. 

The  point  of  the  pipette  is  dipped  in  water,  and  this  is  sucked 
up  into  the  pipette  by  placing  the  mouth  at  the  upper  end  until 
the  water  is  above  the  pencil  marking.  The  top  of  the  pipette  is 
then  closed  with  the  finger,  the  water  adhering  to  the  outside 
carefully  wiped  off,  and  that  inside  is  allowed  to  run  into  a  beaker, 
with  the  point  of  the  pipette  against  the  walls,  until  the  upper 
meniscus  in  the  stem  is  exactly  on  the  mark.  The  contents  of 
the  pipette  are  then  allowed  to  run  into  a  tared  beaker  which  is: 
covered  with  a  watch  glass,  or  into  a  glass-stoppered  weighing- 
beaker,  allowing  the  water  to  flow  along  the  walls  of  the  beaker.. 
Now  on  weighing  the  beaker  again  it  is  perhaps  found  that  the  gain 

*  A  solution  of  potassium  dichromate  in  concentrated  sulphuric  acid  can 
be  used. 


526  VOLUMETRIC  ANALYSIS. 

in  weight  is  9.9257  gms.  or  9.9770-9.9257  =  0.0513  gms.  too  little. 
A  second  mark  is  therefore  made  a  little  higher  up  on  the  paper 
attached  to  the  stem  of  the  pipette,  and  the  above  process  is 
repeated.  If  necessary  a  third  mark  is  made  until  finally  the 
weight  of  the  water  does  not  vary  more  than  5  mgms.  from  that 
computed. 

The  strip  of  paper  is  then  cut  off  at  exactly  the  correct  mark,  a 
strip  of  gummed  paper  is  placed  round  the  pipette  at  this  point, 
and,  after  the  gum  has  dried,  it  is  covered  with  a  layer  of  beeswax 
and  etched  with  hydrofluoric  acid  as  described  on  p.  523.  After 
the  mark  has  been  etched  upon  the  pipette,  it  is  filled  with  water 
up  to  the  mark  and  emptied  into  the  tared  flask.  This 
operation  is  repeated  three  times  and  the  mean  value  is  taken  as 
correct. 

Pipettes  may  be  emptied  in  several  ways: 

1.  By  allowing  the  contents  to  run  out  freely  with  the  pipette 
held  vertically.     At  the  end  the  end  of  the  pipette  is  touched  to 
the  sides  of  the  beaker.     A  drop  of  the  liquid  will  then  always 
remain  in  the  pipette. 

2.  The  solution  is  allowed  to  run  out  while  the  point  of  the 
pipette  is  held  against  the  side  of  the  vessel  into  which  the  liquid 
is  being  delivered. 

All  other  methods  of  emptying  pipettes,  especially  that  of 
blowing  at  the  last,  are  to  be  abandoned.  At  all  events,  it  is 
always  necessary  to  use  the  pipette  in  the  same  way  as  in  the  calibra~ 
lion. 

The  "  kaiserl.  Normaleichungskommision "  allows  the  fol- 
lowing error  in  pipettes. 

Contents  of  pipette  .    100        50         25          20  10  2  1      c.c. 

Error  in  c.c 0.07     0.05     0.025     0.025     0.02      0.006     0.006 

Error  in  per  cent 0.07     0.1       0.10      0.125     0.2        0.3        0.6 

It  is  possible,  however,  to  prepare  pipettes  which  are  more 
accurate  than  this;  thus  the  author  by  using  pipettes  as  recom- 
mended above  obtained  the  following  values: 


CALIBRATION  OF  BURETTES.  527 

50  c.c.  Pipette:  49.9904,  49.9910,  49.9926.  Mean  49.9913. 

/*  =  0.002%,     F  =  0.001%. 

20  c.c.  Pipette:  20.0059,  20.0068,  20.0055.  Mean  20.0061. 

/=  0.003%,     F  =  0.002%, 

and  in  the  same  way: 

10  c.c.  Pipette:          /=0.008%,     F=0.004. 
5  c.c.  Pipette:          /=0.011,     F  =  0.006%. 


Calibration  of  Burettes. 

In  volumetric  titrations  it  is  advisable  to  begin  each  titration 
with  the  solution  at  the  zero  point  of  the  burette.  It  is  proper, 
therefore,  to  calibrate  burettes  in  the  same  way.  The  burette  is 
filled  to  the  zero  point  and  a  definite  volume,  e.g.,  5  c.c.,  is  allowed 
to  run  into  a  tared  beaker,  as  described  for  pipettes  on  p.  525, 
allowing  the  tip  of  the  burette  to  touch  the  side  of  the  beaker. 
After  determining  the  weight  of  the  water,  the  burette  is  filled 
again  to  the  zero  point  and  then  10  c.c.  are  withdrawn  in  exactly 
the  same  way.  This  process  is  repeated  for  each  5  c.c.  until 
finally  the  50  c.c.  mark  is  reached,  each  time  determining  the 
weight  of  the  water  withdrawn.  In  withdrawing  liquid  from 
a  burette  until  a  given  point  is  reached,  without  waiting  for  the 
burette  to  drain,  evidently  the  amount  actually  withdrawn 
depends  upon  the  rate  at  which  it  flows  from  the  burette.  It  is 
advisable,  therefore,  to  have  the  tip  so  narrow  that  it  will  take 
80  seconds  for  50  c.c.  to  run  out.  It  is  true  that  the  burette  is  not 


*  By  /  is  understood  the  average  error  of  the  single  determination.     It 

/S(d2+d12+d22+   .  ) 
is  computed  by  the  formula  /=±  \  —  (cf.  Kohlrausch: 

71  —  1 

Leitfaden  der  prakt.  Physik.),  in  which  n  represents  the  number  of  deter- 
minations made,  and  d,  dl9  d^ .  .  .  represent  the  deviation  of  each  from  the 
arithmetical  mean  and  S  (d2  +  d^ + df  + .  .  .)  the  sum  of  the  squares  of 


the  errors.     F-*±\—        ~T~L~7\ and  rePresents  the  probable  error 

of  the  mean. 


528  VOLUMETRIC  ANALYSIS. 

drained  completely  in  this  time,*  but  according  to  Wagner  f  it 
is  sufficiently  so  for  practical  purposes. 

Burettes  with  rubber  tubing  at  the  bottom  gradually  change 
with  regard  to  the  amount  delivered  on  account  of  the  rubber 
losing  its  elasticity.  For  this  reason  the  tubing  should  be  made 
quite  short  and  when  it  begins  to  get  old  it  should  be  renewed. 

The  corrections  obtained  as  above  are  best  tabulated  by 
means  of  a  plot  in  which  the  burette  readings  are  taken  as 
abscissae  and  the  corrections  as  ordinates.  By  connecting  the 
points,  a  curve  is  obtained  by  means  of  which  the  correct  reading 
of  all  parts  of  the  burette  can  be  obtained  at  a  glance. 

Method  of  Reading  Burettes. 

Although  in  the  case  of  graduated  flasks  and  pipettes  the 
marks  are  carried  around  the  whole  circumference  of  the  tube,  in 
b  the  case  of  burettes  this  is  not  usually  done,J  so  that  it 
is  a  matter  of  some  difficulty  to  determine  with  certainty 
the  exact  position  of  the  lowest  part  of  the  meniscus. 
To  avoid  a  parallax  error  a  number  of  means  have  been 
devised.  Thus  floats  are  often  used  such  as  are  shown 
in  Fig.  84,  a  and  6 ;  the  former  represents  that  of  Beuttel 
IG*  *  and  the  latter  that  of  Rey.  Around  the  bulb  of  a  a 
circle  is  etched,  and  if  the  eye  is  in  the  correct  position,  it  appears 
to  the  observer  as  a  straight  line.  The  liquid  in  the  burette  is  at 
the  zero-mark,  when  the  projection  line  from  the  circle  on  the  float 
exactly  coincides  with  the  line  at  the  zero-point  on  the  burette. 
In  the  case  of  dark-colored  liquids  it  is  difficult  to  see  the  circle  in 
the  case  of  the  float  a,  but  this  difficulty  is  overcome  in  b  by  the 
circle  being  etched  upon  the  upper  bulb  (in  the  figure  the  latter 
is  drawn  too  small) .  Such  floats  are  weighted  so  that  the  upper 
bulb  rises  above  the  level  of  the  liquid  in  the  burette;  it  is, 
therefore,  easier  to  make  a  reading  with  the  float  devised  by 
Rey  than  with  that  of  Beuttel.  In  refilling  the  burette,  the 
former  float  assumes  an  inclined  position;  it  must,  therefore, 


*  W.  Schlosser,  Chem.  Ztg.,  1904,  4. 
f  Habilitationsschrift,  Leipzig,  1898. 

J  Such  burettes  can  be  purchased,  however,  and  very  accurate  readings 
can  be  made  with  them. 


CALIBRATION  OF  BURETTES. 


529 


be  removed,  dried  off,  and  again  carefully  introduced  into  the 
liquid.* 

Schellbach  has  in  vented  another  method  of  avoiding  the  parallax 
error,  by  providing  the  back  of  the  burette  with  a  dark  vertical 


a  b 

FIG.  85 

line  upon  a  background  of  milky  glass  as  is  shown  in  Fig.  85. 
When  the  eye  is  in  the  correct  position,  this  dark  line  is  apparently 
drawn  out  into  two  points 
as  shown  in  6,  whereas  if  the 
eye  is  too  low  the  appear- 
ance a  is  obtained,  or  c  if 
the  eye  is  too  high. 

Kreitling  f  has  proved, 
however,  that  the  use  of 
floats  is  likely  to  lead  to 
error,  and  experiments  have 
also  shown  that  the  Schell- 
bach burettes  are  not  alto- 
gether reliable.  Better  than 
these  is  the  Bergmann's  screen  as  improved  by  Gockel.J  If  burettes 

*  This  difficulty  is  overcome  by  Diethelm  by  placing  below  the  large 
bulb  a  second  "  flattened-out "  bulb,  and  in  this  case  the  float  will  not 
attach  itself  to  the  sides  of  the  burette,  so  that  it  is  not  necessary  to  remove 
it  in  refilling  the  burette. 

fZ.  angew.  Chem.,  1900,  829,  990;   1902,  4. 

JChem.  Ztg.,  1901,  1084.     Z.  angew.  Chem.,  1898,  1856. 


53°  VOLUMETRIC  ANALYSIS. 

are  used  on  which  the  divisions  extend  at  least  half  around  the  tube, 
and  the  eye  is  placed  so  that  the  line  on  the  back  of  the  burette 
coincides  with  that  on  the  front,  then  with  the  use  of  this  screen 
very  exact  results  are  obtained. 

Bergmann's  screen,  which  can  be  used  to  advantage  with  all 
kinds  of  vessel,  consists  of  a  wooden  test-tube  holder  painted  a 
dull  black  (Fig.  86).  The  reading  is  made  easier  if  a  piece  of 
ground  glass  or  a  strip  of  oiled  paper  is  held  behind  the  burette, 
or  fastened  to  the  screen  itself. 

The  "  kaiserl.  Normalaichungskommission  "  gives  as  the 

ALLOWABLE  ERROR  FOR  BURETTES. 

Contents 100        75         50        30         10          2  c.c. 

6.08     0.06     0.04     0.03     0.02     O.OOSc.C. 


Normal  Solutions. 

By  a  normal  solution  is  understood  one  which  contains  one 
"gram-equivalent"  of  the  active  reagent  dissolved  in  one  liter  of 
solution.  By  "  gram-equivalent "  is  meant  the  amount  of  substance 
corresponding  to  one  gram-atom  (1.008  gms.)  of  hydrogen.  For 
convenience  in  computation  the  concentration  of  solutions  used 
for  volumetric  purposes  are  expressed  in  terms  of  their  normal- 
ity; i.e.,  a  solution  is  2  normal,  \  normal,  fa  normal,  etc.  The 
letter  N  is  used  as  an  abbreviation  for  normal. 

A  normal  solution  of  the  following  substances  will  contain 
dissolved  in  1000  c.c.: 

Hydrochloric  acid,  1  gm.-mol.  HC1  =  36.468  gms.  =  1  gm.-atom  H. 
Nitric  acid  (as  an  acid),  1  gm.-mol.  HN(>3  =  63.018  gms.  =  l  gm.- 
atom  H. 

QO  QOA 

Sulphuric  acid,  i  gm.-mol.  H2SO4= — '- — =49.043  gms.  =  l  gm.- 
atom  H. 

Potassium  hydroxide,  1  gm.-mol.  KOH  =  56.108  gms.  =  l  gm.- 
atom  H. 

Sodium   carbonate,  £  gm.-mol.   Na2CO3=  —^-  =  53.00  gms.  =  l 

gm.-atom  H. 
Silver  nitrate,  1  gm.-mol.  AgN03=  169.89  gms.  =  l  gm.-atom  H. 


NORMAL  SOLUTIONS.  531 

How  much  potassium  permanganate  or  potassium  dichromate 
must  be  dissolved  in  1  liter  of  solution  to  obtain  a  normal  solu- 
tion of  each  salt  for  oxidation  purposes? 

First  the  oxidation  equations  must  be  written: 

1.  In  acid  solutions  potassium  permanganate  is  reduced  to 
salts  of  potassium  and  manganous  oxides  with  loss  of  oxygen; 
the  latter  is  taken  up  by  the  substance  oxidized  : 

2KMnO4  =  KjO+  2MnO  +  5O. 


Two  gram-molecules  of  KMnO4,  therefore,  correspond  to  5  gm.- 
atoms  of  oxygen,  or  10  gm.  -atoms  of  hydrogen.  For  the  normal 

1  58  0^5 

solution  -J-  molecular  weight  in  grams  of  KMnO4,  —  ^  —  =31.61 

0 

gms.,  should  be  dissolved  in  1  liter. 

2.  Potassium  dichromate  is  reduced  to  salts  of  potassium 
and  chromic  oxides: 

K2Cr2O7  =  K2O+  Cr203+  3O. 

One  gram-molecule  of  K2Cr2O7  loses  3  gm.-atoms  of  oxygen, 
corresponding  to  6  gm.-atoms  of  hydrogen.  One  liter  of  normal 
solution  will  contain  -J-  molecular  weight  in  grams  of  K2O2O7, 


If,  however,  the  potassium  dichromate  is  not  used  as  an  oxi- 
dizing agent,  but  as  a  precipitant,  e.g.  for  the  precipitation  of 
barium  as  barium  chromate,  the  case  is  different  : 

K2O2O7+  2BaCl2+  2NaC2H3O2  +H2O  = 

-  2KC1+  2NaCl+  2HC2H3O2+  2BaCrO4. 

*  The  same  results  will  be  obtained  by  considering  the  change  of  valence 
which  the  manganese  and  chromium  undergo  on  reduction.  The  former 
is  reduced  (in  acid  solutions)  from  a  valence  of  seven  to  a  valence  of  two, 
corresponding  to  five  atoms  of  hydrogen.  In  the  dichromate,  each  of  the 
two  atoms  of  chromium  is  changed  from  a  valence  of  six  to  a  valence  of 
three,  equivalent  to  six  atoms  of  hydrogen.  Consequently  from  this  point 
of  view  it  is  evident  that  |  of  the  molecular  weight  of  permanganate  and 
\  of  a  molecular  weight  of  bichromate  must  be  used  to  prepare  one  liter 
of  normal  solution.  —  [Translator.] 


532  VOLUMETRIC  /IN /I LYSIS. 

One  gram-molecule  of  K^C^Or,  therefore,  precipitates  2  gm.- 
atoms  of  barium,  and  this  corresponds  to  4  gm. -atoms  of  hydrogen, 
so  that  J  gm. -molecule  of  K2Cr207  will  now  be  sufficient  to  make  1 

294  4 
liter  of  normal  solution,  — -7^-  =  73. 6    gms. 

It  is  evident,  then,  that  the  amount  of  substance  necessary  to 
make  a  normal  solution  depends  upon  the  purpose  for  which  it 
is  to  be  used. 


Preparation  of  Normal  Solutions. 

The  required  amount  of  substance  should  be  dissolved  in 
water  at  15°  and  diluted  to  a  volume  of  1  liter  while  at  this  tem- 
perature. In  most  cases,  however,  the  water  is  not  at  the  normal 
temperature  of  15°,  so  that  it  is  customary  to  dissolve  the  substance 
in  water  at  the  laboratory  temperature  and  then  dilute  the  solution 
up  to  the  mark  in  a  liter  flask.  After  thoroughly  mixing  the 
solution,  its  temperature  is  taken  by  a  sensitive  thermometer. 
If  the  temperature  is  above  15°,  as  is  usually  the  case,  the  volume 
of  the  solution  would  be  less  than  1  liter  if  it  were  cooled  to 
exactly  15°,  so  that  the  solution  as  made  up  is  a  little  too  strong. 
The  error  can  be  computed  as  follows: 

Not  only  the  solution,  but  the  glass  of  the  flask  should  have 
been  at  the  normal  temperature  of  15°.  The  coefficient  of  cubical 
expansion  for  glass  may  be  taken  as  a,  and  that  of  the  solution 
as  ft.  The  volume  of  the  flask,  and  that  of  the  solution  is  equal  to 
1000[l+a(J-15)]  c.c.,  but  this  volume  of  solution  at  t°  would 
assume  at  15°  a  volume  of 


innn 
1000 


-  15) 


Schlosser  *  has  worked  out  the  following  table  to  show  how 
much  greater  or  less  a  given  volume  is  at  different  temperatures 
than  it  would  be  at  exactly  15°. 

*  Chem.  Ztg.,  1904,  4;  1905,  510. 


PREPARATION  OF  NORMAL  SOLUTIONS. 


533 


TABLE  FOR  THE  REDUCTION  OF  THE  VOLUME   OF   WATER,   NORMAL,   AND   TENTH 
NORMAL    SOLUTIONS   TO    THE    NORMAL   TEMPERATURE    OF    15°    C. 


Tem- 
perature. 

Water  and 

Vio  n- 
Solutions. 

Vin. 
HCl. 

Vtn. 

Oxals. 

Vin- 
H2S04 

HNoi 

Vin. 
Na,COi 

Vin. 
NaOH 

5° 

+  0.60 

+  1.26 

+  1.33 

+  1.94 

+  2.00 

+  2.03 

+  2.18 

6 

0.60 

1.18 

1.25 

1.79 

1.84 

1.87 

1.99 

7 

0.59 

1.10 

1.16 

1.63 

1.68 

1.69 

1.80 

8 

0.56 

1.00 

1.05 

1.46 

1.50 

1.50 

1.60 

9 

0.52 

0.88 

0.94 

1.28 

1.31 

1.31 

1.39 

10 

0.46 

0.76 

0.81 

1.09 

1.11 

1.11 

1.18 

11 

0.40 

0.63 

0.67 

0.89 

0.91 

0.90 

0.96 

12 

0.33 

0.48 

0.52 

0.68 

0.69 

0.69 

0.73 

13 

0.22 

0.33 

0.35 

0.46 

0.46 

0.47 

0.50 

14 

+  0.12 

+  0.17 

+  0.18 

+  0.23 

+  0.23 

+  0.24 

+  0.25 

15 

0.00 

0.00 

0.00 

0  00 

0.00 

0.00 

0.00 

16 

-0.13 

-0.18 

-0.20 

-0.24 

-0.25 

-0.24 

-0.25 

17 

0.27 

0.36 

0.40 

0.49 

0.50 

0.49 

0.51 

18 

0.42 

0.56 

0.61 

0.75 

0.76 

0.75 

0.78 

19 

0.59 

0.76 

0.82 

1.02 

1.03 

1.02 

1.05 

20 

-0.76 

-0.97 

-1.05 

-1.30 

-1.30 

-1.29 

-1.33 

21 

0.95 

1.19 

1.29 

1.58 

1.58 

1.57 

1.62 

22 

1.94 

1.41 

1.54 

1.86 

1.87 

1.85 

1.92 

23 

1.35 

1.64 

1.80 

2.15 

2.17 

2.14 

2.23 

24 

1.56 

1.88 

2.07 

2.45 

2.47 

2.44 

2.54   . 

25 

1.79 

2.14 

2.34 

2.76 

2.78 

2.75 

2.85 

26 

2.02 

2.40 

2.62 

3.08 

3.10 

3.06 

3.17   , 

27 

2.27 

2.67 

2.90 

3.41 

3.43 

3.38 

3.50 

28 

2.52 

2.95 

3.19 

3.75 

3.76 

3.70 

3.83 

29 

2.75 

3.23 

3.49 

4.09 

4.10 

4.04 

4.17 

30        -3.06 

-3.52 

-3.82 

-4.43 

-4.44 

-4.38 

-4.52 

The  use  of  the  tables  on  pp.  533-535  can  be  best  explained  by 
a  few  examples : 

1.  A  liter  flask  is  calibrated  to  contain  exactly  1000  true  c.c. 
at  15°.     A  normal  solution  of  sodium  hydroxide  is  prepared  at 
25°.     The  table  shows  that  the  solution  at  25°  would  occupy  at 
15°  2.85  c.c.  less,  so  that  in  order  to  make  the  solution  exactly 
normal,  2.85  c.c.  of  water  should  be  added. 

2.  In  a  titration  47.35  c.c.  of  normal  sodium  hydroxide  solution 
were  used  which  was  at  a  temperature  of  19°;    this  amount  of 
solution  would  at  the  normal  temperature  occupy  a  volume  of 

._Q_     47.35X0.76 

4'-3° — =47'31  c'c- 


534 


VOLUMETRIC  A 'NA LYSIS. 


TABLE  FOR  THE  REDUCTION  OF  THE  VOLUME  OF  A  N/10 

(Correction  is  given  in 


Burette  Reading, 

5° 

u 

-0 

8° 

9° 

10° 

11° 

12° 

13° 

14° 

15° 

16° 

in  c.c. 

1 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

-0 

2 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

3 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

4 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

5 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

6 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

7 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

8 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

9 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

10 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

+  0 

-0 

11 

1 

1 

1 

0 

0 

0 

0 

0 

0 

0 

0 

12 

1 

1 

1 

1 

0 

0 

0 

0 

0 

0 

0 

13 

1 

1 

1 

1 

1 

0 

0 

0 

0 

0 

0 

14 

1 

1 

1 

1 

1 

0 

0 

0 

0 

0 

0 

15 

1 

1 

1 

1 

1 

0 

0 

0 

0 

0 

16 

1 

1 

1 

1 

1 

0 

0 

0 

0 

0 

17 

1 

1 

1 

1 

1 

0 

0 

0 

0 

0 

18 

1 

1 

1 

1 

1 

0 

0 

0 

0 

0 

19 

1 

1 

1 

1 

1 

1 

0 

0 

0 

0 

20 

+1 

+  1 

+ 

+1 

+1 

+  1 

+  1 

+  1 

+  0 

+  0 

+  0 

-0 

21 

1 

1 

1 

1 

0 

0 

0 

0 

22 

1 

1 

1 

1 

0 

0 

0 

0 

23 

1 

1 

1 

1 

1 

0 

0 

0 

0 

24 

1 

1 

1 

1 

1 

0 

0 

0 

0 

25 

1 

1 

1 

1 

1 

0 

0 

0 

0 

26 

1 

1 

1 

1 

1 

0 

0 

0 

0 

27 

2 

2 

1 

1 

1 

0 

0 

0 

0 

28 

2 

2 

2 

1 

1 

1 

0 

0 

0 

29 

2 

2 

2 

2 

1 

1 

1 

0 

0 

0 

30 

+  2 

+  2 

+  2 

+  2 

+  1 

+ 

+  1 

+1 

+1 

+  0 

+  0 

-0 

31 

2 

2 

2 

2 

2 

1 

1 

1 

0 

0 

0 

32 

2 

2 

2 

2 

2 

1 

1 

1 

0 

0 

0 

33 

2 

2 

2 

2 

2 

1 

1 

0 

0 

0 

34 

2 

2 

2 

2 

2 

2 

1 

1 

0 

0 

0 

35 

2 

2 

2 

2 

2 

2 

1 

J 

0 

0 

0 

36 

2 

2 

2 

2 

2 

2 

1 

1 

0 

0 

0 

37 

2 

2 

2 

2 

2 

2 

1 

1 

0 

0 

0 

38 

2 

2 

2 

2 

2 

2 

1 

0 

0 

0 

39 

2 

2 

2 

2 

2 

2 

1 

0 

0 

0 

40 

+  2 

+  2 

+  2 

+  2 

+  2 

+  2 

+  2 

+  : 

+  1 

+  0 

+  0 

-0 

41 

2 

2 

2 

2 

2 

2 

2 

1 

.  0 

0 

0 

42 

2 

2 

2 

2 

2 

2 

2 

1 

0 

0 

0 

43 

2 

2 

2 

2 

2 

2 

2 

1 

0 

0 

0 

44 

3 

3 

3 

2 

2 

2 

2 

1 

0 

0 

0 

45 

3 

3 

3 

2 

2 

2 

2 

i 

1 

0 

0 

1 

46 

3 

3 

3 

2 

2 

2 

2 

i 

1 

0 

0 

1 

47 

3 

3 

3 

3 

2 

2 

2 

i 

1 

0 

0 

1 

48 

3 

3 

3 

3 

2 

2 

2 

i 

1 

0 

0 

1 

49 

3 

3 

3 

3 

2 

2 

2 

i 

1 

0 

0 

1 

50 

+  3 

+  3 

+  3 

+  3 

+  3 

+  2 

+  2 

+  2 

+  1 

+  0 

+  0 

-1 

1 

*  Computed  according  to 


PREPARATION  OF   NORMAL  SOLUTIONS. 


535 


SOLUTION    TO    THE    NORMAL    TEMPERATURE   OP    15°   C. 

TOO^  cubic  centimeters.*) 


17° 

18° 

19° 

20° 

21° 

22° 

23° 

24° 

25° 

26° 

27° 

28° 

29° 

30° 

—  0 

-0 

-0 

-0 

-0 

-0 

-0 

-0 

-0 

-0 

-0 

-0 

-0 

-0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

1 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

1 

1 

1 

1 

0 

0 

0 

0 

0 

0 

0 

1 

1 

1 

1 

1 

1 

0 

0 

0 

0 

0 

0 

1 

1 

1 

1 

1 

1 

2 

0 

0 

0 

0 

0 

1 

1 

1 

1 

1 

1 

2 

2 

0 

0 

0 

0 

1 

1 

1 

1 

1 

1 

2 

2 

2 

0 

0 

0 

1 

1 

1 

1 

1 

2 

2 

2 

2 

2 

0 

0 

0 

1 

1 

1 

1 

1 

2 

2 

2 

2 

2 

3 

-0 

-0 

-0 

-1 

-1 

-1 

-1 

-1 

-2 

-2 

-2 

-2 

-3 

-3 

0 

0 

1 

1 

1 

1 

2 

2 

2 

2 

3 

3 

3 

0 

0 

1 

1 

1 

2 

2 

2 

3 

3 

3 

3 

4 

0 

0 

1 

1 

1 

2 

2 

2 

3 

3 

3 

3 

4 

0 

0 

1 

1 

2 

2 

2 

2 

3 

3 

3 

4 

4 

0 

1 

1 

1 

2 

2 

2 

3 

3 

3 

4 

4 

4 

0 

1 

1 

1 

2 

2 

2 

3 

3 

4 

4 

4 

5 

0 

1 

1 

1 

2 

2 

2 

3 

3 

3 

4 

4 

5 

5 

0 

1 

1 

1 

2 

2 

2 

3 

3 

4 

4 

4 

5 

5 

0 

1 

1 

1 

2 

2 

2 

3 

3 

4 

4 

5 

5 

6 

-0 

-1 

-1 

-1 

-2 

-2 

-3 

-3 

-4 

-4 

-4 

-5 

-5 

-6 

0 

1 

1 

2 

2 

2 

3 

3 

4 

4 

5 

5 

6 

6 

1 

1 

2 

2 

2 

3 

3 

4 

4 

5 

5 

6 

7 

1 

1 

2- 

2 

3 

3 

4 

4 

4 

5 

6 

6 

7 

1 

1 

2 

2 

3 

3 

4 

4 

5 

5 

6 

7 

7 

1 

1 

2 

2 

3 

3 

4 

5 

5 

6 

6 

7 

8 

1 

1 

2 

2 

3 

3 

4 

0 

5 

6 

6 

7 

8 

1 

1 

1 

2 

2 

3 

4 

4 

5 

5 

6 

7 

7 

8 

1 

1 

2 

2 

3 

3 

4 

4 

5 

6 

6 

7 

8 

8 

1 

1 

2 

2 

3 

3 

4 

4 

5 

6 

6 

7 

8 

9 

-1 

-1 

-2 

o 

-3 

-3 

-4 

-5 

-5 

-6 

-7 

-7 

-8 

-9 

1 

•2 

2 

3 

3 

4 

5 

5 

6 

7 

8 

9 

9 

1 

2 

2 

3 

4 

4 

5 

6 

6 

7 

8 

9 

10 

1 

2 

2 

3 

4 

4 

5 

6 

7 

7 

8 

9 

10 

1 

2 

2 

3 

4 

4 

5 

6 

7 

8 

8 

9 

10 

1 

2 

3 

3 

4 

5 

5 

6 

7 

8 

9 

10 

11 

1 

2 

3 

3 

4 

5 

6 

6 

7 

8 

9 

10 

11 

1 

2 

3 

3 

4 

5 

6 

7 

7 

8 

9 

10 

11 

2 

2 

3 

4 

4 

5 

6 

7 

8 

0 

9 

10 

12 

2 

2 

3 

4 

4 

5 

6 

7 

8 

9 

10 

11 

12 

_ 

-2 

-2 

-3 

-4 

-4 

-5 

-6 

-7 

-8 

-9 

-10 

-11 

-12 

2 

2 

3 

4 

5 

5 

6 

7 

8 

9 

10 

11 

12 

2 

2 

3 

4 

5 

6 

6 

7 

8 

9 

11 

12 

13 

2 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

2 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

2 

3 

3 

4 

5 

6 

7 

8 

9 

10 

11 

13 

14 

2 

3 

3 

4 

5 

6 

7 

8 

9 

10 

12 

13 

14 

1 

2 

3 

4 

4 

5 

6 

7 

8 

9 

11 

12 

13 

14 

1 

2 

3 

4 

4 

5 

6 

7 

8 

10 

11 

12 

13 

14 

1 

2 

3 

4 

5 

6 

6 

S 

9 

10 

11 

12 

14 

15 

-1 

-2 

-3 

-4 

-5 

-6 

-6 

-8 

-9 

-10 

-11 

-12 

-14 

-15 

the  table  on  page  493. 


53<5  VOLUMETRIC  ANALYSIS. 

3.  If  a  normal  solution  of  common  salt  is  prepared  at  25°,  it 
is  evident  that  the  volume  of  one  liter  when  reduced  to  the  normal 
temperature  would  be  1000-1.79  =  998.21  c.c.  The  solution  is, 
therefore,  too  strong,  for  it  contains  as  much  sodium  chloride  as 
should  be  present  in  1000  c.c.  at  the  normal  temperature.  1  c.c. 
of  this  solution  is  equivalent  to  1.0018  c.c.  of  a  normal  solution 
prepared  at  15°. 

This  number  is  called  the  factor  of  the  solution,  for  if  the  number 
of  cubic  centimeters  actually  used  is  multiplied  by  it,  the  result 
represents  the  corresponding  number  of  cubic  centimeters  of 
exactly  tenth-normal  solution. 

Similarly  in  all  future  experiments,  the  actual  volume  should 
be  reduced  to  the  normal  temperature  when  the  greatest  accuracy 
is  desired.  This  reduction  can  be  accomplished  by  means  of  the 
tables  on  pages  534  and  535  for  tenth-normal  solutions,  and  by 
the  table  on  page  533  for  more  concentrated  solutions. 

If  20  c.c.  of  a  tenth-normal  solution  is  used  at  25°  this  would 
correspond  at  the  normal  temperature  of  15°  to  20—0.04  =  19.96 
c.c.  (cf.  p.  535). 

Now  calibrated  vessels  serve  not  only  for  the  measurement  of 
water  but  for  other  dilute  and  concentrated  liquids.  The  question 
arises  as  to  whether  the  volumes  as  determined  by  weighing  water 
are  accurate  for  those  liquids  which  differ  greatly  from  water  as 
regards  viscosity,  adhesion  and  capillarity.  In  the  case  of  vessels 
calibrated  for  contents,  the  only  difference  is  that  arising  from  the 
different  nature  of  the  meniscus;  but  even  in  the  most  unfavorable 
instances  no  appreciable  error  is  occasioned.  Schlosser  and 
Grimm  *  have  carefully  studied  the  amount  of  error  in  vessels 
calibrated  for  delivery.  According  to  them  there  is  no  correction 
needed  for  tenth-normal  solutions  except  in  the  case  of  iodine. 
With  normal  solutions  a  correction  is  needed  at  the  most  only 
with  hydrochloric  and  oxalic  acids  and  with  liquids  in  the  nature 
of  alkalies  and  ferric  chloride  (1  c.c.  =0.012  gm.  Fe).  In  the 
case  of  concentrated  liquids  it  is  those  containing  alcohol  in  which 
the  deviations  are  most  marked.  Thus  Boutron  and  Boudet  found 
that  with  an  alcohol  soap  solution,  0.255  c.c.  less  was  delivered 

*  Chem.  Ztg.,  1906,  1071. 


PREPARATION  OF  NORMAL  SOLUTIONS.  537 

from  a  100  c.c.  pipette  than  of  water,  and  from  a  25-c.c.  pipette, 
0.103  c.c.  less.  With  concentrated  alkalies  and  acids  the  devia- 
tions were  also  quite  marked;  thus  with  95%  sulphuric  acid, 
0.442  c.c.  less  were  delivered  from  a  100-c.c.  pipette  and  0.085  c.c. 
less  from  a  10-c.c.  pipette  than  when  water  was  used.  The 
amount  of  time  allowed  for  the  draining  of  the  pipette  exerts  an 
important  effect  in  this  connection.  If  the  pipette  is  allowed  to 
drain  for  a  long  time  the  negative  correction  becomes  smaller 
and  may  even  become  positive.  It  would  be  well,  then,  for  the 
chemist  to  determine  the  amount  of  time  required  to  drain  a 
pipette  with  water  and  with  any  other  liquid,  and  if  the  difference 
exceeds  two  seconds  to  then  determine  the  contents  of  the  pipette 
for  the  other  liquid.  If  it  be  desired  to  avoid  this  difficulty,  the 
pipettes  may  be  graduated  both  for  contents  and  for  delivery. 
The  pipette  is  then  filled  with  the  special  liquid  to  the  mark 
corresponding  to  the  former  graduation,  it  is  allowed  to  drain, 
and  then  the  remaining  solution  is  carefully  washed  out. 


SUBDIVISIONS  OP  VOLUMETRIC  ANALYSIS. 
I.    Acidimetry  and  Alkalimetry. 
II.    Oxidation  and  Reduction  Processes. 
III.    Precipitation  Processes. 

I.  ACIDIMETRY  AND  ALKALIMETRY. 

This  covers  the  analysis  of  acids  and  bases.  In  order  to  deter- 
mine the  amount  of  acid  present,  an  alkaline  solution  of  known 
strength  is  required;  and  conversely,  in  the  analysis  of  a  base, 
an  acid  solution  is  required.  In  both  cases  the  "end-point"  of 
the  reaction  is  determined  with  the  help  of  a  suitable  indicator. 
The  accuracy  of  the  result  depends  largely  upon  the  choice  of 
the  indicator,  so  that  at  this  place  a  few  words  will  be  said  with 
regard  to  the  indicators  most  frequently  used  for  detecting  the 
presence  of  acids  or  alkalies. 


S38  VOLUMETRIC  ANALYSIS. 


INDICATORS. 

The  indicators  used  in  acidimetry  and  alkalimetry  are  dyestuffs 
which  are  of  one  color  in  acid  solutions  and  another  color  in  dilute 
alkali.  They  are,  as  a  rule,  weak  acids;  though  some  of  them  are 
weak  bases.  It  has  been  found  that  in  organic  compounds  the 
color  can  usually  be  traced  to  a  particular  arrangement  of  atoms 
called  a  chromophor.  The  change  in  color,  therefore,  is  caused 
by  a  slight  rearrangement  of  the  atoms  in  the  molecule.  Thus, 
if  the  salt  of  an  indicator  acid  is  yellow  and  on  treatment  with 
acid  it  turns  red,  this  is  due  to  the  fact  that  when  the  free  indi- 
cator acid  is  set  free  by  the  action  of  the  stronger  acid,  it  under- 
goes a  change  whereby  a  slight  change  takes  place  in  the  way 
the  atoms  are  linked  together  in  the  molecule,  and  in  fact  thereby 
loses  temporarily  the  ability  to  dissociate  electrolytically  as  an 
acid.  It  is  not  sufficient,  however,  to  assume  that  this  change 
of  color  is  caused  solely  by  the  fact  that  the  ions  have  a  color  other 
than  that  of  the  un dissociated  molecule;  on  the  contrary  it  has 
been  shown  in  certain  cases  that  the  ions  have  the  same  color 
that  the  undissociated  molecule  has  before  the  rearrangement  of 
the  atoms  in  the  molecule  has  taken  place.  On  the  other  hand, 
as  regards  the  proper  use  of  indicators  it  is  necessary  simply  to 
bear  in  mind  how  salts  of  weak  acids  behave  in  the  presence  of 
stronger  acids  and  how  the  acids  themselves  behave  in  the  pre- 
sence of  alkali. 

The  number  of  indicators  which  have  been  discovered  and 
used  more  or  less  is  very  large,  but  it  will  be  sufficient  here  to 
consider  only  methyl  orange,  methyl  red,  lacmoid,  litmus,  and 
phenolphthalein. 


INDICATORS.  539 


i.  Methyl  Orange.* 

Under  methyl  orange,  Lunge,  |  who  first  proposed  the  use  of 
this  indicator,  understood  either  the  free  sulphonic  acid  of  dimethyl- 
amido-azo-benzene  or  its  sodium  or  ammonium  salt. 

In  the  free  state  the  free  suiphonic  acid  is  obtained  in  the  form 
of  reddish-violet  scales,  soluble  in  considerable  water.  If  some  of 
the  solid  is  dissolved  in  as  little  water  as  possible,  a  distinct  reddish- 
orange  colored  solution  is  obtained;  but  on  the  further  addition  of 
water  this  color  gradually  changes  to  yellow.  If  a  trace  of  an  acid 
is  added  to  the  yellow  solution,  it  becomes  red  again  and  on  further 
dilution  with  water  the  color  changes  to  orange  and  finally  to 
yellow  once  more,  if  too  much  acid  was  not  added.  This  color 
change  can  be  easily  explained. 

In  the  sensitive  neutral  solution  there  is  a  condition  of  equi- 
librium between  two  isomeric  forms  of  methyl  orange  as  expressed 
by  the  equation: 


HS03-C6H4-X:X.C6H4X(CH3)2<=> 

<=±SO3  •  C'6H4  -  NH  -  N :  C6H4 :  N  (CH3)  2 


The  formula  on  the  left  represents  the  yellow  substance  and  the 
color  is  due  to  the  azo  group  X :  X;  whereas  the  formula  on  the 
right  represents  the  red  substance  which  has  for  its  chromophor 
the  quinoid  group  iCeH^..  The  formula  on  the  left  has  a  sul- 
pbonic  group  which  imparts  acid  properties  to  the  molecule  and 
at  the  other  end  is  an  X(CH3)2  group  which  has  weakly  basic 
properties.  The  formula  on  the  right,  therefore,  represents  an 


*  This  dyestuff  is  known  commercially  as  helianthin,  orange  III,  tro- 
paolin  D,  Poirrier's  orange  III,  dimethylaniline  orange,  mandarine  orange, 
and  gold  orange. 

tBerichte,  II  (1878),  p.  1944;  Zeitschr.  f.  ch.  Industrie,  1881,  p.  348; 
Handbuch  fur  Sodaindustrie,  I  (1879),  p.  52;  II  (1893),  p.  151. 


540  VOLUMETRIC  ANALYSIS. 

inner  salt  inasmuch  as  the  acid  and  base  forming  groups  are  here 
united. 

The  sodium  salt  of  methyl  orange  is  yellow  and  has  the  formula 


NaSO3  •  C6H4N :  NC6H4N  (CH3)  2 

and  when  decomposed  by  acids  the  free  sulphonate  at  once  reverts 
to  the  red  form: 

S03  •  C6H4  •  NH .  N :  C6H4 :  N  (CH3)  2 


Methyl  orange  is  an  excellent  indicator  for  weak  bases,  but 
cannot  be  used  for  the  titration  of  weak  acids.* 

If  it  is  desired  to  titrate  a  solution  containing  sodium  hydrox- 
ide with  a  tenth-normal  acid,  a  little  methyl  orange  is  added  to 
the  alkaline  solution  and  the  acid  is  added  until  the  solution  is 
colored  a  distinct  red.  The  latter  color  will  not  appear,  however, 
until  an  excess  of  the  acid  has  been  added.  This  causes  a  slight 
error  in  the  analysis  which  is  greater  in  proportion  to  the  amount 
of  indicator  employed,  and  the  more  dilute  the  solution. 

It  is  apparent  that  the  weaker  the  acid  character  of  the  indi- 
cator the  more  sensitive  it  will  be,  and  the  opposite  is  true  of  in- 
dicators which  are  bases. 

From  what  has  been  said  the  following  rule  holds : 

In  any  titration  the  smallest  amount  possible  of  indicator 
should  be  used,  and  inasmuch  as  the  change  of  color  is  propor- 
tional to  the  concentration  and  not  to  the  absolute  amount  of 
acid  present,  the  titrated  solution  should  have  as  nearly  as  pos- 
sible the  same  concentration  as  was  the  case  in  the  standardiza- 
tion of  the  normal  solution. 

When  a  normal  acid  is  used  for  the  titration,  the  change  of 
color  is  very  sharp  when  the  volume  of  the  solution  titrated 

*Cf.  Stieglitz,  J.  Am.  Chem.  Soc.,  25,  1117. 


INDICATORS.  541 

amounts  to  about  100  c.c.  Even  with  a  fifth  normal  solution  the 
change  of  color  is  very  distinct,  but  less  so  with  tenth-normal 
solutions,  but  these  can  be  titrated  provided  the  standardization 
was  made  at  the  same  dilution  as  that  used  in  the  analysis. 

How  is  it  with  the  end-point  in  the  titration  of  an  acid  with 
an  alkaline  hydroxide  solution? 

If  a  few  drops  of  methyl  orange  are  added  to  100  c.c.  of  water, 
the  latter  will  be  colored  distinctly  yellow.  If  we  imagine  that 
the  solution  contains  the  same  amount  of  gaseous  hydrochloric 
acid  as  is  contained  in  10  c.c.  of  a  tenth-normal  solution  of  this 
acid,  the  solution  will  be  colored  a  deep  red.  In  order  that  the 
solution  shall  assume  its  original  yellow  color,  it  is  only  necessary 

N 

to  add  exactly  10  c.c.  of  —  alkali  hydroxide  solution,  but  no  ex- 
cess of  alkali,  because  the  water  is  itself  sufficient  to  decompose 
the  dyestuff  sufficiently  to  produce  the  yellow  color. 

It  is  evident,  then,  that  it  is  not  a  matter  of  indifference  in 
the  analysis  whether  the  titration  is  completed  by  the  addition 
of  acid  or  by  the  addition  of  alkali.  In  the  former  case,  for  the 

N  N 

titration  of  T  c.c.  of  ~  alkali  solution,  T-H  c.c.  of  —  acid  would 

be  necessary. 

Methyl  orange  is  more  sensitive  toward  alkali  than  it  is  toward 
acid,  but  many  prefer  to  finish  the  titration  by  the  addition  of 
acid,  for  most  eyes  can  detect  the  change  from  yellow  to  red  with 
greater  accuracy.  In  principle  it  is  more  accurate  to  accomplish 
the  titration  the  other  way,  as  was  recommended  by  F.  Glaser. 

Preparation  of  Methyl-orange  Solution. — The  solution  of  0.02 
gm.  of  solid  methyl  orange  *  dissolved  in  100  c.c.  of  hot  water  is 
allowed  to  cool,  and  any  deposited  meta-sulphonic  acid  is 
filtered  off. 

Use. — Methyl  orange  is  suitable  for  the  titration  of  strong 
acids  (HC1,  HNO3,  H2SO4)  as  well  as  phosphoric  and  sulphurous 
acids.  Hydrochloric  and  nitric  acids  can  be  titrated  with  this 

*  If  the  free  acid  is  not  at  hand,  0.022  gm.  of  the  sodium  salt  is  dissolved 

N 
in  100  c.c.  of  water,  0.67  c.c.  ^  HC1  is  added,  and  after  standing  iome  time 

any  deposited  crystal*  are  filtered  off. 


542  VOLUMETRIC  ANALYSIS. 

indicator  with  a  sharper  end-point  than  is  the  case  with  sulphuric 
acid  If  free  phosphoric  acid  is  titrated  with  sodium  hydroxide 
using  this  indicator,  the  solution  changes  from  red  to  yellow  when 
one-third  of  the  phosphoric  acid  has  been  neutralized: 

H3PO4+  NaOH  =  NaH2PO4+  H2O. 

The  primary  phosphates  are  neutral  toward  methyl  orange, 
while  the  secondary  and  tertiary  phosphates  react  alkaline  toward 
it.  With  half -normal  solutions,  the  end-point  of  the  reaction  is 
fairly  sharp,  with  tenth-normal  solutions  it  is  less  so ;  in  the  latter 
case  an  excess  of  about  0.3  c.c.  of  the  tenth-normal  alkali  is  nec- 
essary to  cause  the  change  from  red  to  yellow. 

Sulphurous  Acid. — In  titrating  sulphurous  acid  with  sodium 
hydroxide,  the  yellow  color  is  obtained  when  half  the  acid  has 
been  neutralized, 

H2SO3+  NaOH  =  NaHSO3+  H2O, 

so  that  NaHSO3  is  neutral  toward  this  indicator. 

The  weak  acids  HCN,  CO2,  H2S,  As2O3,  B2O3,  CrO3  when  pres- 
ent in  considerable  amount  do  not  act  upon  the  indicator.  CO2 
and  H2S  produce  an  orange-red  coloration  only  when  present  in 
large  amounts.  For  this  reason  the  alkali  salts  of  these  acids  can 
be  titrated  with  accuracy  by  means  of  this  indicator. 

Organic  acids  cannot  be  titrated  with  methyl  orange. 

The  strong  and  weak  bases  NaOH,  KOH,  NH4OH,  Ca(OH)2, 
Sr(OH)2Ba(OH)2,  and  Mg(OH)2  can  be  titrated  with  great  accu- 
racy by  means  of  this  indicator,  and  the  same  is  true  of  the  amine 
bases  (methyl  and  ethyl  amines,  etc.);  on  the  other  hand,  such 
weak  bases  as  pyridine,  aniline,  and  toluidine  cannot  be  titrated. 

Nitrous  acid  ordinarily  cannot  be  titrated  with  this  indicator 
because  the  acid  destroys  it.  If,  however,  an  excess  of  alkali  is 
first  added  to  the  solution  of  nitrous  acid,  then  the  methyl  orange, 
the  titration  can  be  accomplished  with  accuracy. 

B.    THE  SODIUM  SALT, 

N(CH3)_— C6H4— N=N— C0H4SO3Na. 

This  sodium  salt  can  be  used  as  an  indicator  in  the  same  way 
as  the  free  acid ;  it  should  be  mentioned,  however,  that  the  com- 


INDICATORS.  543 

mercial  salt  often  contains  small  amounts  of  sodium  carbonate 
as  impurity,  which  causes  it  to  be  slightly  less  sensitive  than  the 
free  acid. 

The  change  of  the  yellow  color  of  the  solution  in  this  case 
takes  place  when  the  salt  has  been  decomposed  by  the  addition 
of  an  equivalent  amount  of  a  stronger  acid,  and  the  dissociation 
of  the  free  acid  diminished  by  increasing  the  concentration  of  the 
hydrogen  ions.  As  a  matter  of  fact,  however,  the  amount  of 
acid  necessary  to  effect  this  change  in  a  solution  containing  a 
drop  of  the  indicator  solution  is 'inappreciable. 

2.  Methyl  Red.  * 

( CH3)2N— C6H4— X  =  X— C6H4— COOH. 
Para-dimethyl-amido-azo-benzene-o-carboxylic  acid. 

This  valuable  indicator  is  suitable  for  titrating  weak  organic 
bases  and  ammonia.  The  aqueous  solution  of  methyl  red  is 
orange,  but  if  a  few  drops  are  added  to  50-100  c.c.  of  water,  the 
latter  is  colored  a  pale  yellow.  The  addition  of  a  drop  of  0.1  X. 
HC1  at  once  turns  the  liquid  a  violet  red  without  passing  through 
any  intermediate  shade  and  by  the  addition  of  a  drop  of  ammonia 
the  solution  becomes  nearly  colorless  again.  Methyl  red  is  not 
very  sensitive  toward  carbonic  acid,  but  more  so  than  is  methyl 
orange,  so  that  it  is  less  suitable  for  the  titration  of  carbonates. 
The  chief  advantage  of  this  indicator  lies  in  the  sharp  color 
change  from  a  very  pale  yellow  to  a  violet  red,  even  in  titrating 
ammonia. 

Preparation  of  the  Indicator.  About  0.02  g.  of  the  free  acid 
is  dissolved  in  100  c.c.  of  hot  water,  the  solution  allowed  to  cool, 
and  then  filtered.  Two  or  three  drops  of  this  solution  are  added 
for  every  100  c.c.  of  the  solution  to  be  titrated. 

H.  W.  Langbeck  f  recommends  the  use  of  ortho-nitrophenol 
as  indicator,  but  it  has  no  advantages  over  methyl  orange  and 
methyl  red.  It  is  not  at  all  sensitive  towards  carbonic  acid. 
It  is  turned  yellow  by  alkalies  and  colorless  by  acids. 

*  E.  Rupp  and  R.  Loose,  Berichte,  41,  3905  (1908). 
f  Chem.  News,  43,  162. 


544  VOLUMETRIC  ANALYSIS. 

3.  Lacmoid,  or  Resorcin  Blue, 
C12H9O8N. 

Lacmoid  is  prepared  by  heating  resorcin  with  sodium  nitrite 
at  not  too  high  a  temperature.  The  constitution  of  the  dye  has 
not  been  completely  established.  Pure  lacmoid  is  soluble  in 
water  (the  impure  product  is  difficultly  soluble),  but  more  soluble 
in  alcohol,  glacial  acetic  acid,  actone,  and  phenol,  and  less  so  in 
ether.  To  determine  whether  a  sample  of  commercial  lacmoid 
is  suitable  for  use  as  an  indicator,  a  little  of  it  is  boiled  with  water; 
if  the  water  is  colored  an  intense  and  beautiful  blue,  it  can  be 
used.  In  this  case  the  alcoholic  solution  will  be  of  a  pure  blue 
color,  and  not  with  a  tinge  of  violet,  as  is  the  case  with  the  impure 
substance. 

Preparation  of  Pure  Lacmoid. — The  solution  of  the  good  com- 
mercial product  in  hot  96  per  cent,  alcohol  is  filtered  and  allowed 
to  evaporate  in  vacuo  over  concentrated  sulphuric  acid. 

Preparation  of  the  Indicator. — A  solution  is  used  containing 
0.2  gm.  of  the  purified  lacmoid  in  100  c.c.  of  alcohol. 

Behavior  of  Lacmoid  toward  Acids  and  Bases. — If  the  solution 
after  it  has  been  colored  reddish  by  acid  is  treated  with  a  solu- 
tion of  an  alkali  hydroxide,  the  red  color  is  gradually  changed 
to  a  violet-red,  and  on  further  addition  of  alkali,  it  suddenly 
changes  to  a  pure  blue.  If  the  violet  solution  is  diluted  with 
considerable  water,  it  becomes  blue. 

Uses. — Lacmoid  is  suitable  for  the  titration  of  strong  acids 
and  bases  as  well  as  for  ammonia,  but  is  not  suited  for  the  titration 
of  nitrous  acid  or  weak  acids. 

4.  Litmus. 

The  chief  coloring  principle  of  litmus,  the  azolitmin,  is  a  dark- 
brown  powder  only  slightly  soluble  in  water  and  insoluble  in 
alcohol  and  ether.  With  alkalies  it  forms  a  readily  soluble  blue 
salt.  Besides  the  azolitmin,  there  are  other  dyestuffs  present  in 
litmus  which  are  soluble  in  alcohol  with  a  red  color. 

Commercial  litmus  is  obtained  in  small  cubes  mixed  with  con- 
siderable calcium  carbonate;  the  dyestuffs  are  then  in  the  form 
of  their  calcium  salts,  soluble  in  water.  If  the  commercial  mate- 


INDICATORS.  545 

rial  is  dissolved  in  water,  a  solution  of  blue  and  reddish-violet 
coloring  matter  is  obtained,  which  becomes  red  on  the  addition  of 
acid.  On  making  alkaline  again,  a  pure  blue  color  is  not  obtained 
at  first,  but  a  reddish- violet,  which  becomes  blue  on  the  addition 
of  considerable  alkali.  Such  a  solution,  therefore,  is  far  from 
being  a  sensitive  indicator  and  cannot  be  used  for  accurate  work. 
A  number  of  different  methods  have  been  proposed  for  obtaining 
a  sensitive  litmus  solution,  and  that  of  F.  Mohr*  will  be  de- 
scribed. 

Purification  of  Litmus. — The  cubes  of  litmus  are  placed  in  a 
porcelain  dish  (without  powdering),  covered  with  85  per  cent,  alcohol, 
and  digested  on  the  water-bath  for  some  time  with  frequent  stirring. 
The  solution  is  decanted  off  and  the  operation  is  repeated  three 
times.  By  this  means  the  undesired  coloring  matter  is  removed. 
The  residue  is  now  extracted  with  hot  water,  and  as  it  is  very 
difficult  to  filter  the  solution,  it  is  poured  into  a  tall  cylinder,  and 
after  standing  several  days  the  clear  liquid  is  siphoned  off.  The 
solution  is  concentrated  to  about  one-third  of  its  volume  and  acidified 
with  acetic  acid  in  order  to  decompose  the  potassium  carbonate 
present.  It  is  then  evaporated  to  a  syrupy  consistency  upon  the 
water-bath  and  the  mass  covered  with  a  large  amount  of  90  per  cent 
alcohol.  By  this  means  the  blue  coloring  matter  is  precipitated, 
while  the  remainder  of  the  violet  substance  remains  in  solution 
with  the  potassium  acetate.  The  residue  is  filtered  off  and  dis- 
solved in  sufficient  hot  water  so  that  three  drops  of  the  solution 
will  be  necessary  to  impart  a  distinct  color  to  50  c.c.  of  water. 

Use. — Litmus  can  be  used  for  the  titration  of  inorganic  and 
strong  organic  acids,  alkali  and  alkaline-earth  hydroxides,  and 
ammonia,  as  well  as  for  the  titration  of  carbonates  in  hot  solm- 
tion. 

5.  Phenolphthalein. 

Phenolphthalein  is  a  very  weak  acid  forming  red  salts  which 
contain  the  strongly  chromophoric  quinoid  group  :CeH4:.  The 
free  acid,  however,  is  unstable  and  when  set  free  from  one  of  its 
colored  salts  reverts  instantly  into  a  colorless  lactoid  form,  con- 
taining no  chromophor  group: 

HOOC  •  C6H4 •  C(C6H4OH) : C6H4 : CMOOC - C5H4 •  C(C6H4OH)2 

I          I 

*  Lehrbuch  der  Chemicsh-Analytische  Titrirmethode. 


546  VOLUMETRIC  ANALYSIS. 

In  the  case  of  the  free  acid,  therefore,  the  condition  of  equi- 
librium favors  the  lactoid  form  and  only  minimal  traces  of  the 
quinoid  acid  are  present.  This  trace  of  quinoid  acid  is  ionized 
and  is  in  equilibrium  with  its  ions  : 

HOOC  •  C6H4  •  C(C6H4OH)  :  C6H4  :  O<=» 

'  +  OOC  •  C6H4  •  C(C6H4OH)  :  C6H4  :  O' 


The  addition  of  an  alkali  causes  the  hydrogen  ions  to  disappear, 
so  that  more  of  the  quinoid  molecules  must  be  ionized  to  preserve 
equilibrium,  and  the  quinoid  molecules  in  turn  be  reproduced  from 
the  lactoid  as  fast  as  the  former  are  converted  into  the  salt. 
Phenolphthalein  is  a  very  sensitive  indicator  towards  acids,  but 
on  account  of  being  such  a  weak  acid  it  does  not  form  stable 
salts  with  weak  bases. 

Preparation  of  the  Indicator.  —  One  gram  of  pure  phenolphthalein 
is  dissolved  in  100  c.c.  of  86%  alcohol. 

Uses.  —  Phenolphthalein  is  particularly  suited  for  the  titration 
of  organic  and  inorganic  acids  and  strong  bases,  but  not  for  the 
titration  of  ammonia. 

If  the  red-colored  solution  containing  phenolphthalein  and  a 
little  alkali  is  treated  with  an  excess  of  concentrated  alkali  hydrox- 
ide solution,  the  red  color  disappears,  but  returns  on  diluting  the 
solution  with  water.  Phenolphthalein,  therefore,  cannot  be  used 
as  an  indicator  for  the  titration  of  concentrated  alkali  without 
previous  dilution  with  water. 

Phenolphthalein  is  the  most  sensitive  indicator  we  possess 
toward  acids,  far  more  sensitive  than  methyl  orange,  for  in  this 
case  not  only  can  the  presence  of  weak  acids  be  detected,  but  very 
small  amounts  can  be  titrated  with  accuracy. 

Ordinary  distilled  water  usually  contains   carbon   dioxide,  as 

N 
can  be  shown  by  slowly  adding  —  barium  hydroxide  solution,  drop 

by  drop,  to  100  c.c.  of  water  containing  a  drop  of  the  indicator 
solution.  Where  the  alkali  first  meets  the  water,  a  red  color  is 
produced  which  disappears  on  stirring,  so  that  often  as  much  as 
0.5  to  1.8  c.c.  of  the  alkali  must  be  added  before  a  permanent  red 
color  is  obtained.  The  disappearance  of  the  red  shows  the  pres- 


INDICATORS.  547 

ence  of  acid  (in  this  case  carbonic  acid),  and  its  amount  corre- 
sponds to  the  alkali  neutralized. 

Phosphoric  Acid. — If  a  solution  of  phosphoric  acid  containing 
phenolphthalein  is  titrated  with  normal  sodium  hydroxide  solu- 
tion, a  permanent  coloration  is  produced  when  two-thirds  of  the 
phosphoric  acid  is  neutralized : 

H3PO<+  2XaOH  =  Xa2HPO4+  2H2O. 

Apparently  NajHPO,  reacts  neutral  toward  phenophthalein, 
but  this  is  not  quite  correct,  for  a  pure  solution  of  disodium  phos- 
phate is  colored  by  phenolphthalein  a  pale  pink,  and  on  diluting 
with  water  the  intensity  of  the  color  increases  owing  to  progres- 
sive hydrolysis: 

Xa2  HPO4+  H  OH  «=»  NaOH'  +  Na  H2PO4. 

During  the  titration  of  phosphoric  acid  with  sodium  hydrox- 
ide, a  pale-pink  color  is  obtained  somewhat  too  soon,  and  this  color 
gradually  increases  in  intensity  until  finally  a  maximum  is  reached ; 
the  latter  point  is  taken  as  the  end-point.  It  is  possible  that  this 
hydrolysis  could  be  prevented  by  the  addition  of  a  large  excess  of 
sodium  chloride  and  cooling  to  about  zero  Centigrade. 

Carbonic  Acid. — If  the  solution  of  a  neutral  alkali  carbonate 
is  treated  with  phenolphthalein  a  red  color  is  obtained,  showing 
the  presence  of  hydroxyl  ions  in  the  solution,  due  to  hydrolysis : 

Xa  Xa  CO3+  H  OH<=>Na  OH-f  Xa  HCO3. 

If  hydrochloric  acid  is  added  to  such  a  solution  which  is  not 
too  dilute  and  is  at  a  temperature  of  0°  C.,  decolorization 
is  effected  when  half  of  the  soda  has  been  neutralized.  At  ordi- 
nary temperatures  a  sharp  end-point  cannot  be  obtained;  the 
.color  gradually  fades.  Pure  sodium  bicarbonate  dissolved  in 
ice-cold  water  is  not  colored  by  the  addition  of  phenolphthalein; 
if  it  is  warmed  to  the  temperature  of  the  room  it  turns  red,  but 
on  cooling  the  color  disappears  (Kiister). 

Silicic  acid  seems  to  be  without  influence  upon  phenolphtha- 
lein, for  alkali  silicates  (the  water-glasses)  can  be  titrated  with, 
accuracy. 

Chromic  Acid  and  Acid  Chromates  are  changed  by  the  addi- 
tion of  alkali  to  neutral  chromates  and  the  latter  have  no  action, 
upon  phenolphthalein. 


548  yOLU METRIC  ANALYSIS. 

Alkali  aluminates  can  be  titrated  accurately  with  this  indicator, 
for  aluminium  hydroxide  does  not  affect  it. 

'  Almost  all  the  problems  involved  in  acidimetry  and*  alkalim- 
etry can  be  solved  by  the  use  of  one  or  the  other  of  these  two 
indicators:  methyl  orange  and  phenolphthalein.  For  further 
information  with  regard  to  the  countless  other  indicators  which 
have  been  proposed,  the  student  is  referred  to  Glaser's  "Indica- 
toren  der  Acidimetrie  und  Alkalimetrie,"  Wiesbaden,  1901.* 

NORMAL  SOLUTIONS. 

For  the  standardization  of  the  solutions  used  in  acidimetry 
and  alkalimetry,  a  great  many  different  methods  have  been  pro- 
posed, all  of  which  more  or  less  satisfactorily  answer  the  purpose. 
It  was  Gay-Lussac  who  first  proposed  the  use  of  chemically-pure, 
calcined  sodium  carbonate,  and  in  simplicity  and  accuracy  this 
method  has  never  been  excelled,f  so  that  we  will  content  ourselves 
with  its  description. 

The  chemically-pure  sodium  carbonate  must  form  a  clear  solu- 
tion with  water  and  should  contain  neither  sulphuric  nor  hydro- 
chloric acids.  It  is  possible  to  obtain  the  pure  substance  com- 
mercially, but  as  a  rule  it  must  be  purified.  For  this  purpose 
about  300  gms.  of  crystallized  sodium  carbonate  are  dissolved  in 
250  c.c.  of  water  at  25-30°  C.,  and  quickly  filtered  into  a  two-liter 
flask  of  Jena  glass.  After  replacing  the  air  by  carbon  dioxide,  { 
the  flask  is  closed  by  means  of  a  perforated  rubber  stopper  through 
which  a  short,  right-angled  glass  tube  is  passed,  and  the  latter  is 
connected  by  means  of  a  long  piece  of  rubber  tubing  with  a 
Kipp-carbon  dioxide  generator.  The  contents  of  the  flask  are 
shaken  until  no  more  carbon  dioxide  will  be  absorbed;  this  usu- 
ally takes  from  half  to  three-quarters  of  an  hour.  In  proportion 
as  carbon  dioxide  is  absorbed,  sodium  bicarbonate  is  deposited. 
The  solution  is  cooled  to  0°  C.,  while  the  carbon  dioxide  is  continu- 
ally passed  through  it ;  the  thick  mass  of  crystals  is  transferred  to  a 

*  See  also  J.  Wagner,  Zeitschr.  fiir  anorg.  Chem.,  XXVII  (1901),  p.  138. 

t  According  to  Sorensen,  the  standardization  takes  place  with  equal 
accuracy  by  means  of  anhydrous  sodium  oxalate,  which  after  weighing  is 
heated  until  the  carbonate  is  formed;  cf.  page  597. 

%  The  carbon  dioxide  is  passed  through  a  solution  containing  sodium 
bicarbonate  before  it  reaches  the  flask. 


NORMAL  HYDROCHLORIC  ACID.  549 

filter-plate  which  is  covered  with  a  piece  of  hardened  filter-paper  and 
sucked  as  dry  as  possible.  The  sodium  bicarbonate  thus  obtained 
often  contains  considerable  chloride  and  sulphate.  It  is  wrashed 
back  into  the  flask  by  means  of  50  c.c.  of  distilled  water  (that  has 
been  cooled  to  0°  C.  and  saturated  with  carbon  dioxide),  vigor- 
ously shaken,  and  the  mother- liquor  once  more  removed  by  suc- 
tion. This  operation  is  repeated  until  finally  3  gms.  of  the  salt 
will  no  longer  give  the  test  for  chlorides  or  sulphates. 

The  pure  sodium  bicarbonate  thus  obtained  is  dried  on  the 
water-bath  and  preserved  for  further  use  in  a  tightly-stoppered 
bottle. 

Normal  Hydrochloric  Acid. 

1000  c.c.  contain  1  HC1  =  36.468  gms. 

Pure,  concentrated  hydrochloric  acid  is  diluted  until  its  spe- 
cific gravity  is  about  1.020,  and  in  this  way  a  solution  is  obtained 
that  is  slightly  more  than  normal  in  strength.  To  obtain  an  ex- 
actly normal  solution,  it  is  titrated  against  a  weighed  amount  of 
chemically-pure  sodium  carbonate,  and  from  the  result  obtained 
the  amount  of  water  to  be  added  can  be  computed.  About  8 
gms.  of  the  pure,  L\V  sodium  bicarbonate  are  placed  in  a  large  plati- 
num crucible,  and  the  latter  is  inserted  in  an  inclined  position 
within  a  hole  in  a  piece  of  asbestos  board  and  over  a  small  flame 
(cf.  p.  358).  The  contents  of  the  crucible  are  stirred  frequently 
with  a  short  piece  of  heavy  platinum  wire,  and  only  the  bottom  of 
the  crucible  is  heated  to  redness.  The  mass  must  not  be  al- 
lowed to  sinter  together  or  fuse,  for  in  that  way  an  appreciable 
amount  of  the  normal  carbonate  would  be  decomposed.  After 
heating  for  about  half  an  hour  the  crucible  is  cooled  in  a  desicca- 
tor, weighed,  and  to  make  sure  that  a  constant  weight  has  been 
obtained,  the  heating  is  repeated  once  or  twice  more.* 

*  If  it  is  feared  that  some  of  the  carbon  dioxide  may  be  expelled  from  the 
normal  carbonate,  the  bicarbonate  may  be  heated  for  half  an  hour  at  270- 
300°.  This  can  be  easily  accomplished  by  embedding  the  platinum  crucible, 
which  contains  the  bicarbonate,  in  sand,  so  that  the  latter  extends  up  on  the 
outside  of  the  crucible  as  high  as  the  bicarbonate  on  the  inside,  and  then 
heat  slowly  to  230°.  The  heating  is  then  continued  for  about  half  an  hour,  but 
taking  care  that  the  thermometer  in  the  sand  beside  the  crucible  does  not 
register  above  300°. 


55°  VOLUMETRIC  ANA LYSIS. 

The  amount  necessary  to  neutralize  35-40  c.c.*  of  normal  acid 
(about  2  gms.)  is  weighed  out  from  a  glass-stoppered  weighing- 
tube  into  a  beaker,  dissolved  in  about  100  c.c.  of  distilled  water, 
and  enough  methyl  orange  is  added  (from  5-6  drops)  to  impart  a 
pale-yellow  color  to  the  solution.  The  hydrochloric  acid  at  17- 
18°  C.  is  added  from  a  burette,  with  constant  stirring,  until  the 
color  of  the  solution  is  changed  from  yellow  to  orange.  The 
burette  is  then  read  and  a  drop  more  of  the  acid  is  added  to  see 
whether  this  will  produce  a  pure  pink  color.  If  this  is  not  the  case, 
more  hydrochloric  acid  is  added  until  this  point  is  reached,  and 
in  this  way  the  number  of  cubic  centimeters  of  the  acid  that  are 
required  to  neutralize  the  weighed  amount  of  the  sodium  carbo- 
nate is  determined.  Assuming  that  for  the  neutralization  of 
2.1132  gms.  of  Na2C03,  39.20  c.c.  of  hydrochloric  acid  at  19°  were 
necessary,  how  strong  is  the  acid? 

If  the  acid  were  exactly  normal,  according  to  definition  (p. 

530)  1000  c.c.  would  neutralize  -  ft2      3  =  ^^  =  53.00  gms.  of 

sodium  carbonate,  so  that  the  amount  weighed  out  would  req.uire 

for  neutralization  at  15° 

• 

53-00:1000  =  2.113::r 
2113 


53.00 


=39.87  c.c. 


This  would  be  equivalent  to  39.90  c.c.  at  19°.f 

As,  however,  only  39.20  c.c.  were  necessary  it  is  evident  that 
our  solution  is  too  strong,  and  for  each  39.20  c.c.  of  the  acid, 
39.90  —  39.20  =  0.70  c.c.  of  water  must  be  added  to  make  it  normal, 
and  to  1  liter: 

39.2:  0.70=  1000  :x 
700 


=  17.86  c.c.  water. 


*  It  is  best  not  to  weigh  out  more  substance  than  can  be  titrated  with 
one  buretteful,  and  not  too  small  an  amount  should  be  taken,  for  in  the 
latter  case  the  error  in  reading  is  too  great. 

f  According  to  the  table  on  page  533,  1000  c.c.  X.  HC1  at  19°  =  1000-0.76 
c.c.  at  15°;  therefore  (1000-0.76):  1000  =  39.83  :x,  x  =  39.90  c.c. 


NORMAL   HYDROCHLORIC  ACID  551 

A  perfectly  dry  liter  flask  is,  therefore,  filled  exactly  to  the 
mark  with  acid,  and  17.9  c.c.  of  water  are  added  from  a  burette 
(or  measuring-pipette),  the  solution  is  thoroughly  mixed,  and  the 
strength  of  the  solution  is  verified  by  a  second  titration  with  a 
weighed  amount  of  sodium  carbonate.  Further,  it  is  to  be  rec- 
ommended that  the  beginner  should  convince  himself  of  the  accu- 
racy of  the  result  by  determining  the  amount  of  chlorine  present 
gravimetrically  as  silver  chloride.  10  c.c.  of  normal  acid  yield 
1.433S  gins.  AgCl. 

For  practical  purposes  it  is  quite  unnecessary  to  spend  the 
time  necessary  for  the  preparation  of  an  exactly  normal  solution, 
but  its  normality  *  is  determined,  and  if  the  number  of  cubic  centi- 
meters used  is  multiplied  by  this  factor,  the  corresponding  amount 
of  normal  solution  will  be  obtained.  Thus  in  the  above  case  39.20 
c.c.  of  acid  were  used  to  do  the  work  that  would  require  39.90  c.c. 

39  90 
of  normal  acid.     The  solution  is,  therefore,  '—  -  =1.018  X.     Or, 

tJU.^O 

if  instead  of  using  40.10  c.c.  it  was  found  that  40.15  c.c.  of  acid 

40  10 
were  required,  the  solution  would  be  -  ^  =  0.9987  X.     Whatever 

"iU.lO 

the  normality  may  be,  it  is  written  upon  a  label  and  pasted  upon 
the  bottle  containing  the  acid. 

For  most  purposes;  a  normal  solution  is  too  strong,  so  that 
£,  4  and  TV  X  solutions  are  used.  Obviously  a  tenth-normal  solu- 
tion can  be  prepared  by  diluting  100  c.c.  of  a  normal  solution  to 
1  liter,  etc. 

In  order  to  titrate  a  —  acid  solution  with  sodium  carbonate 

about  0.2  gm.  of  the  salt  is  placed  in  a  white  porcelain  dish  and  dis- 
solved in  50  c.c.  of  water,  methyl  orange  is  added  until  a  pale-yellow 
color  is  obtained,  and  acid  is  added  until  the  color  becomes  orange. 
The  carbon  dioxide  is  then  expelled  by  heating  to  boiling,  after  which 
the  solution  is  cooled  and  once  more  titrated  until  an  orange  color 
is  obtained;  the  second  titration  requires  but  about  0.1-0.2  c.c. 
more,  but  in  this  way  the  correct  end-point  is  obtained.  At  this 
dilution  the  carbon  dioxide  exerts  an  imperceptible  action  upon 
the  indicator. 

*  By  normality  is  understood  the  relation  to  a  normal  solution. 


552  VOLUMETRIC  ANALYSIS. 

Normal  Nitric  and  Sulphuric  Acid  Solutions. 

These  are  prepared  in  the  same  way  as  wa*s  described  in  the 
preparation  of  normal  hydrochloric  acid. 

—  Oxalic  Acid. 
10 


«™«  ,24          ,       126.05 

1000  c.c.  contain  -     2   ^Q        —  --  ^—  =  6.303  gms. 

An  oxalic  acid  solution  of  this  strength  can  be  prepared  by 
dissolving  exactly  6.303  gms.  of  pure,  crystallized  oxalic  acid  ^in 
water  at  17°.5  and  diluting  to  a  volume  of  1  liter.  The  com- 
mercial acid,  however,  must  always  be  purified. 

The  chief  impurities  found  in  the  commercial  product  are  cal- 
cium and  potassium  oxalates.  In  order  to  remove  these  salts,  500 
gms.  are  dissolved  in  500  c.c.  of  pure,  boiling  hydrochloric  acid 
of  specific  gravity  1.075  in  a  porcelain  dish.  If  an  insoluble  resi- 
due should  be  obtained,  the  solution  is  filtered  through  a  hot-water 
funnel  and  the  filtrate  received  in  a  porcelain  evaporating-dish, 
the  latter  placed  upon  ice  and  cooled  as  quickly  as  possible.  The 
fine  crystals  thus  obtained  are  placed  in  a  funnel  provided  with  a 
platinum  cone  and  the  mother-liquor  completely  removed  by  suc- 
tion. The  above  process  is  repeated,  and  the  crystals  obtained 
the  second  time  are  washed  with  a  little  ice-cold  water,  re- 
crystallized  three  times  from  hot  water,  and  their  purity  tested. 
A  solution  of  2  gms.  of  the  purified  acid  should  give  no  sign 
of  a  turbidity  with  silver  nitrate,  and  another  portion  of  5  gms. 
should  leave  no  weighable  residue  after  ignition  in  a  weighed 
platinum  dish.  After  having  been  dried  as  completely  as  pos- 
sible by  suction,  the  crystals  are  spread  out  upon  several  layers 
of  blotting-paper  and  allowed  to  stand  in  the  air  for  several  days; 
they  then  have  the  formula  H2C2O4  +  2H2O.  The  strength  of  the 


N 
solution  is  tested  by  titration  with  ^  sodium  hydroxide  solution 

using   phenolphthalein,   as   indicator    (see   p.   553),   or  with    — r 
potassium  permanganate  solution  (see  p.  598). 


NORMAL   SODIUM  HYDROXIDE  SOLUTION.  553 

Normal  Sodium  Hydroxide  Solution. 
1000  c.c.  contain  1  NaOH  =  40.01  gms. 

About  45  gms.  of  the  commercial  caustic  soda  are  roughly 
weighed  out,  the  carbonate  on  the  surface  is  washed  off  as  much  as 
possible  by  a  stream  of  water  from  the  wash-bottle,  and  the  alkali 
is  dissolved  in  a  little  more  than  a  liter  of  water.  The  solution  is 
then  allowed  to  stand  for  about  one  hour  beside  the  hydrochloric 
acid  against  which  it  is  to  be  titrated,  in  order  that  both  solutions 
may  be  at  the  same  temperature.  About  40  c.c.  of  the  solution  are 
measured  off  from  a  burette,  and  titrated  with  normal  hydrochloric 
acid  after  the  addition  of  a  few  drops  of  methyl  orange  solution. 
The  titration  is  repeated  several  times  with  fresh  amounts  of  the 
sodium  hydroxide  and  from  the  mean  of  the  results  the  amount  of 
water  to  -be  added  is  calculated.  If,  for  example, 

40  c.c.  NaOH  =  41.23  c.c.  N.  HC1, 

it  is  evident  that  1.23  c.c.  of  water  must  be  added  to  each  40  c.c. 
of  the  alkali  to  make  the  solution  exactly  normal,  and  for  one  liter 

40:1.23  =  1000:x 

•joorj 

x=~  ^  =  30.75  c.c.  water. 

4(J 

After  the  solution  has  been  diluted  with  water  until  it  is  exactly 
normal,  it  must  be  tested  once  more  with  the  hydrochloric  acid, 
and  from  it  tenth-normal  and  fifth-normal  hydroxide  solutions 
can  be  prepared. 

The  solutions  thus  obtained  always  contain  carbonate,  so  that 
they  are  not  suitable  for  titration  with  phenolphthalein,  but  with 
methyl  orange  the  results  obtained  are  the  same  as  if  all  of  the 
sodium  was  present  as  the  hydroxide.  With  phenolphthalein 
accurate  results  can  be  obtained  from  a  boiling-hot  solution,  or  by 
preparing  a  solution  of  alkali  free  from  carbonate. 


554  VOLUMETRIC  A NA LYSIS. 

Titration  of  Alkali  containing  Carbonate  with  Phenolphthalein  in 

Hot  Solutions. 

The  alkali  is  measured  into  a  porcelain  dish,  a  drop  of  phenol- 
phthalein  added,  and  hydrochloric  acid  of  approximately  the  same 
strength  is  run  into  the  solution  until  the  red  color  disappears. 

The  solution  is  then  heated  to  boiling,  when  the  red  color 
soon  reappears;  it  is  cooled  by  placing  the  dish  in  cold  water,* 
hydrochloric  acid  is  again  added  until  decolorized,  and  the 
process  is  repeated  until  finally  the  red  color  does  not  reap- 
pear on  boiling.  This  method  of  titration  is  tedious,  but  the 

N 
results  obtained  are  accurate.     On  titrating  —  acids  with  methyl 

orange  as  indicator,  there  is  no  sharp  change  from  yellow  to 
pink,  as  is  the  case  with  normal  and  half -normal  solutions,  but 
first  a  brownish-orange  color  is  obtained  which  becomes  pink  on 
the  addition  of  more  acid.  The  correct  end-point  is  the  change 
from  yellow  to  yellowish  brown.  Only  when  considerable  car- 
bonate is  present  will  this  change  occur  before  enough  acid  has 
been  added,  for  in  this  case  the  carbon  dioxide  exerts  an  action 
upon  the  methyl  orange. 

The  disturbing  action  of  carbon  dioxide  is  best  prevented  by 
first  titrating  in  the  cold,  then  heating  to  remove  the  carbon 
dioxide,  again  titrating  the  cold  solution  with  acid.  If  only  a 
small  amount  of  carbonate  is  present,  it  exerts  no  appreciable 
effect  upon  methyl  orange. 

The  titration  of  oxalic  acid  with  alkali  which  contains  carbonate 
is  best  effected  with  phenolphthalei'n  in  hot  solution.  The  process 
is  carried  out  as  follows:  About  40  c.c.  of  the  sodium  hydroxide 
are  accurately  measured  into  a  porcelain  dish,  a  few  drops  of 
phenolphthalein  added,  and  oxalic  acid  run  in  from  a  burette 
until  the  solution  is  decolorized. 

The  solution  is  then  heated  upon  the  water  bath  until  the 
red  color  reappears,  whereupon  it  is  decolorized  by  oxalic  acid 


*  With  phenolphthalein  the  titration  can  be  finished  in  the  hot  solu- 
tion, but  the  end-point  is  not  so  sharp. 


NORMAL  SODIUM  HYDROXIDE  SOLUTION.  555 

and  the  process  continued  until  finally  the  color  does  not  reappear 
on  heating  the  solution.  This  point  is  reached,  however,  only 
after  the  solution  has  been  evaporated  to  dryness  and  th^ 
residue  taken  up  with  a  few  cubic  centimeters  of  distilled  water. 
A  slight  red  color  will  appear  after  this  first  evaporation,  but 
it  will  be  discharged  by  the  fraction  of  a  drop  of  oxalic  acid 
and  will  not  reappear  upon  a  second  evaporation. 

Remark. — Formerly  the  author  was  accustomed  to  heat  the 
oxalic  acid  solution  over  a  free  flame,  but  since  Christie  has  found 
in  this  laboratory  that  it  was  impossible  to  reach  an  end  point  in 
this  way,  the  use  of  a  free  flame  has  been  avoided. 

Sorensen  met  with  the  same  difficulty  and  attributed  the 
reappearance  of  the  red  color  to  the  following^action  having 
taken  place: 

2Xa2C2O4  +  H2O  =  Na2CO3  -f  2HCOONa  +  CO2 

Sodium  formate 

It  seems  more  probable,  however,  that  this  reappearance  of 
the  red  color  after  the  alkali  is  all  neutralized  is  not  due  to  the 
decomposition  of  sodium  oxalate  in  the  solution  but  to  its  beipg 
overheated  on  the  sides  of  the  dish,  whereby  it  is  decomposed  into 
sodium  carbonate  and  carbon  monoxide: 

Xa2C204=Xa2C03  +  CO 

Such  a  decomposition  does  not  occur  when  the  heating  takes 
place  upon  a  water  bath. 


Preparation  of  Sodium  Hydroxide  Solution  Free  from  Carbonate. 

This  is  best  effected  as  proposed  by  Kiister.*  About  40  c.c.  of 
pure  alcohol  is  placed  in  a  small  round-bottomed  flask,  heated  to 
boiling  on  the  water-bath,  and  little  by  little  2.5  gms.  of  bright 
metallic  sodium  are  added,  the  latter  being  freed  from  petroleum 


*  Zeit.  f.  anorg.  Chem.,  13,  134. 


556 


VOLUMETRIC  ANALYSIS. 


by  rubbing  between  pieces  of  blotting-paper.  The  reaction  be- 
tween the  boiling  alcohol  and  the 
sodium  is  at  first  very  violent  and 
large  amounts  of  hydrogen  and 
alcohol  vapors  are  evolved.  Dur- 
ing this  time  the  flask  is,  there- 
fore, kept  covered  with  a  watch- 
glass.  Gradually  the  reaction  be- 
gins to  diminish  and  finally  stops. 
In  the  flask  there  will  be  a  deposit 
of  sodium  alcoholate  and  some 
undissolved  sodium  on  account  of 
the  insufficient  amount  of  alcohol. 
Small  amounts  of  water  free  from 
carbon  dioxide  *  are  now  added, 
a  test-tube  full  at  a  time.  The 
alcohol  is  almost  all  boiled  away, 
and  in  order  to  completely  remove 
it,  a  current  of  air  free  from  car- 
bon dioxide  is  passed  through  the 
solution  until  the  odor  of  alco- 
hol can  no  longer  be  detected. 
The  solution  is  then  quickly 
cooled  by  the  addition  of  water 
free  from  carbon  dioxide,  imme- 
diately placed  in  a  liter  flask,  and 
diluted  to  the  mark  with  pure 
water  at  17-18°  C.  This  solution 

will  give  the  same  value  when  titrated  with  phenolphthalem  in  a  cold 
solution  as  when  the  latter  is  hot.f  With  methyl  orange  correct 
results  are  also  obtained  if  the  orange  color  is  taken  as  the  end-point. 
Such  a  solution  quickly  absorbs  carbon  dioxide  from  the  air. 
Tn  order  to  prevent  this,  it  is  placed  in  a  bottle  as  shown  in  Fig. 

*  This  is  accomplished  by  boiling  the  water  while  a  current  of  air  free 
from  carbon  dioxide  is  passed  through  it. 

t  Provided  the  hydrochloric  acid  solution  was  prepared  with  water  free 
from  carbonate,  otherwise  too  little  acid  will  be  necessary  when  the  titration 
takes  place  in  the  cold. 


FIG.  87. 


BARIUM  HYDROXIDE  SOLUTION.  557 

S7  which  is  Connected  with  a  soda-lime  tube,  Ar,  and  with  the  burette 
by  means  of  the  tubes  p  and  r.  The  burette  is  filled  by  squeezing 
the  tube  at  a.  In  this  way  a  solution  can  be  kept  free  from 
carbon  dioxide  for  a  long  time.  In  order  to  determine  whether 
the  solution  is  free  from  carbonate,  two  parallel  titrations  are  made 
with  phenolphthalei'n  as  an  indicator,  one  in  the  cold  and  the 
other  in  the  hot  solution.  If  the  results  agree  the  solution  is  free 
from  carbonate.  Otherwise  it  is  necessary  either  to  prepare  a  fresh 
solution  or  to  make  a  corresponding  correction  in  each  analysis 
after  determining  the  amount  of  carbonate  present  as  described 
on  p.  563. 

In  many  cases  it  is  better  to  use  a  jV  normal  barium  hydrox- 
ide solution ;  as  long  as  it  remains  clear  it  is  free  from  carbonate. 

Preparation  of  —  Barium  Hydroxide  Solution. 

.   .    Ba(OH)2-f-8H2O     315.51 
1000  c.c.  contain gp- *-  =     9Q     =15.776  gms. 

The  crystallized  barium  hydroxide  of  commerce  always  con- 
tains barium  carbonate,  so  that  the  solution  cannot  be  prepared 
by  simply  weighing  out  the  necessary  amount  and  diluting  to  1 
liter.  About  20  gms.  of  the  commercial  product  are  dissolved  in 
the  necessary  amount  of  distilled  water  within  a  large  flask.  The 
flask  is  closed  and  shaken  until  the  crystals  have  completely  dis- 
appeared and  a  light,  insoluble  powder  of  barium  carbonate 
remains.  The  solution  is  allowed  to  stand  for  two  days,  until  the 
barium  carbonate  has  completely  settled,  when  it  is  siphoned  into 
a  bottle  through  which  a  current  of  air  free  from  carbon  dioxide  has 
been  passed  for  two  hours  previous,  after  which  the  bottle  is  con- 
nected with  a  soda-lime  tube  and  with  the  burette  as  shown  hi 

N 
Fig.   87.     For    the    titration,  50  c.c.  —  hydrochloric    acid    are 

placed  in  an  Erlenmeyer  flask,  a  little  phenolphthalem  is  added , 
and  the  solution  titrated  by  the  addition  of  the  barium  hydroxide, 
solution.  The  normality  found  should  be  written  upon  the  label. 

It  is  not  advisable  to  make  the  solution  exactly  — ,  for  it  usu- 
ally becomes  turbid  on  dilution. 


558  VOLUMETRIC  ANALYSIS. 


A.  ALKALIMETRY. 

I.  Determination  of  Alkali  Hydroxides. 

i 

Rule. — If  the  substance  to  be  analyzed  is  a  solid,  an  accurately 
weighed  amount  is  dissolved  in  enough  water  so  that  the  solution 
is  at  about  the  same  concentration  as  that  of  the  acid  to  be  used 
in  the  titration.  If,  on  the  other  hand,  a  solution  of  an  alkali 
hydroxide  in  water  is  to  be  analyzed,  the  specific  gravity  of  the 
solution  is  determined  by  weighing  in  a  pycnometei  or  by  means 
of  an  areometer,  and  then  diluted  to  the  amount  desired. 

(a)  Determination  of  Sodium  Hydroxide  in  Commercial 
Caustic  Soda, 

NaQH- 40,01. 

N 
For   the    titration    a  —  hydrochloric  acid    solution    can  be 

N 
used.     Consequently  in   this   case   an   approximately  — r    normal 

solution  of  the  alkali  is  prepared.  As  sodium  hydroxide  absorbs 
water  and  carbon  dioxide  from  the  air.  the  sample  for  analysis  is 
weighed  out  in  a  tared  watch-glass  and  dissolved  in  water  to  a 
definite  volume.  After  thoroughly  mixing  the  solution  a  pipetted 
portion  is  treated  with  methyl  orange  and  titrated  in  the  cold  with 

N 

—  hydrochloric  acid. 

Example. — 4.6623  gms.  sodium  hydroxide  were  dissolved  in 
1000  c.c.  of  solution  and  25  c.c.  of  the  latter,  corresponding  to 

N 

0.11656  gm.  sodium  hydroxide,  required  28.66  c.c.  —  hydro- 
chloric acid  for  neutralization. 

N 
Since   1000   c.c.  of  —  acid  correspond   to  4.001  gms.  NaOH, 

it  is  evident  that  1  c.c.  ^  acid -~~« 0.004006  gm.  NaOH,  and 
28.66  c.c.  acid  correspond  to  0.004001  X  28.66  =  0. 1 147  gm.  NaOH. 


ALKALIMETRY.  559 

This  amount  of  NaOH  was  contained  in  25  c.c.  of  solution, 
equivalent  to  0.1166  gm.  of  the  solid  substance,  so  that  the  per 
cent,  of  sodium  hydroxide  present  can  be  calculated: 

0.1166  :0.1147  =  100:z 

98'38  per  cent-  NaOH- 


(6)  Determination  of  Sodium  Hydroxide  Present  in  Caustic 
Soda  Solution. 

N 

For  the  titration  assume  that  a  —  solution  is  at  hand  I 

Jt 

1000  c.c.  =^5=20.00  gms.  NaOH. 

The  alkali  solution  to  be  analyzed  has  a  specific  gravity  of  1.285 
at  15°  C.;  and  by  consulting  the  table  (see  the  supplement)  we 
find  that  the  solution  should  contain  25.80  per  cent.  NaOH  by 
weight;  i.e.,  100  gms.  of  the  solution  should  contain  25.80  gms. 
NaOH.  Usually  instead  of  weighing  out  the  solution  it  is  meas- 
ured and  the  per  cent,  by  volume  is  computed. 

As  128.5  gms.  of  the  alkali  occupy  a  volume  of  100  c.c.  we  have 

100:  25.8  =  128.5  :x 

z=33.153  gms.  NaOH  in  100  c.c. 

N 
Now  as  1  liter  of  -^  sodium  hydroxide  contains  20.00  gms.  NaOH, 

we  can  compute  how  much  of  the  alkali  must  be  taken  to  be  di- 
luted to  1000  c.c.  in  order  to  make  a  —  solution: 

100:33.153  =x:  20.00 
2003 


33.153 


=60.32  c.c. 


We  measure,  therefore,  60  c.c.  into  a  liter  flask,  dilute  with 
water  just  to  the  mark,  shake  thoroughly,  and  by  means  of  a  pi- 
pette 25  c.c.  of  the  solution  are  removed  and  titrated  with  half- 
normal  acid,  using  methyl  orange  as  an  indicator. 


560  VOLUMETRIC  ANALYSIS. 

N 
Assume  that  25  c.c.  of  the  solution  require  24.3  c.c.  of  -~  acid 

N  \aOH 

for  neutralization.     As   1000  c.c.  —  acid    contain  -  —  -  —  =20.0 

gms.   NaOH,  it  is  evident  that   1  c.c.  of  the  acid  corresponds 

N 
to  0.02000  gm.   NaOH,   and  24.3   c.c.  —  acid  is  equivalent  to 

0.02000X24.3  =  0.4860  gm.  NaOH. 

25  c.c.  of  the  dilute  alkali,  therefore,  contain  0.4860  gm. 
NaOH,  and  1000  c.c.  of  the  dilute  solution,  or  60  c.c.  of  the  original 
alkali,  contain  0.4860x40=19.44  gms.  NaOH,  and  100  c.c.  of 
the  original  solution  contain 

60:  19.44  =  100  :x 

32.40  gms.  NaOH. 


In  order  to  obtain  the  per  cent,  by  weight,  this  number  must 
be  divided  by  the  specific  gravity. 
In  the  assumed  case  we  have: 

00       C 

=  25.21  per  cent.  NaOH. 


Remark.  —  The  tit  ration  of  alkali  hydroxides  with  methyl 
orange  as  an  indicator  will  only  give  correct  results  when  the 
alkali  hydroxide  is  free  from  carbonate,  which  with  commercial 
material  is  never  the  case.  The  above  results  are  too  high,  for 
they  represent  the  total  amount  of  alkali,  i.e.  the  amount  of 
NaOH+Na2CO3,  though  the  latter  is  expressed  in  terms  of  NaOH. 
For  an  accurate  determination  of  alkali  hydroxide  in  the  presence 
of  alkali  carbonate,  see  p.  563. 

(c)  Determination  of  Ammonia  in  Aqueous  Ammonia. 
The  procedure  is  the  same  as  under  (6). 

(d)  Determination  of  Ammonia  in  Ammonium  Salts. 

A  weighed  amount  of  the  ammonium  salt  is  placed  in  the 
flask  K  (Fig.  24,  p.  59),*  dissolved  in  about  200  c.c.  of  water,  and 

*  Or  better,  the  apparatus  shown  in  Fig.  78,  p.  454,  may  be  used. 


TIT  RATION  OF  PYRID1NE  BASES.  561 

treated  with  10  c.c.  of  a  boiled  solution  of  10  per  cent,  caustic 
soda.  The  solution  is  distilled,  and  the  distillate  received  in  a 
known  amount  of  normal  acid  in  the  receiver  V,  as  described  on 
p.  59.  The  excess  of  acid  is  titrated  with  normal  alkali,  using 
methyl  orange  as  an  indicator  and  the  ammonia  calculated  from 
the  difference  between  the  amount  of  acid  now  found  and  that 
originally  in  the  receiver. 

Example. — The  amount  of  ammonia  in  a  sample  of  commer- 
cial ammonium  sulphate  is  to  be  determined.  As  the  technical 
product  is  never  entirely  pure,  a  large  amount  of  the  substance  is 
weighed  out,  and  for  the  sake  of  convenience  this  can  amount  to 
the  gram-equivalent  of  ammonia,  i.e.  17.03  gms.  This  quantity  of 
the  salt  is  dissolved  in  500  c.c.  of  distilled  water,  and  for  the  analy- 
sis, 50  c.c.  of  this  solution  are  taken  (1.703  gms.  of  salt).  This  is 
placed  in  the  flask  K  (Fig.  23,  p.  59),  diluted  with  150  c.c.  of  water, 
and  distilled  after  the  addition  of  10  c.c.  of  10  per  cent,  caustic 
soda  solution.  The  distillate  is  received  in  60  c.c.  of  half-normal 
hydrochloric  acid,  the  excess  of  the  latter  titrated  with  half-normal 
alkali,  and  from  the  difference  the  amount  of  ammonia  calculated. 

N 
For  the  titration,  t  c.c.  of  -jr-  alkali  are  necessary;  consequently 

the  amount  of  ammonia  in  1.703  gms.  of  the  substance  neutral- 
ized 60-Z  c.c.  ~  acid.  This  corresponds  to  (60-0  X0.008516  gm. 

XHa  and  in  per  cent. 

1.703:  (60-0x0.008515=100:z 
(60-00.8515     60-* 
X= T703 =  ~2~~  =  Percent-  NH3- 

(e)  Titration  of  Pyridine  Bases.     Method  of  K.  E.  Schulze* 
1000  c.c.  X.  acid  =  C5H,N  =  79.05  gms.  pyridine. 

The  pyridine  bases  are  so  weak  that  they  cannot  be  titrated 
with  ordinary  indicators.  If,  however,  an  aqueous  pyridine  solu- 
tion is  treated  with  an  aqueous  solution  of  ferric  chloride,  the  iron 
is  precipitated  as  ferric  hydroxide: 

FeCl3 + 3QH.N + 3HOH  =  3(CSH5N,  HC1)  +  Fe(OH),. 
*  Berichte,  20  (1887),  p.  3391. 


562  VOLUMETRIC  ANALYSIS. 

If  normal  sulphuric  acid  is  very  carefully  added  with  constant 
stirring  until  the  precipitate  redissolves,  each  cubic  centimetre  of 

C  H  N 

the  acid  required  will  correspond  to  -^  -  =  0.07905  gm.  pyridine. 

1UUU 

Procedure. — 5  c.c.  of  pyridine  are  dissolved  in  100  c.c.  of  water, 
25  c.c.  of  the  resulting  solution  are  treated  with  1  c.c.  of  5  per  cent, 
aqueous  ferric  chloride  solution,  and  the  precipitate  of  reddish- 
brown  ferric  hydroxide  is  titrated  with  normal  sulphuric  acid  until 
completely  dissolved. 

2.  Determination  of  Alkali  Carbonates. 

Alkali  carbonates  can  be  titrated  in  the  cold  by  using  methyl 
orange  as  an  indicator,  the  end-point  being  taken  as  the  change 
from  yellow  into  reddish  orange.  When  fifth-,  half-,  and  normal 
acids  are  used  this  is  the  correct  end-point,  but  with  tenth-normal 
acids  this  change  is  obtained  a  little  too  soon,  for  large  amounts  of 
carbonic  acid  exert  a  slight  action  upon  the  indicator.  In  this  case 
the  difficulty  is  best  overcome  by  titrating  the  solution  until  the 
orange  color  is  obtained,  then  heating  to  boiling  to  expel  the  car- 
bon dioxide,  cooling,  and  again  titrating  until  the  now  yellow 
solution  becomes  orange  again.*  With  phenolphthalem,  accurate 
results  may  be  obtained  by  titrating  the  hot  solution  (cf.  p.  554). 
According  to  Warder, f  sodium  bicarbonate  solution  reacts  neu- 
tral toward  phenolphthalei'n  in  the  cold,  so  that  when  a  sample  of 
sodium  carbonate  is  titrated  in  the  cold,  with  phenolphthalei'n 
as  an  indicator,  an  end-point  is  obtained  when  the  carbonate  is 
changed  to  bicarbonate: 

Na2C03  +  HC1  =•  NaCl  +  NaHCO3.J 

If  the  acid  is  allowed  to  run  upon  the  carbonate  solution,  a 
part  of  the  carbon  dioxide  from  the  sodium  bicarbonate  is  lost, 
so  that  too  much  acid  must  be  added  before  the  end-point  is 
reached.  On  the  other  hand,  correct  results  may  be  obtained  if 

*  Kiister  recommends  in  titrating  carbonates  with  methyl  orange,  that 
a  blank  experiment  be  made  to  see  how  much  effect  an  equal  amount  of 
water  saturated  with  carbon  dioxide  has  upon  the  same  amount  of  indicator 
solution.  (Zeitschr.  fur  anorg.  Chem.,  XIII,  p.  140.) 

t  Zeitschr.  f.  analyt.  Ch.,  21,  p.  102. 

j  Zeitschr.  f.  anorg.  Ch.,  XIII,  p.  140. 


DETERMINATION  OF  ALKALI  CARBONATES.  563 

the  titration  is  carried  out  at  0°  in  the  presence  of  NaCl  (cf.  p. 
547).  This  is  important,  for  in  this  way  a  convenient  method  is 
obtained  for  determining  the  amount  of  hydroxide  in  the  presence 
of  carbonate. 

3.  Determination  of  Alkali   Carbonate  and   Hydroxide  in  the 
Presence  of  one  Another. 

(a)  Method  of  C.  Winkler. 

Of  the  many  methods  which  have  been  proposed  for  this  deter- 
mination that  of  Winkler  is  the  best. 

In  one  portion  the  total  amount  of  alkali  present  is  determined 
by  titration  with  acid,  using  methyl  orange  as  an  indicator,  and 
the  hydroxide  in  a  second  portion  is  determined  as  follows:  The 
solution  is  treated  with  barium  chloride  solution,  when  the  follow- 
ing reaction  take  place : 

Na2CO3  +  BaCl2=  2NaCl  +  BaCO3  (insoluble). 
2NaOH  +  BaCl2  =  2NaCl  +  Ba  (OH)  2  (soluble) . 

The  sodium  of  the  carbonate  is  transformed  into  neutral 
sodium  chloride,  while  insoluble  barium  carbonate  is  precipitated 
from  the  solution;  the  sodium  hydroxide,  however,  yields  an 
equivalent  amount  of  barium  hydroxide.  If  the  solution  contain- 
ing phenolphthalei'n  is  slowly  titrated  with  hydrochloric  acid  with 
constant  stirring,  decolorization  is  effected  as  soon  as  the  hydroxide 
is  neutralized.  The  amount  of  acid  used  corresponds  to  the  amount 
of  hydroxide  originally  present. 

Example: 

1.  20  c.c.  (Na^COa-f  NaOH)  require  T  c.c.      acid  for  Na^COg-f  NaOH, 


2.  20c.c.(Na2CO3  +  NaOH)  t        ^"     "  NaOH  alone, 

so  that 

20  c.c.  (Na2CO3+NaOH)  require  T-t  c.c.  ^  acid  for  Na^CO,; 
20  c.c.  of  the  solution,  therefore,  contain 

(a)  tx  0.004001  gm.  NaOH, 
(6)  (T-t)  X 0.005300  gm.  Na^COs. 

Remark. — It  has  been  proposed  to  add  an  excess  of  barium 
chloride  solution  to  the  mixture  of  alkali  carbonate  and  hydroxide 


564  VOLUMETRIC  ANALYSIS. 

contained  in  a  measuring-flask,  then  dilute  to  the  mark,  thoroughly 
mix,  and  filter  through  a  dry  filter;  for  the  titration  an  aliquot 
part  of  the  filtrate  is  taken.  This  method,  however,  will  only  give 
accurate  results  when  the  water  used  for  the  dilution  is  absolutely 
free  from  carbon  dioxide,  and  this  will  be  the  case  only  when  it 
is  previously  boiled  with  a  current  of  air  free  from  carbon  dioxide 
passing  through  it.  Further,  no  attention  is  paid  to  the  volume 
occupied  by  the  precipitated  barium  carbonate,  and  in  the  case 
of  a  large  amount  of  the  latter,  a  considerable  error  is  introduced. 
The  method  of  Winkler  does  not  have  these  disadvantages.  Care 
must  be  taken,  however,  with  regard  to  the  addition  of  the  hydro- 
chloric acid  in  the  titration;  unless  it  is  added  very  slowly  some 
of  the  barium  carbonate  will  be  acted  upon  before  the  end-point 
is  reached. 

(6)  Method  of  R.  B.  Warder. 

To  the  cold  *  solution  containing  phenolphthalei'n,  hydro- 
chloric acid  is  added  and  the  liquid  is  gently  stirred  with  a  glass 
rod.  Decolorization  takes  place  when  all  of  the  hydroxide  and 
half  of  the  carbonate  are  neutralized: 

NaOH  +  HC1  =  NaCl-f  H2O, 
Na2CO3+  HC1  =  NaCl-f  NaHC03. 

To  the  colorless  solution,  methyl  orange  is  added,  and  the 
solution  is  again  titrated  with  acid  until  the  other  half  of  the  car- 
bonate is  neutralized,  when  the  solution  turns  brownish-red. 

If  the  amount  of  acid  used  for  the  titration  with  phenolphtha- 
lei'n is  represented  by  T,  and  that  necessary  for  the  titration  with 
methyl  orange  by  t,  then 

2t  c.c.  corresponds  to  the  amount  of  carbonate  present,  and 
T— t  represents  the  amount  of  hydroxide. 

4.  Determination  of  Alkali  Bicarbonates. 

The  solution  is  titrated  in  the  cold  until  an  orange  color  is 
obtained  with  methyl  orange,  or  until  a  colorless  solution  is  obtained 
by  titrating  hot  with  phenolphthalei'n.  (See  page  553.) 

*  The  results  are  accurate  only  when  the  solution  is  at  0°  and  NaCl  is 
present.  Cf.  Kiister.  Zeitschr.  f.  anorg.  Chem.,  XIII,  p.  134  (1897). 


DETERMINATION  OF  ALKALI  CARBONATES.  565 

5.  Determination  of  Alkali  Carbonates  in  the  Presence  of 
Alkali  Bicarbonates. 

(a)  Method  of  C.  Winkler. 

The  total  alkali  is  determined  in  one  portion  by  titration 
with  hydrochloric  acid,  using  methyl  orange  as  an  indicator,  and 
in  a  second  portion  the  amount  of  bicarbonate  is  determined  as 
follows : 

A  definite  volume  of  the  solution  is  treated  with  an  excess 
of  sodium  hydroxide,  by  which  means  the  bicarbonate  is  changed 
to  neutral  carbonate: 

NaHCO3+  NaOH  =  Na2C03+  H2O. 

The  solution  now  contains  sodium  carbonate  with  the  excess 
of  sodium  hydroxide,  and  the  amount  of  the  latter  is  determined 
as  described  under  3.  In  other  words,  barium  chloride  is  added, 
then  phenolphthalein,  and  the  solution  is  titrated  until  colorless. 
The  amount  of  acid  now  used  corresponds  to  the  excess  of  sodium 
hydroxide  added,  and  if  this  amount  is  deducted  from  the  total 
sodium  hydroxide,  the  corresponding  amount  of  bicarbonate 
will  be  obtained. 

Example: 

25  c.c.  Na^COg+NaHCO,  required  T  c.c.  ^  acid  for 

Na2CO3+NaHCO3; 
25  c.c.  Na2C03+NaHCO3+7\  c.c.  ^  NaOH+BaCl, 

N 
required  t  c.c.  ^  acid  for  the  excess  of  NaOH; 

25  c.c.  Na2CO3  +  NaHCO3,  therefore,  require   T1  -  t  c.c.  —  acid 
for  the  NaHCO3    and     T-(T^-t)  c.c.  ^  acid  for  the  Na3C03. 

25  c.c.  of  the  original  solution  contain 

(a)  (T\-OX0.008401  gm.NaHCO3, 
(6)  (T-^+OxO.OOoSOOgm.  Na2COr 


566  yOLU METRIC  ANALYSIS. 

Remark.— In  order  to  make  sure  that  enough  sodium  hydrox- 

N 
ide  solution  is  present,  the  same  amount  of  the  T-T  alkali  is  added 

•   N 

as  there  were  cubic  centimeters  used  of  —  acid  in  the  deter- 
mination of  the  total  alkali;  in  this  case,  then,  T=Tl}  and  t,  the 
excess  of  alkali,  corresponds  at  the  same  time  to  the  amount  of 
Na2CO3  present.  The  caustic  alkali  solutions,  even  when  origi- 
nally free  from  carbonate,  gradually  absorb  it  from  the  air,  so  that 
in  every  case  the  amount  of  carbonate  in  the  alkali  should  be  deter- 
mined before  making  the  above  analysis  and  a  corresponding, 
correction  applied  to  the  calculation. 

(b)  Method  of  Warder* 

Using  phenolphthalem  as  indicator,  the  solution  is  titrated 
with  hydrochloric  acid  until  colorless,  and  in  this  way  half  of  the- 
carbonate  is  determined.  Methyl  orange  is  then  added  and  the 
solution  titrated  until  a  brownish-red  color  is  obtained,  and  in 
this  way  the  total  amount  of  the  bicarbonate  together  with  half 
of  the  carbonate  is  determined.  If  t  represents  the  amount  of 
acid  used  in  the  first  titration,  and  T  the  total  amount  used, 
then: 

It  c.c.  of  acid  correspond  to  the  amount  of  carbonate  and 
(T  —  2t)  c.c.  correspond  to  the  bicarbonate. 


6.  Determination  of  Alkaline-earth  Hydroxides. 

The  solution  containing  phenolphthalem  is  titrated  until  color- 
less. 

7.  Determination  of  Alkaline-earth  Carbonates. 

The  carbonate  is  dissolved  in  an  excess  of  the  standard  acid, 
boiled  to  remove  the  carbon  dioxide,  and  the  excess  of  acid  titrated 
with  alkali,  using  methyl  orange  as  indicator  in  cold  solution. 

*  Cf.  Am.  Ch.  Journ.,  3,  No.  1,  and  Chem.  News,  43,  228. 


DETERMINATION  OF  ALKALINE-EARTH  OXIDE.  567 

8.  Determination  of  Alkaline-Earth  Oxide  together  with 
Alkaline-Earth  Carbonate. 

Suppose,  for  example,  it  is  desired  to  determine  the  amount  of 
oxide  and  carbonate  in  a  sample  of  "quicklime."  The  lime  is 
broken  up  into  pieces  about  the  size  of  a  pea,  exactly  14  gms.  are 
accurately  weighed  out  and  slaked  with  boiled  water,  the  paste  is 
washed  into  a  500-c.c.  flask  and  diluted  to  the  mark  with  water 
free  from  carbon  dioxide.  After  thoroughly  mixing,  50  c.c.  of 
the  turbid  liquid  is  transferred  to  a  second  500-c.c.  flask  and 
again  diluted  to  the  mark. 

Determination  of  the  Total  Calcium.  —  50  c.c.  (0.14  gm.  of  sub- 

N 
stance)  of  the  last  solution  are  treated  with  60  c.c.  of  —  hydro- 

chloric acid  and  heated  until  there  is  no  further  evolution  of  car- 
bon dioxide,  the  solution  is  cooled,  and  the  excess  of  the  acid 

X 
titrated  with  ^--  caustic   soda  solution,  using  methyl  orange  as 

an  indicator.  For  this  purpose  t  c.c.  of  the  latter  are  required; 
consequently  60  —  2  c.c.  —  acid  were  necessary  to  neutralize  the 

calcium  hydroxide  and  calcium  carbonate  hi  the  50  c.c.  of  the 
solution  taken  for  analysis. 

Determination  of  the  Calcium  Oxide.  —  A  second  portion  of  the 

N 
freshly-shaken    solution    is    titrated    with  —    hydrochloric    acid 

added  drop  by  drop  to  the  cold  solution,  using  phenolphthalein 
as  an  indicator.  Assume  that  ^  c.c.  of  the  acid  were  necessary  to 
neutralize  the  calcium  oxide. 

Consequently,    for   the   neutralization   of   the   CaCO3+CaO  = 

\  N 

60  —  t  c.c.  —  acid  were  required,  and  for  the  CaO,  ^  c.c.  —  acid 

were  necessary.     For  the  neutralization  of  the  CaC08,  therefore, 

N 
60—  (J-Mi)  c.c.       acid  were  necessary. 


50  c.c.  solution  (0.14  gm.  lime)  contain: 

(a)  <iX0.002805  gm.  CaO, 

(6)  [60  -  (*  +  <i)]X  0.5005  gm.  CaC03, 


568  VOLUMETRIC   ANALYSIS. 

and  in  per  cent. 


0. 14  :tiX  0.002805=  100  \x 
x  = 
and 


<iX  0.2805 
x=  —  =2ti  per  cent.  CaO 


[60  -(*  +  *i)]X  0.5005 
xi  =  -  *  -  per  cent.  CaCO3. 

9.  Determination  of  Alkaline-Earth  Bicarbonates. 

This  determination  finds  a  practical  application  in  the  deter- 
mination of  the  temporary  hardness  of  water. 

The  hardness  of  a  water  is  caused  by  the  presence  of  alkaline- 
earth  salts,  either  those  with  strong  acids  (CaSO4,  MgCl2)  or  bicar- 
bonates.  A  hard  water  is  recognized  by  the  fact  that  it  gives 
with  a  clear  soap  solution  a  turbidity  or  even  a  precipitate,  and 
considerable  soap  must  be  added  before  a  lather  is  obtained  on 
shaking.  As  in  a  majority  of  cases  calcium  salts,  and  chiefly 
calcium  bicarbonate,  predominate  in  such  a  solution,  its  hard- 
ness is  usually  expressed  in  parts  of  calcium  carbonate  (or  calcium 
oxide)  in  100,000  parts  of  water. 

If  the  solution  contains  1  part  of  calcium  carbonate  in  100,000 
parts  of  water  it  is  said  to  possess  one  degree  of  hardness  (French)  ; 
if  such  a  water  contains  n  parts  of  CaC03  in  the  same  quantity  of 
water  it  possesses  n  degrees  of  hardness.  In  Germany  the  hard- 
ness is  expressed  in  parts  of  CaO  per  100,000  parts  of  water,  while 
in  England  the  hardness  is  expressed  in  grains  of  calcium  carbon- 
ate per  gallon.  When  magnesium  salts  are  present,  these  are 
expressed  in  terms  of  the  equivalent  amounts  of  CaCO3  or  CaO. 
The  error  caused  by  this  assumption  is  not  great,  for  the  amount 
of  magnesium  present  is  usually  small  compared  with  the  amount 
of  calcium.  If  a  water  containing  calcium  bicarbonate  and  cal- 
cium sulphate  is  heated  to  boiling,  the  former  is  decomposed  with 
the  precipitation  of  calcium  carbonate, 

Ca  (HC03)2,  =  H20+  C02+  CaC03, 

while  the  calcium  sulphate  remains  in  solution.     In  other  words, 
the  hardness  produced  by  the  presence  of   alkaline-earth  bicar- 


DETERMINATION  OF  ALKALINE-EARTH  BICARBONATES.     569 

bonates  disappears  on  boiling,  and  is  designated,  therefore,  as 
"temporary  hardness"  to  distinguish  it  from  "  permanent  hard- 
ness," which  is  usually  caused  by  alkaline-earth  salts  of  the 
stronger  acids,  usually  calcium  sulphate.  The  sum  of  the  tem- 
porary and  permanent  hardness  of  a  water  represents  the  total 
hardness. 

According  to  C.  Hehner,  the  temporary  as  well  as  permanent 
hardness  may  be  determined  accurately  by  an  alkalimetric  process. 

(a)  Determination  of  Temporary  Hardness. 

100  c.c.  of  the  water  to  be  examined  are  placed  in  a  white  por- 
celain evapora  ting-dish,  a  few  drops  of  methyl  orange  are  added 

N 
and  the  solution  is  titrated  with  —  hydrochloric  acid  until  the 

first  change  from  yellow  to  orange  takes  place.     From  the  amount 
of  hydrochloric  acid  used  the  amount  of  calcium  carbonate  present 
is  calculated. 
Example: 

N 
100  c.c.  water  required  2.5  c.c.  -^  hydrochloric  acid. 

As  1000  c.c.  TJYJ  hydrochloric  acid  neutralize  3  =  5.005  gms. 

\ 

CaCOs,  1  c.c.  ^  hydrocholric  acid  will  neutralize  0.005005  gm. 

"W 

CaCOs    and    2.5    c.c.    of  ^  hydrochloric    acid    corresponds    to 

0.005005X2.5  =  0.0125  gm.  CaC03. 

Then  if  100  c.c.  of  water  contain  0.0125  gm.  CaCO3,  100,000  c.c. 
of  water  will  contain  12.5  gms.  CaCOs. 

The  hardness  of  the  water  corresponds  to  12.5  French  degrees, 
or  as 

CaCO3  :CaO 

100.09:56.09=12.5:o: 

7.0  German  degree, 


57°  VOLUMETRIC  ANALYSIS. 

(b)  Determination  of  the  Permanent  Hardness. 

Another  portion  of  100  c.c.  of  the  water  is  treated  with  an 

N 
excess    of  —^    sodium    carbonate    solution,   evaporated    on    the 

water-bath  to  dryness,  and  taken  up  in  a  little  freshly-boiled, 
distilled  water.  The  residue  is  filtered  and  washed  four  times 
with  hot  water,  while  the  filtrate  is  allowed  to  cool  and  afterwards 

N 
titrated  with  —  hydrochloric  acid.     If  the  amount  of  hydrochloric 

acid  used  for  the  titration  is  deducted  from  the  total  amount  of 
sodium  carbonate  added  to  the  water,  the  difference  represents  the 
amount  of  sodium  carbonate  required  for  the  precipitation  of  the 
alkaline-earth  salts  of  the  strong  acids. 

N 

Example. — 100  c.c.  of  water+10  c.c.  —  Na2C03  were  evapo- 
rated to  dryness,  the  residue  extracted  with  water,  and  the  filtrate 

N 
titrated  with  —  hydrochloric  acid;   this  required  8.7  c.c.  of  HC1. 

Consequently,    for    the    precipitation    of    the    calcium    sulphate 

N 
10  —  8.7=1.3    c.c.    — Na2C03     were     necessary,    which    cone- 

sponds   to 

1.3X0.005=0.0065  gm.  CaCO3  per  100  c.c.  water  and 
6.5  gms.  CaCO3  per  100,000  c.c.  water. 

The  permanent  hardness  amounts  to  6.5  French  degrees  or 
6.5X0.56=3.64  German  degrees. 

Remark. — The  above  methods  of  Hehner  for  the  determina- 
tion of  hardness  will  give  reliable  results  only  when  the  water 
contains  no  alkali  carbonates  in  solution,  as  is  usually  the  case 
with  drinking-waters.  For  the  determination  of  the  amount 
of  alkaline  earth  present  in  many  mineral  waters  it  is  obvious 
that  these  methods  cannot  be  used. 

10.  Determination  of  Alkaline-earth  Salts  of  Strong  Acids. 

The  determination  is  practically  the  same  as  was  indicated 
above.  The  alkaline-earth  salt  is  precipitated  by  means  of  an 


ACIDIMEJRY.  571 

excess  of  titrated  sodium  carbonate  solution,  and  after  filtration 
the  excess  of  the  latter  is  determined  by  titrating  back  with  acid. 

Procedure. — A  solution  containing  calcium  chloride  and  hydro- 
chloric acid  is  to  be  analyzed.  It  is  placed  in  a  measuring-flask, 
treated  with  a  few  drops  of  methyl  orange  and  with  sodium 
hydroxide  solution  until  the  neutral  point  is  reached,  after  which 
an  accurately  measured  amount  of  sodium  carbonate  solution  is 
added,  The  solution  is  heated  until  the  precipitated  calcium 
carbonate  becomes  crystalline,  allowed  to  cool,  diluted  up  to  the 
mark,  mixed,  filtered  through  a  dry  filter,  and  the  excess  of  sodium 
carbonate  titrated  in  an  aliquot  part  of  the  filtrate.  From  the 
amount  of  sodium  carbonate  required  for  the  precipitation  of  the 
calcium  the  amount  of  the  metal  can  be  calculated. 

Remark. — Other  metals  which  are  precipitated  by  sodium  car- 
bonate can  be  determined  in  this  way. 

B.  ACIDIMETRY. 

Acids  are  determined  either  by  titration  with  standard  alkali 
solution  or  a  known  amount  of  the  latter  is  added  and  the  excess 
titrated  with  standard  acid.  The  latter  method  requires  more 
burette  readings  and  is,  therefore,  less  satisfactory  than  the 
former. 

Determination  of  the  Acid  Contents  of  Dilute  Mineral  Acids 
(HC1,  HN03,  H2S04). 

The  specific  gravity  of  the  acid  is  determined  by  means  of  an 
areometer  and  from  the  tables  in  the  back  of  this  book  the  approx- 
imate amount  of  acid  present  is  determined.  A  weighed  amount 
of  the  acid  is  then  diluted  so  that  the  solution  will  have  approxi- 
mately the  same  concentration  as  that  of  the  alkali  to  be  used 
for  the  titration.  It  is  analyzed  by  one  of  the  following  methods: 

1.  An  accurately-measured  portion  of  the  diluted  acid  (20  to 
25  c.c.)  is  placed  in  a  beaker,  methyl  orange  is  added,  and  the 
solution  is  titrated  with  sodium  hydroxide  solution  until  a  yellow 
color  is  obtained. 

2.  The  dilute  solution  to  be  analyzed  is  placed  in  a  burette, 
and  with  it  a  definite  amount  of  normal  alkali  is  titrated. 


572  yOLUMETRIC  ANALYSIS. 

3.    A  definite  volume  of  the  diluted  acid  is  titrated  with  — 

Ba(OH)2  solution  or  with  sodium  hydroxide  free  from  carbonate, 
using  phenolphthalein  as  an  indicator.* 

N 
Example. — For  the  analysis  —  NaOH  is  at  hand. 

The  hydrochloric  acid  to  be  analyzed  had  at  15°  C.  a  specific 
gravity  of  1.122,  corresponding  to  about  24  per  cent.  HC1  by 
weight. 

1000  c.c.  —  sodium  hydroxide  are  equivalent  to  — — = — ^—  = 

2i  A  L 

=  18.23  gms.  HC1,  and  100  c.c.  \  NaOH  neutralize  1.823  gms.  HC1. 

& 

Consequently 

100:24=z:1.823 

182  *•? 
x=       '    =7.595  gms.  of  the  above  acid 

N 
would  be  required  to  make  100  c.c.  of  —  acid,  if  it  contained  exactly 

24  per  cent.  HC1.  About  this  quantity  (say  8  gms.)  is,  therefore, 
weighed  out,  and  as  the  specific  gravity  of  the  solution  is  1.122, 

o 

this  will  require  =7.1  c.c.     About  7  c.c.  of  the  acid  are  placed 

in  a  tared,  glass-stoppered  weighing-tube,  the  tube  and  its  con- 
tents weighed,  the  latter  washed  into  a  100-c.c.  measuring-flask 
and  diluted  with  distilled  water  up  to  the  mark.  After  thoroughly 
mixing,  25  c.c.  of  the  acid  are  measured  off  and  analyzed  by  one 
of  the  above  methods.  Assume  that  the  original  weight  of  the  acid 
amounted  to  7.9623  gms.  and  that  25  c.c.  of  the  diluted  acid  re- 

N 
quired  25.80  c.c.  of  —  alkali,  then  100  c.c.  would  require  25.80X4= 

N 
103.2   c.c.   of—  alkali,  corresponding  to   103.2x0.01823=1.8813 


*  When  phenolphthalein  is  used  as  an  indicator  in  cold,  solutions  the 
acids  must  be  diluted  with  water  free  from  carbonate. 


COMMERCIAL  HYDROUS  STANNIC  CHLORIDE.  573 

gms.  HC1  and  in  per  cent. 

7.962  :  1.881=  100  :x 

I  00    -I 

23.6  per  cent.HCl. 


Remark.  —  Instead  of  weighing  out  the  acid  for  the  analysis, 
it  can  be  measured  and  from  the  per  cent,  by  volume  found  the 
per  cent,  by  weight  calculated.  As,  however,  the  specific  grav- 
ity as  determined  by  an  areometer  is  not  very  accurate,  it  is  better 
to  weigh  the  acid.* 

Analysis  of  Commercial  Hydrous  Stannic  Chloride. 

Stannic  chloride,  as  used  for  a  mordant  in  dyeing,  is  obtained 
as  the  solid  salt  SnCl4  +  5H2O,  or  in  a  concentrated  aqueous  solu- 
tion of  about  50°  Be. 

The  latter  is  obtained  by  dissolving  metallic  tin  in  hydro- 
chloric acid  and  oxidizing  the  stannous  chloride  formed  either 
with  potassium  chlorate  or  potassium  nitrate.  The  preparation 
should  contain  no  free  acid,  especially  nitric  acid,  no  stannous 
chloride,  and  no  iron.  The  substance  is,  therefore,  tested  quali- 
tatively for  these  substances  as  follows  : 

For  stannous  chloride,  by  dissolving  in  water  (or  diluting  the 
concentrated  solution)  and  adding  mercuric  chloride;  a  white 
precipitate  of  mercurous  chloride  shows  the  presence  of  bivalent 
tin. 

For  nitric  acid,  by  means  of  ferrous  sulphate  and  concentrated 
sulphuric  acid. 

For  sulphuric  acid  (caused  by  the  use  of  impure  hydrochloric 
acid  in  the  preparation  of  the  salt)  with  barium  chloride. 

For  iron,  with  potassium  sulphocyanate. 

The  solid  salt  SnCl4+5H2O,  made  by  treating  anhydrous 
stannic  chloride  with  the  calculated  amount  of  water,  is  almost 
always  found  to  be  very  pure. 

*  If  the  specific  gravity  of  the  acid  is  taken  with  a  pycnometer,  using 
all  necessary  precautions  (cf.  Kohlrausch,  Leitfaden  der  praktischen  Physik), 
it  is  a  matter  of  indifference  whether  the  acid  used  for  the  analysis  is  weighed 
or  measured. 


574  VOLUMETRIC  A  "NA  LYSIS. 

The  gravimetric  determination  of  both  the  tin  and  the  chlo- 
rine has  been  described  on  p.  321,  but  here  will  be  given  a  method 
for  determining  the  amount  of  the  latter  volumetrically. 

If  stannic  chloride  is  diluted  with  water,  the  salt  is  hydro- 
lytically  decomposed,  and  the  solution  reacts  acid: 

SnCl4+  4HOH  <=±  Sn(OH)4+  4HC1. 

Consequently  if  methyl  orange  is  added  to  the  diluted  solu- 
tion, the  amount  of  acid  may  be  titrated  with  caustic  soda  solu- 
tion, and  from  the  amount  used  the  chlorine  combined  with  the 
tin  can  be  calculated,  provided  no  other  acid  is  present.  If  the 
stannic  chloride  was  prepared  by  oxidation  with  potassium 
chlorate  or  nitrate,*  the  solution  will  also  contain  chlorine  com- 
bined with  potassium.  The  total  chlorine  can  be  determined  by 
adding  a  few  drops  of  neutral  potassium  chromate  solution  to 
the  solution  which  has  been  titrated  with  sodium  hydroxide,  and 
titrating  with  silver  nitrate  solution.  If  in  this  way  more  chlo- 
rine is  found  than  corresponds  to  the  amount  of  hydrochloric 
acid  neutralized  by  the  alkali,  the  difference  is  expressed  in  terms 
of  potassium  chloride.  If,  on  the  other  hand,  less  chlorine  is  found, 
the  presence  of  some  other  acid  in  the  tin  solution  is  assured. 

To  illustrate  the  accuracy  of  such  an  analysis,  the  following 
results  will  be  given:  A  sample  of  solid  stannic  chloride 
(SnCl4+5H20)  was  analyzed  gravimetrically,  as  described  on 
p.  321.  It  was  found  to  contain  42.02  per  cent,  of  chlorine  and 
34.73  per  cent,  of  tin. 

Two  portions  were  then  analyzed  volumetrically  by  titration 
first  with  sodium  hydroxide  and  then  with  silver  nitrate  : 

N 
A.  0.8533  gm.  of  tin   salt  required  20.06  c.c.  —  sodium  hy- 

N  N 

droxide  and  20.34  c.c.  —  silver  nitrate.     As  1  c.c.  -^  solution  corre- 
2.  2i 

N 
spends  to  0.01773  gm.  of  chlorine,  it  is  evident  that  20.06  c.c.  -^ 


*  The  potassium  nitrate  is  acted  upon  by  the  excess  of  hydrochloric  acid 
present  forming  the  chloride,  and  the  excess  of  the  acid  is  afterwards  removed 
by  evaporation  as  much  as  possible. 


COMMERCIAL  HYDROUS  STANNIC  CHLORIDE.  S75 

sodium  hydroxide  represent  20.06X0.01773  =  0.3556  gm.  chlorine 

N 
or  41.67  per  cent.  Cl,  and  20.34  c.c.  -^  silver  solution  show  20.34X 

0.01773  =  0.3605  or  42.25  per  cent.  Cl. 

N 
B.  0.8383  gm.  of  tin  salt  required  19.79  c.c.  —  sodium  hydroxide 

N  N 

and  19.92  c.c.  —  silver  nitrate.    19.79  c.c.  —   sodium  hydroxide 
2  2 

represent  19.79  X  0.01773  =  0.3508  gm.  chlorine  or  41.84  per  cent.  Cl. 

N 
19.92    c.c.  —   silver    solution    show   19.92X0.01773  =  0.3531   gm. 

chlorine  or  42.12  per  cent.  Cl. 

The  above  analysis  shows  that  the  tin  salt  was  practically  free 
from  potassium  chloride  by  the  comparative  agreement  of  the 
results  obtained  by  titration  with  sodium  hydroxide  with  those  of 
the  silver  nitrate  titration.  In  the  absence  of  free  hydrochloric 
acid,  the  tin  can  be  determined  from  the  amount  of  chlorine  found* 

4C1       Sn 
141.84: 119.0  =  41.75  *:z 

x  =  35.03  per  cent,  tin  instead  of  34.73  per  cent,  as  found  gravi- 
metrically. 

Remark. — It  is  only  permissible  to  compute  the  amount  of  tin 
present  from  the  amount  of  chlorine  found  by  titration  when 
there  is  no  free  hydrochloric  acid  present.  It  is  never  possible 
to  know  whether  this  is  the  case  or  not,  so  that  the  volumetric 
determination  is  only  useful  as  a  check  upon  the  gravimetric 
method. 

Determination  of  the  Acid  Contents  of  Fuming  Acids. 

Highly  concentrated  acids  must  be  always  weighed  and  not 
measured,  in  order  to  avoid  loss  by  evaporation.  The  weighing 
is  best  accomplished  by  means  of  the  Lunge-Rey  pipette,  shown 
in  Fig.  88. 

*  41.75  is  the  mean  of  the  values  obtained  by  titration  with  alkali. 


576  VOLUMETRIC  ANALYSIS. 

The  lower  tube  is  removed,  J  c.c.  of  water  is  placed  within 
it,  and  this  is  weighed  together  with  the  dry  upper  pipette,  but  the 
two  parts  are  left  unconnected.  The  lower  stop-cock  is  closed,  the 
upper  one  opened,  and  a  slight  vacuum  is  produced 
in  the  bulb  by  sucking  through  the  upper  tube  and 
then  closing  the  stop-cock.  The  dry  point  of  the 
pipette  is  now  introduced  into  the  fuming  acid 
(in  the  case  of  solid  pyrosulphuric  acid  it  is  first 
liquefied  by  warming  slightly)  and  the  lower  stop- 
cock is  opened.  As  soon  as  the  widened  part  of  the 
pipette  below  the  lower  bulb  is  i  to  }  full,  the  stop- 
cock is  closed,  taking  care  that  none  of  the  liquid 
reaches  up  to  it. 

The  acid  on  the  outside  of  the  pipette  is  care- 
fully wiped  off  with  filter-paper;  the  two  parts  of  the 
pipette  are  now  connected  for  the  first  time   and 
again  weighed.     The  amount  of  acid  taken  for  the 
analysis  should  amount  to  from  0.5  to  1  gm.     The 
point   of   the   pipette   is   then    dipped   into    about 
100  c.c.  of  distilled   water   contained   in  a  beaker, 
and,  by  opening  first  the  upper  stop-cock  and  then 
the  lower,  the  acid  is  allowed  to  run  into  the  water.     The  amount 
remaining  in  the  two  parts  of  the  pipette  is  also  washed  into  the 
beaker. 

If  the  acid  to  be  analyzed  is  hydrochloric  or  sulphuric  acid, 
methyl  orange  is  added  and  the  solution  is  titrated  with  a  half- 
normal  sodium  hydroxide.  If  it  is  nitric  acid,  an  excess  of  sodium 
hydroxide  is  first  added,  then  a  little  methyl  orange,  and  the  titra- 

N 
tion  is  completed  with  —  hydrochloric  acid.*     When  one  of  the 

above  pipettes  is  not  available,  the  weighing  out  of  the  sample 
for  analysis  can  be  effected  as  follows:  A  thin- walled  bulb  with 
about  1  c.c.  capacity  is  blown  between  two  ends  of  capillary  tubing. 
After  weighing,  the  upper  piece  of  capillary  tubing  is  connected 
with  a  small,  ordinary  pipette,  at  the  ends  of  which  are  attached 


*  In  this  way  the  action  of  the  ever-present  nitrous  acid  upon  the  indi- 
cator is  avoided. 


ACID  CONTENTS  OF  FUMING  ACIDS.  577 

pieces  of  rubber  tubing,  and  the  latter  are  closed  with  pinch-cocks. 
The  bulb  is  filled  as  follows: 

The  lower  pinch-cock  is  closed,  the  upper  one  opened,  and  a 
vacuum  produced  by  sucking  through  the  upper  tube  and  then 
closing  the  pinch-cock.  The  lower  point  of  the  weighed  tube  is 
introduced  into  the  acid  and  the  lower  pinch-cock  opened.  When 
the  small  bulb  is  one-third  full  the  pinch-cock  is  closed,  the  upper 
end  of  the  capillary  tubing  is  melted  together,  and,  after  wiping 
off  the  acid  from  the  outside,  the  lower  end  is  likewise  sealed, 
and  the  bulb  weighed.  About  100  c.c.  of  water  are  placed  hi  a 
flask  with  a  closely  fitting  ground-glass  stopper,  the  weighed  bulb 
is  thrown  in,  and  it  is  broken  by  shaking.  In  this  way  the  very 
strongest,  fuming  sulphuric  acid  can  be  dissolved  in  water  with- 
out loss.  On  the  other  hand,  the  pipette  shown  in  Fig.  88  is  not 
so  good  for  the  weighing  out  of  an  acid  containing  70  per  cent,  or 
more  of  SOS.  If  the  acid  is  not  too  concentrated,  this  bulb  may 
be  emptied  as  was  described  for  the  pipette. 

For  the  analysis  of  the  solid  anhydride,  Stroof  places  a  little 
in  a  dry  weighing  tube,  and  concentrated  sulphuric  acid  of  known 
strength  is  added  until  a  fuming  acid  of  about  70  per  cent.  S03 
is  obtained.  To  effect  solution,  the  mixture  is  warmed  to  about 
30°  to  40°  C.  in  a  loosely  stoppered  bottle.  The  acid  thus 
obtained  is  analyzed  as  above.* 

Computation  of  the  S03  Contents  of  a  Fuming  Sulphuric  Acid. 

The  above  titration  gives  not  only  the  sulphuric  anhydride 
present,  but  also  the  never-failing  S02.  In  a  separate  portion, 
therefore,  the  amount  of  the  latter  is  determined  by  titration 

with  a  ^  iodine  solution  (see  lodimetry),  an  equivalent  amount 

is  subtracted  from  the  total  amount  of  sodium  hydroxide  used, 
and  from  the  difference  the  total  S03  present  is  computed. 

With  regard  to  the  S02,  the  following  reactions  take  place 
during  the  titrations: 

H2SO3+  NaOH=  NaHSO3+H2O 
H3S03-f  H20  +  21=  2HI +H2S04 

*G.  Finch  (Chem.  Ztg.,  1910,  297)  and  R.  H.  Vernon  (ibid.,  1910,  702) 
use  a  different  apparatus  and  larger  samples,  thus  getting  more  accurate 
results. 


57^  VOLUMETRIC  ANALYSIS. 

It  is  to  be  noted  in  the  first  reaction  that,  although  sulphurous 
acid  is  a  dibasic  acid,  the  end-point  is  reached,  with  methyl  orange 
as  an  indicator,  when  the  first  hydrogen  atom  has  been  neutralized. 
From  the  two  equations,  then,  it  is  evident  that  2  gin.  atoms  of 

N 
iodine  are  equivalent  to  1  gm.  molecule  of  NaOH,  or  1  c.c.  ^  iodine 

N 
solution  is  equivalent  to  \  c.c.  —  sodium  hydroxide. 

N  N 

Since,  in  general,  5  c.c.  -^  solution=  1  c.c.—  solution,  then 

1  c.c.  TQ  solution=-J  c.c.  —  solution, 

N  N 

and  in  the  given  case  1  c.c.  y^r  iodine-=-]V  c.c.  —  sodium  hydroxide; 

N 

so  that  if  T  c.c.  -^  alkali  were  used  in  the  first  titration  of  the 
& 

N 
total  acid  present,  and  t  c.c.  of  -r-r  iodine  solution  for  the  oxidation 

of  the  sulphurous  acid,  it  is  plain  that  ^—77:  represents  the  amount 

of  alkali  necessary  for  the  neutralization  of  the  total  sulphuric  acid. 
The  S08  is  determined  by  an  indirect  analysis. 
We  will  assume  that  the  fuming  acid  consisted  of 

H2S04=s 
S0.-y 
S02=a 


100 
then  100-a=3+y. 

In  order  to  determine  x  and  y  a  second  equation  is  necessary, 
and  this  is  found  from  the  titration  of  the  total  sulphuric  acid. 
Assume  that,  after  the  deduction  corresponding  to  the  amount  of 
S02  has  been  made,  the  .total  amount  of  H2S04  was  found  to  be 
p  per  cent.,  then: 

1.  x+y   =100-a 

2.  x+my=  p 

p+a-100 
m-1 


ACID  CONTENTS  OF  FUMING  ACIDS.  S79 


s=100-(a+y)  =  per  cent.  H2SO4 

H2SO4     98.086 
In  equation  2,      m  =    gQ     =  8QQ7-  =  1  .2250 

and 

m-  1  =  0.2250 

Example*  —  3.5562  gms.  of  fuming  acid  were  diluted  to  500  c.c., 
and  of  this  amount  100  c.c.  =0.71  12  gm.  were  taken  for  analysis. 

1.  100    c.c.    required    5.40    c.c.    ^  iodine  =  5.4  X  0.003203  = 

0.01730  gm.  S02=2.43  per  cent.  S02=a. 

N 

2.  100  c.c.  required  34.40  c.c.  -^  sodium  hydroxide. 

From  the  latter  must  be  deducted  0.54  c.c.  to  correspond  to 
the  amount  of  alkali  necessary  for  the  SO2.    34.40  -  0.54  *  33.86  c.e. 

33.86x0.02452=0.8305  gm.  H2SO4=  116.7  per  cent.-p. 

If  these  values  are  introduced  in  the  above    equations  we 
obtain 

119.16-100     19.16 


0.2250 
and 

z  =  100-  (85.15  +  2.43)  =  12.42  per  cent. 

The  acid  contains,  therefore: 

H2SO4=    12.42  f 
SO3=  85.15 
SO2=     2.43 

100.00 

*  Lunge,  Zeitechr.  f.  angew.  Ch.,  1895,  p.  221. 

t  Like  ail  indirect  analyses,  the  results  obtained  are  not  absolutely  accu- 
rate. Almost  all  fuming  acids  contain  solid  constituents  which  are  neg- 
lected in  the  above  calculation.  It  would  be  more  accurate  to  determine 
the  amount  of  the  latter  in  a  separate  portion,  by  weighing  the  residue  on 
ignition. 


580  VOLUMETRIC  A 'NA LYSIS. 

Preparation  of  Concentrated  Sulphuric  Acid  Mixtures  (M.  Gerster.) 

It  is  often  necessary  to  prepare  fuming  sulphuric  acid  of 
definite  concentration. 

Given : 

(a)    Fuming  sulphuric  acid  (A)  with  a  per  cent,  free  SO3. 

(6)  Sulphuric  acid  (B)  with  c  per  cent.  H2SO4  and  100— c  per 
cent,  water. 

A  fuming  acid  containing  b  per  cent,  free  SO3  is  desired. 

To  obtain  the  latter,  100  gms.  of  the  acid  A  are  mixed  with 
x  gms.  of  the  acid  B.  It  must  be  remembered,  however,  that  the 
water  in  the  acid  B  requires  S03  in  order  to  form  100  per  cent. 
H2SO4: 

H20  +  S03=H2S04. 

The  acid  B  requires  for  the  water  present  hi  each  100  gms 
H2O:SO3=(100-c):2/ 

(100-c)  S03    (100 -c)  80.06 
y=     -E&-  -1832-      =4-44(1°0-c)gms.SO, 

If   100  gms.  of  the  acid  B  require  4.44  (100-c)  gms.  SO3  from  A> 
then 
x  gms.  of  the  acid  B  require  0.0444  (100— c)  x  gms.  SOS  from  A. 

Now 

A  +  B 

(100+z):[a-0.0444(100-c)z]=100:& 
100  (a -b) 


444+ 6-  4.44  c 


gms.  of  B. 


Example.— The  fuming  acid  A   contains  25.5  per  cent,  free 
S03=a. 

Sulphuric  acid  B  contains  98.2  per  cent.  H2SO4=c. 

The  acid  desired  is  to  contain  19.0  per  cent.  SO3=6. 

If  these  values  are  inserted  in  the  above  equations,  we  obtain 

100(25.5-19.0)         650 


TITRATION  OF  HYDROXYLAMINE  SALTS,  ETC.  581 

We  must  add,  therefore,  24.07  gms.  of  the  98.2  per  cent,  sulphuric 
acid  to  100  gms.  of  the  fuming  acid  in  order  to  obtain  an  acid 
containing  19.0  per  cent,  of  free  S03. 

Titration  of  Hydroxylamine  Salts. 

Hydroxylamine  hydrochloride  reacts  neutral  towards  methyl 
orange  and  acid  towards  phenolphthalein.  If  the  latter  is  added 
to  an  aqueous  solution  of  the  salt,  and  the  titration  is  made  with 

N 

•— r  alkali,  the  end-point  will  be  obtained  when  the  total  amount  of 

acid  present  has  been  neutralized  by  the  alkali.  It  is  impossible 
to  determine  the  amount  of  free  hydrochloric  acid  present 
when  phenolphthalein  is  used,  but  it  can  be  done  with  methyl 
orange.  Romijn  *  recommends  for  the  titration  of  the  acid  a 

~  borax  solution. 

Hydrofluoric  Acid. 

1000  c.c.  normal  alkali=HF=20.01  gms.  HF. 

Hydrofluoric  acid  can  be  titrated  with  phenolphthalein  as  an 
indicator,  but  not  with  litmus  or  methyl  orange.  The  acid  is 
measured  out  into  a  platinum  dish  by  means  of  a  pipette  which  is 
coated  with  beeswax,  an  excess  of  sodium  hydroxide  free  from 
alkali  is  added,  and  the  excess  of  the  latter  is  titrated  in  hot 
solution  with  an  acid  of  knowTi  strength.! 


Hydrofluosilicic  Acid. 

The  titration  of  this  acid  may  take  place  according  to  either 
of  the  following  reactions: 

I.     H2SiF6  +  2KOH  =  K2SiF6  +  2H2O, 
II. 


*  Z.  anal.  Chem.,  36  (1897),  19.     This  method  has  not  been  tested  in 
the  author's  laboratory. 

t  Cf.  Winteler,  Z.  angew.  Chem.,  1902,  p,  33. 


582  VOLUMETRIC  ANA LYSIS. 

According  to  Equation  I. 
1000  c.c.  normal  KOH  or  Ba(OH)2  =  72.16  gms.  H2SiF6. 

Author's  Method. 

If  hydrofluosilicic  acid  is  titrated  in  the  cold  with  caustic 
potash,  using  phenolphthalein  as  indicator,  a  red  color  appears 
after  a  time,  but  disappears  later  on  account  of  the  excess  alkali 
reacting  according  to  the  equation : 

K2SiF6+4KOH=6KF  +  Si(OH)4. 

This  last  reaction,  however,  takes  place  so  slowly  that  it  is 
impossible  to  obtain  a  distinct  end  point.  If,  however,  the 
solution  is  diluted  with  an  equal  volume  of  alcohol,  then  2  or  3 
drops  of  phenolphthalein  added,  fit  can  be  titrated  with  tenth- 
normal  potassium  or  barium  hydroxide.  The  insoluble  potassium 
or  barium  fluosilicate  separates  out,  and  is  not  acted  upon  by  an 
excess  of  the  alkali,  so  that  a  sharp  end  point  is  obtained.  Sodium 
hydroxide  forms  a  soluble  salt  so  that  the  titration  cannot  be 
made  with  this  reagent. 

Indirect  Method  of  Penfield* 

Penfield  treats  the  solution  to  be  titrated  with  an  excess  of 
KC1,  dilutes  with  an  equal  volume  of  alcohol,  and  then  titrates 
the  hydrochloric  acid  set  free  in  the  reaction, 

H2SiF6  +  2KC1  =  K2SiF6  +  2HC1, 

with  tenth-normal  sodium  hydroxide  solution,  using  cochineal 
as  indicator.     Methyl  red  is  preferable  to  the  cochineal. 

According  to  Equation  II. 
1000  c.c.  normal  NaOH=  24.05  gm.  H2SiF6. 

(a)  Method  of  Sahlbom  and  Hinrichsen.-f 

The  solution  is  titrated  at  the  temperature  of  the  water-bath 
with  tenth-normal  sodium  hydroxide  solution,  using  phenol- 
phthalein as  indicator. 

*  Chem.  News,  39,  179. 
t  t  Ber.,  39,  2609  (1906). 


DETERMINATION  OF  ORGANIC  ACIDS.  S83 

(b)  Method  of  Schucht  and  M oiler* 

The  solution  to  be  titrated  is  treated  with  an  excess  of  neutral 
calcium  chloride  solution  (25  c.c.  of  4N.  CaCl2)  and  titrated  with 
tenth-normal  sodium  hydroxide,  using  methyl  orange  as  indi- 
cator. The  following  reaction  takes  place  in  the  cold: 

H.,SiF6  +  3CaCl2  +  GNaOH  =  3CaF2 + GNaCl  +  Si  (OH)  4 + 2H2O . 

During  the  titration  the  solution  remains  clear,  for  the  CaF2 
and  the  Si(OH)4  remain  in  colloidal  solution.  Phenolphthalein 
should  not  be  used  as  indicator,  as  it  is  hard  to  decide  upon  the 
correct  end  point. 

In  the  titration  of  salts  of  hydrofluosilicic  acid,  however, 
the  titration  must  always  be  carried  out  with  phenolphthalein  as 
indicator: 

Na2SiF6 + 3CaCl2  +  4XaOH = 3CaF2 + 6NaCl + Si  (OH)  4. 

In  this  case 

1000  c.c.  of  normal  XaOH  =  47.08  gms.  of  Xa2SiF6. 

Determination  of  Organic  Acids. 

Methyl  orange  cannot  be  used  for  the  titration  of  organic  acids; 
but  either  phenolphthalein  or  litmus  may  be  employed.  If  car- 
bonic acid  is  present  at  the  same  time,  the  titration  is  made  in  a  hot 
solution  (cf.  p.  554).  It  is  best  to  dilute  the  organic  acid  with 
water  free  from  carbon  dioxide,  add  phenolphthalein,  and  titrate 
with  half-normal  barium  hydroxide  in  the  cold. 

To  illustrate. — It  is  desired  to  analyze  a  sample  of  acetic  anhy- 
dride. The  only  impurity  that  the  distilled  product  is  likely  to 
contain  is  acetic  acid,  so  that  it  is  a  question  of  determining  the 
amount  of  acid  and  anhydride  in  the  presence  of  one  another. 
Such  a  problem  can  be  solved  only  by  an  indirect  analysis. 
The  mixture  is  weighed  out  in  a  small  glass  bulb  and  then 
thrown  into  an  accurately-measured  amount  of  standard  barium 
hydroxide  solution.  The  latter  is  contained  in  a  flask  which  is 

*  Ber.,  39,  3693.     This  method  has  not  been  tested  by  the  author. 


584  VOLUMETRIC  ANALYSIS. 

connected  with  a  return-flow  condenser  and  at  the  top  of  the 
condenser  a  soda-lime  tube  is  fitted.  The  contents  of  the  flask 
are  warmed  gently  until  the  oil  has  completely  dissolved;  it  is 
thereby  changed  to  acetic  acid, 

vyXigLx'v/  N^/^V  i  TT  /~V__O/^TT  r*r\r\ET 
CH3CO/C  ZCHjOUUH, 

and  the  latter  is  neutralized  by  the  alkali.  After  the  reaction  is 
complete,  a  drop  of  phenolphthalein  is  added  and  the  solution 
is  decolorized  by  the  addition  of  a  titrated  acid.  From  the  amount 
of  the  latter  used,  tho  excess  of  the  alkali  is  known,  and  if  this 
is  deducted  from  the  total  amount  of  alkali  in  the  flask,  the  amount 
necessary  for  the  complete  neutralization  of  the  acetic  acid,  whether 
originally  present  as  the  free  acid  or  in  the  form  of  its  anhydride, 
can  be  calculated: 

C4H.O,   C2H402 

1.  x     +     y     =  p  (original  weight) ; 

2.  mx     +     y    =  q  (weight  acetic  acid  after  the  action  of  water); 

and  from  this  x  can  be  calculated. 

1 

2C2H4O2     120.06 
and  in  these  equations  m=  n  ^  n_  =  ino  nc;  =  1.1765     and 

1 


m-l 


=  5.665. 


Example. — The  absolutely  clear  preparation  of  acetic  anhy- 
dride from  a  well-known  firm  gave  the  following  results,  0.9665  gm. 
being  taken  for  the  analysis: 

N 
200  c.c.  of  barium  hydroxide  solution  required  187.79  c.c.  —  HC1; 

200  c.c.  of  barium  hydroxide  +  0.9665  gm.  of  substance 

N 
required  6.03  c.c.  —  HC1; 

so  that  the  0.9665  gm.  of  substance  was  equivalent  to  181.76  c.c. 


DETERMINATION  OF  ORGANIC  ACIDS.  585 

X  N 

^-  HC1,  and  this  amount  of  —  Ba(OH)2  solution  would  have  been 

required  to  neutralize  it.     This  corresponds  to 

181.76X0.006003  =  1.0911  gms.  acetic  acid=g. 

If,  now  the  values  of  p  and  q  are  introduced  in  the  previous 
equations,  we  obtain 

z  =  5.665(1.0911-0.9665)  =0.7059  gm.  anhydride, 

and  in  per  cent. 

0.9665: 0.7059  =  100  :x 

£  =  73.04  per  cent,  acetic  anhydride. 
The  preparation,  therefore,  contained 

Acetic  anhydride  =   73.04  per  cent. 

26.96  per  cent. 
Acetic  acid  =  ^ 

100.00  per  cent. 

Remark. — Acetic  acid  anhydride  is  also  hydrolyzed  by  water 
at  the  ordinary  temperature.  If  a  weighed  amount  of  the  sub- 
stance is  shaken  with  water  in  a  flask  until  no  more  drops  of 
anhydride  are  to  be  recognized,  and  the  acetic  acid  formed  is 
then  titrated  with  barium  hydroxide,  using  phenolphthalem  as 
indicator,  correct  results  are  obtained  if  the  water  used  is  entirely 
free  from  carbon  dioxide.  It  is  always  safer,  however,  to  earn- 
out  the  determination  as  outlined  above. 

In  some  factories  the  analysis  of  acetic  acid  anhydride  is 
carried  out  by  the  method  of  Menschutkin  and  Wasiljeff.  This 
is  based  upon  the  fact  that  when  acetic  acid  anhydride  is  treated 
with  freshly  distilled  aniline,  acetanilide  is  formed  in  accordance 
with  the  following  equation. 

23  r   ! V>  +  C6H5NH2  =  C6H5N(C2H30)  H  +  CH3GOOH 
Url3  •  \j\j/ 


586  VOLUMETRIC  ANALYSIS. 

whereas  acetic  acid  itself  does  not  form  acetanilide  under  the  same 
conditions.  Two  or  three  grams  of  commercial  acetic  anhydride 
are  shaken  in  a  dry  weighing  beaker  with  from  4  to  6  cc.  of  freshly 
distilled  aniline.  The  anhydride  immediately  begins  to  combine 
with  the  aniline,  liberating  considerable  heat.  After  cooling,  the 
solidified  contents  of  the  weighing  beaker  are  rinsed  by  means  of 
absolute  alcohol  into  an  ordinary  beaker,  phenolphthalei'n  is 
added  and  the  total  amount  of  aectic  acid  present  titrate^  with 
half-normal  alkali. 
We  have  then 


x       +       y     =  p', 
mx       +       y     =  q  (acetic  acid) ; 

from  which  can  be  computed 

*-£=2-2.428. 

1  —  rr.. 

In  this  equation 

C2H402     60.03 


It  is  true  that  concordant  results  are  obtained  by  this  method, 
but  they  are  much  too  high:  in  fact  as  much  as  14  to  16  per  cent. 
too  high.  This  is  due  to  the  fact  that  although  acetic  acid  itself 
does  not  react  with  aniline  in  the  cold,  it  does  react  very  readily 
when  heated.  When,  therefore,  a  mixture  of  acetic  anhydride 
and  acetic  acid  are  allowed  to  remain  in  contact  with  aniline,  there 
is  so  much  heat  liberated  from  the  reaction  between  the  anhydride 
and  the  aniline  that  a  part  of  the  acetic  acid  itself  reacts  takes 
part  in  the  formation  of  acetanilide : 

CH3C02H  +  C6H5NH2  =  H20  4-  C6H5N  (C2H30)  H, 
so  that  evidently  too  little  acetic  acid  is  found  in  the  subsequent 


DETERMINATION  OF  SULPHUROUS   ACID.  587 

tit  rat  ion  and  consequently  too  high  values  are  obtained  for  the 
amount  of  anhydride  present. 


Determination  of  Sulphurous  Acid. 

For  the  determination  of  sulphurous  acid  by  itself,  the  analy- 
sis is  always  accomplished,  as  recommended  by  Volhard,  by  an 
iodimetric  process,  i.e.,  it  is  oxidized  to  sulphuric  acid.  In  many 
cases,  however,  it  is  necessary  to  titrate  the  sulphurous  acid  with 
alkali  (cf.  p.  577),  and  here  the  choice  of  an  indicator  is  important. 
for  the  end-point  is  very  different  in  the  case  of  methyl  orange 
from  that  obtained  when  phenolphthalein  is  used: 


a  +  2NaOH  =  Na2SO3  +  2H2O  (with  phenolphthalein)  , 
H2SOs+NaOH  =  NaHSO3+  H2O  (with  methyl  orange). 

NaHS03  reacts  acid  toward  phenolphthalein,  but  neutral 
toward  methyl  orange,  so  that  twice  as  much  alkai  would  be 
added  in  the  first  case.  The  most  accurate  results  are  obtained 
with  the  use  of  methyl  orange,  for  the  carbon  dioxide  which  is 
almost  always  present  does  not  exert  much  of  an  effect  upon  this 
indicator,  whereas  it  does  upon  phenolphthalein. 

Determination  of  Orthophosphoric  Acid. 

NaH2PO4  reacts  acid  toward  phenolphthalein,  and  neutral  toward 
methyl  orange,  while  Na2HP04  is  neutral  toward  the  former  indi- 
cator and  basic  toward  the  latter. 

Therefore,  on  titrating  free  phosphoric  acid  with  alkali  one  of 
the  following  reactions  will  take  place  : 


1.  H3PO4+2NaOH  =  Na2HPO4  +  2H2O  (phenolphthalein). 

2.  H3PO4  +  NaOH=XaH2PO4+H20  (methyl  orange). 

The  first  reaction  is  not  sharp,  because  pure  Na2HP04  is  disso- 
ciated to  a  slight  extent,  so  that  it  becomes  alkaline  to  phenol- 
phthalein : 

Na^HPO.+H  OH<=±NaH2P04  +  Na  OH. 


588  yOLU METRIC  ANA 'LYSIS. 

To  prevent  this  hydrolysis,  the  titration  is  best  effected  in  a 
cold,  concentrated  solution  containing  sodium  chloride. 

Alkalimetric  Determination  of  Phosphorus  in  Iron  and  Steel  * 
1000  c.c.  normal  NaOH=  1.348  gm.  P. 

The  phosphorus  is  precipitated  in  a  2  gm.  sample  with  ammonium 
molybdate  according  to  p.  437,  filtered,  washed  with  1  per  cent, 
nitric  acid  and  then  with  1  per  cent,  potassium  nitrate  solution 
until  the  washings  no  longer  react  acid.  The  filter  and  precipitate 
are  then  transferred  back  to  the  Erlenmeyer  flask  in  which  the 
precipitation  took  place,  covered  with  an  excess  of  tenth-normal 
NaOH  (T  c.c.),  stirred  until  solution  is  complete,  and  then  the 
excess  of  alkali  titrated  with  tenth-normal  nitric  acid,  (t  c.c.) 
using  phenolphthalein  as  indicator. 

The  reactions  taking  place  are  as  follows : 

2[(NH4)3PO4-12MoO3]  +  46NaOH=2(NH4)2HP04+(NH4)2Mo04 

+  23Na2MoO4+22H2O, 

from  which  it  is  clear  that  46  gms.  mols.  of  NaOH  are  equivalent 
to  2  gm.  atoms  of  P  and  1  c.c.  of  the  tenth-normal  NaOH= 
0.0001348  gm.  P. 

Since  the  precipitate  was  produced  from  2  gm.  steel,  and  T  c.c. 
of  NaOH  and  t  c.c.  of  HNOs  were  used,  the  percentage  of  phos- 
phorus is 

(T -t)X  0.01348  _^p 
— 2~  -  /or. 

Remark. — To  obtain  accurate  results,  it  is  advisable  to  deter- 
mine the  percentage  of  phosphorus  in  a  steel  gravimetrically, 
then  to  standardize  the  alkali  against  this  steel,  carrying  out  the 
titration  exactly  as  described  above. 

Determination  of  Boric  Acid. 

Free  boric  acid  has  no  action  upon  methyl  orange,  conse- 
quently alkali  borates  may  be  titrated  with  hydrochloric  and 
nitric  acids,  using  this  indicator;  with  sulphuric  acid  the  results 

*  See  Blair,  Analysis  of  Iron  and  Steel.  The  method  was  proposed  by 
J.  O.  Handy. 


DETERMINATION  OF  BORIC  ACID.  589 

are  not  as  satisfactory,  for  there  is  in  this  case  no  sharp  color 
change.  If  phenolphthalein  is  used  as  the  indicator,  the  red 
color  fades  gradually  and  the  end  point  cannot  be  determined 
with  certainty.  If,  on  the  other  hand,  sodium  hydroxide  ii 
slowly  run  into  an  aqueous  solution  of  boric  acid  containing  phe- 
nolphthalein, after  some  time  a  pale-pink  color  is  noticeable  which 
becomes  deeper  on  the  addition  of  more  alkali.  The  first  pink 
color  is  formed  before  all  of  the  boric  acid  has  been  neutralized, 
for  sodium  borate  is  perceptibly  hydrolyzed.  Free  boric  acid  cannot 
be  titrated  by  itself,  but  if,  as  proposed  by  Jorgensen,*  a  sufficient 
amount  of  glycerol  f  (or  mannitolt)  is  added  to  the  solution,  the 
hydrolysis  is  prevented,  so  that  when  1  mol.  of  NaOH  is  present 
for  1  mol.  of  H3BC>3  the  solution  suddenly  changes  from  colorless 
to  red;  probably  a  stronger  acid  is  formed  by  the  addition  of 
the  glyoerol,  the  glyceryl-boric  acid  (C3H5O2OH)B(OH). 

If  the  solution  does  not  contain  sufficient  glycerol  the  color 
change  takes  place  too  soon,  as  can  be  shown  by  the  addition  of 
more  glyoerol.  If  the  red  color  disappears  on  adding  the  lat- 
ter, more  alkali  is  added  until  it  reappears.  The  right  end-point 
is  reached  when  the  red  color  no  longer  disappears  on  the  addition 
of  glyoerol.  Inasmuch  as  commercial  glycerol  reacts  acid,  it 
must  be  just  neutralized  with  alkali  before  being  used  for  this 
determination.  Furthermore,  in  order  to  obtain  accurate  results 
it  is  necessary  that  the  solutions  should  be  absolutely  free  from 
carbonate. 

Application.     Determination  of  Boric  Add  in  an  Alkali  Borate 

Free  from  Carbonate.^ 

About  30  gms.  of  the  borate  are  dissolved  in  water  free  from 
carbon  dioxide,  diluted  to  1  liter,  and  the  total  alkali  is  determined 

in  an  aliquot  part  by  titration  with  —  hydrochloric  acid,  using 
methyl  orange  as  an  indicator.  A  fresh  portion  of  the  borate  is 

*  Zeitschr.  f .  Nahrungsm.  IX,  p.  389,  and  Zeitschr.  f.  angew.  Ch.,  1897,, 
p.  5. 

t  Zeitschr.  f.  angew.  Ch.,  1896,  p.  549. 

t  Jones,  Am.  J.  Sci.  [4]  7,  147  (1899). 

§  M.  Honig  and  G.  Spitz,  Zeitschr.  f.  angew.  Ch.,  1896,  p.  54Q. 


590  VOLUMETRIC  A NA LYSIS. 

taken  and  exactly  neutralized  by  the  amount  of  hydrochloric 
acid  found  necessary  by  the  previous  titration;  by  this  means  the 
solution  will  contain  free  boric  acid.  After  adding  about  50  c.c.  of 
glycerol  for  each  1.5  gms.  of  the  borate,  the  solution  is  titrated 

N 
with  —  sodium  hydroxide,  using  phenolphthalei'n  as  indicator. 

After  the  end-point  is  reached,  10  c.c.  more  of  glycerol  are  added, 
and  this  usually  causes  the  solution  to  become  colorless.  The  end- 
point  with  sodium  hydroxide  is  again  obtained  and  the  process 
repeated  until  finally  the  addition  of  glycerol  causes  no  further 
action  upon  the  end-point. 

If  the  borate  contained  carbonate,  the  portion  taken  for  analysis 
is  neutralized  with  acid  as  before,  then  boiled  for  a  few  minutes, 
taking  the  precaution  of  connecting  the  flask  containing  the  solu- 
tion with  a  return-flow  condenser.*  After  the  carbon  dioxide  is 
expelled,  the  sides  of  the  condenser  are  washed  down  with  water 
and  the  titration  with  sodium  hydroxide  made  as  before.f 

For  the 

Determination  of  Boric  Acid  in  Insoluble  Silicates. 

see  E.  T.  Wherry  and  W.  H.  Chapin,  J.  Am.  Chem.  Soc.,  30,  1687 
(1908). 

Determination  of  Carbonic  Acid. 

(a)  Determination  of  Free  Carbonic  Acid. 

To  determine  the  amount  of  free  carbonic  acid  present  in  a 
dilute  aqueous  solution,  an  excess  of  titrated  barium  hydroxide 

N 
solution  is  added,  and  the  excess  is  determined  by  means  of  —  HC1, 

using  phenolphthalei'n  as  an  indicator: 

H2C03+  Ba(OH)2 =BaCO3+  2H2O 
1  c.c.  5L  HC1  =  0.0022  gm.  C02. 

*  The  condenser  serves  to  keep  back 'any  boric  acid  escaping  with  the 
steam. 

t  Instead  of  the  glycerol,  about  one  gram  of  mannitol  may  be  used  to 
advantage. 


DETERMINATION  OF  CARBONIC  ACID.  59r 

(6)  Determination  of  Carbon  Dioxide  Present  as  Bicarbonate. 

N 
The  solution  is  titrated  with  ^—  HC1  in  the  presence  of  methyl 

orange: 

NaHC03  +  HC1  =  NaCl+  H2COS 

1  c.c.       HC1  =  0.0044  gm.  CO2. 


(c)  Determination  of  Carbon  Dioxide  Present  as  Carbonate. 

N 
The  titration  is  effected  with  ^  HC1  and  methyl  orange:* 

Na2C03+  2HC1  =  2NaCl+  H2C03 
1  c.c.       HC1  =  0.0022  gm.  C02. 


(d)  Determination  of  Free  Carbonic  Acid  in  the  Presence  of 
Bicarbonate. 

N 
One  portion  is  titrated  with  —  HC1,  using  methyl  orange  as 

indicator,  and    the    amount    of    bicarbonate    is  determined    as 
under  (6). 

A  second  portion  is  treated  with  an  excess  of  barium  chloride,f 
then  with  an  excess  of  barium  hydroxide,  and  the  excess  of  the 
latter  titrated  back  with  HC1,  using  phenolphthalein  as  indicator. 

N 
If  the  amount  of  —  acid  used  for  the  first  titration  is  deducted 

N 
from  the   amount  of  ^  barium  hydroxide  solution  found  to  be 

necessary  by  the  last  titration.  the  difference  multiplied  by  0.0022 
will  give  the  amount  of  free  carbonic  acid.J 

*  Alkaline-earth  carbonates  are  dissolved  in  an  excess  of  standard  acid 
and  the  excess  titrated  back  with  standard  alkali. 

t  The  addition  of  barium  chloride  is  only  necessary  when  free  carbonic 
acid  is  titrated  in  the  presence  of  alkali  bicarbonates.  Without  it  free  alkali 
would  then  be  formed:  NaHCO3  +  Ba(OH)2  =  BaCO3  +  H2O  +  NaOH. 

J  This  method  cannot  be  used  when  magnesium  salts  are  present. 


592  VOLUMETRIC  ANA  LYSIS. 

(e)  Determination   of    Bicarbonate   in   the   Presence   of   Carbonate. 
Method  of  C.  Winkler. 

In  one  portion  the  total  alkalinity  is  determined  by  titration 

N 
with  —  HC1,  using  methyl  orange  as  indicator.     This  requires 


In  a  second  portion  the  bicarbonate  is  determined  by  adding 

N 
an  excess  of  —  r  NaOH,  then  neutral  barium  chloride  solution, 

and  afterward  titrating  the  excess  of  the  former  with  phenol- 

N 
phthalein  and  —  HC1.    We  will  assume  that  for  this  purpose 

N  N 

Tj  c.c.  YQ  NaOH   and  t  c.c.  —  HC1   were  used,  then   evidently 

N 
(Tl—t)  c.c.  —  r  NaOH  were  necessary  to  convert  the  bicarbonate 

into  carbonate: 

NaHC03  +  NaOH  =  NazCO8  +  H3O. 

INaOH  corresponds,  consequently,  to  1CO2,  or 
1  c.c.  -^  NaOH  =  0.0044  gm.  CO2, 

and  therefore  (  7^-0-0.  0044  =CO2  as  bicarbonate. 
For  the  decomposition  of  the  normal  carbonate 

-Tl)  c.c.  ^- 

were  necessary,  and  from  the  equation 

Na,COa  +  2HC1  =  2NaCl  +  HaO  +  CO, 

it  is  evident  that 

2HCl=ieO3 
and 

1  c.c.  ~j  HC1  =  0.0022  gm.  CO2. 
The  carbon  dioxide  as  carbonate  =(T+t-T^  -0.0022  gm. 


DETERMINATION  OF  CARBONIC  ACID  IN   THE  AIR.          593 

Remark.— It  has  been  proposed  to  determine  volumetrically 
the  free  and  bicarbonate  carbonic  acid  in  drinking  and  mineral 
waters;  with  the  former  accurate  results  can  be  obtained,  but 
with  the  latter  this  is  not  the  case.  In  the  determination  of  the 
total  alkalinity  not  only  the  bicarbonate  but  also  the  ever-present 
silicate  and  borate  are  likewise  determined,  so  that  this  in  many 
cases  causes  considerable  error  in  the  analysis  of  mineral  waters. 
Thus  in  analyzing  a  sample  of  mineral  water  containing  in  reality 
4.63  gms.  of  carbonic  acid  as  bicarbonate  per  kilogram,  the  titra- 
tion  showed  5.42  gms.,  a  difference  of  0.61  gm,  CO2l 


Determination  of  Carbonic  Acid  in  the  Air.    Method  of 

Pettenkofer. 

Principle. — A  large,  measured  volume  of  air  is  treated  with 
an  excess  of  titrated  barium  hydroxide  solution  whereby  the  car- 
bon dioxide  is  quantitatively  absorbed,  forming  insoluble  barium 
carbonate.  Phenolphthalein  is  added,  and  the  excess  of  barium 
hydroxide  is  determined  by  titration  with  hydrochloric  acid 
until  the  solution  is  colorless.  From  the  amount  of  alkali 
used  to  absorb  the  carbon  dioxide,  the  amount  of  the  latter  is 
calculated. 

Requirements. — 1.  A  calibrated  flask  of  5  liters  capacity. 

2.  Standard  solutions  of  barium  hydroxide  and  hydrochloric 
acid.  The  acid  is  prepared  so  that  1  c.c.  =  0.25  c.c.  G02  at  0°  C. 

and  760  mm.  pressure;  this  is  accomplished  by  diluting  224.7  c.c.  — 

hydrochloric   acid   to    1    liter.     The   barium   hydroxide   solution 
should  be  of  about  the  same  strength. 

Procedure. — The  flask,  with  its  capacity  etched  upon  it,  is 
placed  in  the  space  from  which  the  air  is  to  be  taken,  and  by  means 
of  a  bellows,  the  mouth  of  which  is  connected  with  a  piece  of 
rubber  tubing,  the  air  in  the  flask  is  changed ;  about  100  strokes  are 
made  with  the  bellows.  The  flask  is  then  stoppered  with  a  rub- 
ber cap,  and  at  the  same  time  the  temperature  and  barometer 
readings  are  noted. 


594  yOLUMETRIC  ANALYSIS. 

By  means  of  a  pipette,  100  c.c.  of  barium  hydroxide  solution 
are  run  into  the  flask,  the  rubber  cap  replaced  on  the  bottle,  and 
the  solution  is  gently  shaken  back  and  forth  in  the  flask  for  fif- 
teen minutes.  The  turbid  liquid  is  then  poured  into  a  dry  flask, 
25  c.c.  are  pipetted  out,  phenolphthalein  is  added,  and  hydrochloric 
acid  slowly  run  in  with  constant  stirring  until  the  solution  is 
colorless.  This  requires  n  c.c.,  so  that  for  the  100  c.c.  of  alkali 
solution,  4Xft  c.c.  would  be  necessary.  The  strength  of  the 
barium  hydroxide  in  terms  of  acid  is  now  accurately  determined; 
25  c.c.  of  barium  hydroxide  require  N  c.c.  of  the  standard  hydro- 
chloric acid,  or  100  c.c.  would  neutralize  4XAT  c.c.  of  acid. 

Calculation.  —  Assume  the  contents  of  the  flask  to  be  V  c.c. 
at  t°  C.  and  B  mm.  pressure.  By  the  introduction  of  100  c.c.  barium 
hydroxide  solution  the  same  volume  of  air  was  replaced,  so  that 
the  amount  of  air  taken  for  analysis  amounts  to  (V—  100)  c.c. 
at  t°  C.  and  B  mm.  pressure.  At  0°  C.  and  760  mm.  pressure  the 
volume  is 

v=  (7-100)  B 
0     760(l+a-0 

100  c.c.  of  barium  hydroxide  solution  require  4  N  c.c.  HC1, 
while  100  c.c.  of  the  alkali  after  treatment  with  VQ  c.c.  of  air  require 
4  n  c.c.  of  the  acid  and  this  corresponds  to  4  (N—n)-  0.25  =  (N—n) 
c.c.  C02  at  0°  C.  and  760  mm.  pressure. 

The  amount  of  C02  present  in  1  liter  of  air  measured  at  standard 
conditions  amounts  to 


lOOO.(AT-n) 
x  --    -  *  gms. 


PERSULPHURIC  ACID.  595 


Persulphuric  Acid. 

1000  c.c.  —  Potassium  Hydroxide 

270.34 

-~  =13.517  gms.  K2S2O8. 


If  an  aqueous  solution  of  either  potassium,  sodium,  or  barium 
persulphate  is  boiled  for  some  time,  the  salt  is  decomposed  in 
accordance  with  the  equation: 

2K2S2O8  +  2H2O  =  2K2SO4  +  2H2S04  +  O2 

into  neutral  sulphate  and  free  sulphuric  acid.  The  latter  can  be 
titrated  with  tenth-normal  potassium  hydroxide  solution. 

Procedure.  —  About  0.25  gm.  of  the  persulphate  is  placed  in  an 
Erlenmeyer  flask  of  Jena  glass,  dissolved  in  about  200  c.c.  of  water, 
and  the  solution  boiled  for  twenty  minutes.  It  is  then  cooled, 
methyl  orange  added,  and  the  solution  titrated  with  tenth-normal 
potassium  hydroxide.  Or,  an  excess  of  the  alkali  may  be  added 
and  the  amount  of  excess  titrated  with  tenth-normal  acid. 

The  results  correspond  with  those  obtained  by  the  ferrous 
sulphate  method  (cf.  p.  629)  provided  the  persulphate  is  not  con- 
taminated with  potassium  bisulphate. 

Remark.  —  Ammonium  persulphate  cannot  be  analyzed  by  the 
above  method  because  when  a  solution  of  this  salt  is  boiled,  two 
reactions  take  place.  The  principal  reaction,  to  be  sure,  is 

2  (XH4)  2S2O8  +  2H2O  =-2  (XH4)  2SO4  +  2H2SO4  +  O2, 

but  the  oxygen  is  evolved  to  some  extent  in  the  form  of  ozone  and 
the  latter  oxidizes  a  part  of  the  nitrogen,  so  that  besides  sulphuric 
acid,  the  solution  will  contain  more  or  less  nitric  acid. 

8  (XH4)  2S208  +  6H2O  =  7  (XH4)  2SO4  +  9H2SO4  +  2HX03. 


H.  OXIDATION  AND  REDUCTION  METHODS. 

All  processes  considered  under  this  heading  are  those  in  which 
the  substance  analyzed  is  either  oxidized  or  reduced  by  means 
of  the  solution  with  which  the  titration  is  made.  As  a  standard 
for  measuring  the  normality,  we  consider  the  oxidation  of  two 
gm.-atoms  of  hydrogen  by  1  gm.-atom  of  oxygen.  The  norma] 
solution,  therefore,  will  be  one  which  for  each  1000  c.c.  gives  up 

or  requires  -~-  —  8  gms.  of  oxygen  =  1  gm.  of  hydrogen. 
& 

OXIDATION  METHODS. 
A.  The  Permanganate  Methods. 

These  are  based  upon  the  fact  that  2  gm.-molecules  of  potas- 
sium permanganate  in  acid  solution  give  up  5  gm.-atoms  of 
oxygen,  equivalent  to  10  gm.-atoms  of  hydrogen: 

2KMnO4  =  K2O+  2MnO+  O5(  =  10H) . 

Or,  what  amounts  to  the  same  thing,  each  atom  of  manganese  is 
reduced  from  a  valence  of  7  to  a  valence  of  2,  so  that  !KMnO4  will 
oxidize  the  equivalent  of  5H. 

The  solution  must  always  contain  enough  sulphuric  acid  in 
order  that  the  metals  will  be  left  in  the  form  of  sulphates  and  not 
as  oxides;  otherwise  less  oxygen  is  available  from  the  perman- 
ganate (cf.  p.  613). 

The  amount  of  potassium  permanganate  required  for  the  prep- 
aration of  a  liter  of  normal  solution  is  shown  by  the  above  equa- 

KMnO4     158.03 

tion  to  be  — E — 4  =  — ^-=31.61  gms. 
o  o 

N  N 

For  the  great  majority  of  oxidation  analyses  —  and  rarely  -^ 

solutions  are  used. 

TV 

The  Preparation  of  --  Potassium  Permanganate  Solution 

was  described  on  p.  90.  6 


OXIDATION  M2THODS.  597 

Standardization  of  Permanganate  Solution. 

1.  Against  Sodium  Oxalate  (Sorensen).* 
1000  c.c.  of  normal  permanganate  solution  =  67.00  gms.  Na2C2O4. 

Sodium  oxalate  can  be  purchased  in  a  very  pure  condition. 
The  traces  of  moisture  present  may  be  expelled  by  heating  the 
oxalate  for  two  hours  at  130°  and  cooling  in  a  desiccator;  but  for 
ordinary  work  this  is  usually  unnecessary. 

A  weighed  amount  of  sodium  oxalate  is  dissolved  in  200  c.c. 
of  distilled  water  at  70°,  about  20  c.c.  of  double-normal  sulphuric 
acid  are  added,  and  the  hot  solution  titrated  with  permanganate. 
At  the  start  the  titration  should  proceed  very  slowly,  waiting- 
after  the  addition  of  each  drop  until  the  color  has  disappeared 
before  p-dding  more  permanganate 

2KMn04  +  5Xa2C2O4  +  8H2SO4  = 

=  K2SO4  +  2MnSO4  +  5Xa2S04  +  10CO2  +  8H20. 

The  purity  of  the  sodium  oxalate  may  be  tested  by  heating 
a  weighed  sample  in  a  covered  platinum  crucible  for  thirty 
minutes  over  a  small  flame,  so  that  the  bottom  of  the  crucible 
is  barely  red.  It  is  best  to  use  an  alcohol  lamp  or  else  insert 
the  crucible  in  a  disk  of  asbestos  as  in  a  sulphur  determina- 
tion. Otherwise  the  sulphur  in  the  illuminating  gas  may  cause 
the  formation  of  some  sodium  sulphate  in  the  crucible.  By 
the  heating  the  oxalate  is  converted  quantitatively  into  car- 
bonate, but  there  is  a  separation  of  some  carbon  which  should 
be  removed  for  the  most  accurate  work.  This  may  be  accom- 
plished by  heating  the  contents  of  the  crucible  to  a  much  higher 
temperature  with  free  access  of  air,  or  more  readily  by  adding  a 
few  cubic  centimeters  of  water,  evaporating  the  solution  to  dry- 
ness  on  the  water  bath,  and  then  very  carefully  heating  the 
crucible  over  a  free  flame.  In  about  ten  minutes  the  carbon 
will  all  disappear  without  the  carbonate  being  melted.  The 
crucible  is  then  allowed  to  cool,  its  contents  dissolved  in  hot 

*  Z.  anal.  Chem.,  42,  352,  512  (1903);  45.  272  (1906). 


598  VOLUMETRIC  ANALYSIS. 

water,  the  crucible  and  cover  thoroughly  washed,  and  the  cold 
solution  titrated  with  tenth-normal  hydrochloric  acid,  using 
methyl  orange  as  indicator. 

1000  c.c.  tenth-normal  hydrochloric  acid  =  6.700  gms.  Na2C204. 

Remark. — Sodium  oxalate  crystallizes  without  water  of  crys- 
tallization, is  not  hygroscopic,  and  is  especially  suited  for  the 
standardization  of  permanganate  solutions.  Sorensen,  in  fact, 
has  strongly  recommended  this  substance  as  a  standard  for 
acidimetry,  although  it  has  no  advantage  over  the  standardization 
by  means  of  sodium  carbonate.  The  titration  of  the  carbonate 
may  be  carried  out  with  methyl  orange  as  an  indicator,  but 
Sorensen  recommends  phenolphthalein  as  somewhat  more  reliable. 


2.  Against  Oxalic  Acid. 

Tenth-normal  oxalic  acid  solution  is  excellent  for  the  standard- 
ization of  a  permanganate  solution.  By  means  of  a  pipette  25  c.c. 
are  measured  into  a  beaker,  10  c.c.  of  dilute  sulphuric  acid  (1:4) 
are  added,  the  solution  is  diluted  with  water  at  about  70°  C.  to 
a  volume  of  200  c.c.,  and  the  permanganate  is  run  into  it,  with 
constant  stirring,  from  a  glass-stoppered  burette.  At  first  the  solu- 
tion is  colored  red  for  several  seconds,  then  it  becomes  colorless 
but  after  the  reaction  is  once  started  the  permanganate  is  rapidly 
decolorized  until  an  excess  is  present.  The  permanent  pink  color 
is  imported  to  the  solution  by  the  permanganate  as  soon  as  all 
the  oxalic  acid  is  oxidized ;  this  is  taken  as  the  end-point. 

The  oxidation  is  expressed  by  the  following  equation: 

2KMn04-f  5H2C2O4+  3H2S04  =  K2SO4+  2MnSO4+  8H,O+  10CO2. 
Since  for  the  oxidation  of  1  gm.  molecule  of  oxalic  acid, 

COOH 

+  0=2C02+H20, 
COOH 

N 
1  gm.-atom  of  oxygen  is  necessary,  and  1  liter  —  oxalic  acid  con- 

N 
tains  -£-$  gm.-molecule  of  the  acid.it  is  evident  that  1000  c.c.  of  — 


OXIDATION  METHODS.  599 

oxalic  acid  are  equivalent  to  -£$  gm.-atom  of  oxygen  =0.8  gm.  and 
1  c.c.  of  the  solution  =  0.0008  gm.  O. 

If  for  the  oxidation  of  25  c.c.  —  -  oxalic  acid  24.3  c.c.  of  per- 
manganate solution  were  required,  these  24.5  cc  correspond  to 
25X0.0008  =  0.0200  gm.  oxygen  or  1  c.c.  KMnO4  =  T  =  0.0008230 


gm.  O. 

Instead  of  expressing  the  concentration  of  the  permanganate 
solution  in  terms  of  oxygen,  it  has  been  the  custom  to  express 
it  in  terms  of  iron.  The  following  consideration  will  show  how 
the  calculation  may  be  made. 

From  the  oxidation  equation 


it  follows  that  1  gm.-atom  of  oxygen  is  necessary  for  the  oxida- 
tion of  2  gm.  -atoms  of  iron;   consequently  i  gm.-atom  of  oxygen 

N 
(=  1H  =  10,000  c.c.  :JYJ  oxalic  acid)  corresponds  to  1  gm.-atom  iron, 

so  that  25  c.c.^  oxalic  acid  =  24.3  c.c.  permanganate  =  25x0.00559 

0  1398 
=  0.1398  gm.  iron;  or  1  c.c.  of  the  permanganate  solution  =   ' 

Z-i.O 

=  0.005751  gm.  Fe. 

Remark.  —  Against  the  use  of  oxalic  acid  solution  for  the  stand- 
ardization of  a  permanganate  solution  is  the  fact  that  the  con- 
centration of  the  aqueous  solution  is  not  permanent;  for  this 
reason,  E.  Riegler  *  proposed  the  addition  of  50  c.c.  of  concen- 
trated sulphuric  acid  to  each  liter  of  the  oxalic  acid,  by  which 
means  the  solution  can  be  kept  unchanged  for  a  much  longer 
length  of  time.  That  this  is  the  case  is  shown  by  the  following 
experiments:  A  solution  of  oxalic  acid  in  water  was  prepared, 
and  also  one  in  dilute  sulphuric  acid.  Both  solutions  were  titrated 
on  the  same  day  with  permanganate  solution  which  had  been 
standardized  against  electrolytic  iron.  At  the  end  of  eight  months 

*  Z.    anal.  Chem.,  1896,  p.  522. 


6oo  VOLUMETRIC  ANALYSIS. 

the  same  solutions  were  titrated  against  a   freshly-standardized 
permanganate  solution,  with  the  following  results: 


Aqueous  Oxalic  Acid. 

Oxalic  Acid  containing 
Sulphuric  Acid. 

Freshly-prepared  . 
After  8  months.  .  . 

1000  c.c.  =1000.6  c.c.  ~  sol. 
1000  c.c.  =  994.9  c.c.    "    " 

1000  c.c.  =1002.5  c.c.  ^  sol 
1000  c.c.  =1001.8    "     "     " 

At  the  end  of  eight  months,  therefore,  the  aqueous  solution 
had  depreciated  0.56  per  cent,  in  strength,  while  the  solution  con- 
taining the  sulphuric  acid  had  only  weakened  to  an  extent  of 
0.12  per  cent,  of  its  original  concentration. 

From  this  it  is  evident  that  a  solution  of  oxalic  acid  contain- 
ing sulphuric  acid  can  be  used  for  the  standardization  of  a  per- 
manganate solution,  provided  the  former  has  not  stood  more  than 
eight  months  since  it  was  prepared.  The  use  of  old  aqueous 
solutions  of  oxalic  acid  is  to  be  discouraged. 

3.  Against  Metallic  Iron. 

It  has  been  a  favorite  practice  to  standardize  permanganate 
solutions  against  iron  wire.  The  wire,  however,  is  never  abso- 
lutely pure,  and  there  is  a  chance  of  the  impurities  reducing 
permanganate  so  that  the  actual  iron  content  of  the  wire  does 
not  suffice  to  show  exactly  how  much  oxidizing  agent  it  will  need. 
Classen,*  therefore,  has  recommended  that  pure  iron  be  prepared 
by  the  electrolyis  of  ferrous  ammonium  oxalate.  This  iron  is 
dissolved  in  dilute  sulphuric  acid  out  of  contact  with  the  air  and 
the  solution  titrated  with  permanganate  (cf.  p.  93). 

The  standardization  of  a  potassium  permanganate  solution  can 
be  correctly  accomplished  by  means  of  iron  wire,  provided  the 
apparent  iron  content  of  the  wire  has  been  determined  by  a  com- 
parison of  the  values  obtained  in  a  titration  with  a  standardization 
by  either  electrolytic  iron  or  sodium  oxalate.  Every  time  a  new 
supply  of  iron  wire  is  purchased,  the  comparison  should  be  made. 

*  Mohr-Classen,  Lehrbuch  der  chem.  anal.  Titrirmethode. 


OXIDATION  METHODS. 


601 


Determination  of  the  Apparent  Iron  Value  of  Iron  Wire. — The 
wire  is  cleaned  as  described  on  p.  98,  and  a  weighed  portion  of 
about  0.2  gm.  is  introduced  into  a  flask  of  not  more  than  250  c.c. 
capacity  as  shown  i.i  Fi^-.  89.  The  air  is  displaced  by  the  intro- 
duction of  a  stream  of  carbon  dioxide,  which  has  passed  through 
a  bottle  containing  water  and  another  containing  copper  sulphate 
solution  (cf.  p.  93,  foot-note);  the  wire  is  then  dissolved  in  55  c.c. 


FIG.  89. 


FIG.  90. 


of  dilute  sulphuric  acid  (1  part  concentrated  acid  to  10  of  water). 
During  the  solution  of  the  wire,  the  flask  is  supported  somewhat 
as  shown  in  the  drawing,  and  is  closed  by  a  rubber  stopper  which 
carries  a  bulb  tube  connected  with  a  Bunsen  valve.*  The  con- 

*  A  Bunsen  valve  consists  of  a  short  piece  of  rubber  tubing  with  a  cut 
along  a  few  centimeters  of  one  side,  and  the  outer  end  of  the  tubing  is  closed 
by  a  glass  rod.  This  valve  prevents  the  entrance  of  air  from  without.  A 
flask  larger  than  250  c.c.  capacity  is  likely  to  be  so  thin  as  to  break  during 
the  cooling  of  the  iron  solution.  In  Fig.  SOa,  instead  of  using  a  glass  rod  at 
the  end  of  the  valve,  a  glass  tube  is  used  which  is  sealed  at  one  end,  and  has 
a  hole  on  one  side.  This  tube  serves  to  prevent  the  collapse  of  the  rubber 
tubing  at  the  place  where  the  slit  is  formed. 


602  VOLUMETRIC  ANALYSIS. 

tents  of  the  flask  are  heated  by  means  of  a  low  flame  until  the  wire 
has  all  dissolved,  after  which  the  solution  is  boiled  gently  for  a 
short  time.  It  is  then  allowed  to  cool,  the  stopper  is  removed,  and 
the  permanganate  added  until  a  color  is  obtained  which  is  per- 
manent for  thirty  seconds. 

Instead  of  using  a  Bunsen  valve,  the  Contat-Gockel  valve  may 
be  used  as  shown  in  Fig.  90.  The  funnel  contains  a  cold,  saturated 
solution  of  sodium  bicarbonate,  through  which  the  hydrogen  from 
the  flask  passes.  When  the  flame  is  removed  sodium  bicarbonate 
solution  is  drawn  into  the  flask,  and  this  causes  the  evolution  of 
carbon  dioxide,  which  prevents  the  entrance  of  more  of  the  solution. 

S.  Christie,  by  following  the  above  procedure,  found  the 
apparent  iron  content  of  a  wire  to  be  99.985  per  cent.,  and  Dr. 
Schudel  found  100.21  for  another  wire. 

It  must  be  mentioned,  however,  that  the  apparent  iron  value 
varies  considerably  with  the  way  in  which  the  solution  of  the  wire 
is  effected.  If  the  volume  of  the  liquid  is  large  (cf.  p.  96),  there 
is  more  chance  of  hydrocarbons  remaining  in  solution,  and  the 
same  is  true  if  the  solution  is  not  boiled  as  in  the  above  direction, 
but  merely  heated  upon  the  water  bath. 

Remarks  Concerning  the  Standardization  by  Means  of  Electro- 
lytic  Iron. — The  objection  has  been  raised  that  electrolytic  iron  is 
contaminated  with  hydrocarbons.  According  to  Avery  and  Benton 
Dales,*  the  iron  obtained  by  the  electrolysis  of  ferrous  ammo- 
nium oxalate  contains  from  0.2  to  0.4  per  cent,  carbon  on  an 
average;  according  to  Skrabal  f  considerably  more.  Verwer  and 
Groll,t  however,  assert  that  electrolytic  iron  contains  no  carbon 
provided  the  bath  still  contains  an  excess  of  iron  at  the  end  of  the 
electrolysis.  Christie  has  carried  out  extensive  experiments  in 
the  author's  laboratory  and  found  that  the  electrolytic  iron  pre- 
pared by  the  Classen  method  does  often  contain  carbon,  but  the 
amount  is  so  small  that  it  may  be  disregarded.  Christie,  further- 
more, standardized  a  solution  of  permanganate  by  four  different 
methods  and  obtained  the  following  values: 

*Ber.,  32,  64  (1899). 

fZ.  anal.  Chem.,  42,  395  (1903). 

$  Ber.,  32,  806  (1899).     See  also  H.  Verwer:  Chem.  Ztg.,  25,  792  (1901). 


STANDARDIZATION  OF  PERMANGANATE  SOLUTION. 


603 


Against. 

Value  1  c.c.  in  Terms  of  Oxygen. 

Electrolytic  iron  

0.0007972 

0.0007960 

Iodine         .        .        ... 

0.0007977 

0.0007982 

Oxalic  acid  

0.0007978 

0.0007967 

Sodium  oxalate  .    .  . 

0.0007970 

0.0007975 

4.  Against  Sodium  Thiosulphate. 
See  lodimetry. 

5.  Against  Hydrogen  Peroxide. 
See  Gasometric  Methods. 

Permanence  of  Potassium  Permanganate  Solutions. 

As  mentioned  on  p.  90,  a  permanganate  solution  will  keep 
indefinitely,  provided  it  is  kept  free  from  dust  and  reducing  vapors. 
In  order  to  test  the  permanence  of  such  a  solution,*  it  was  stand- 
ardized against  electrolytic  iron  and  after  eight  months  it  was 
again  tested,  f  It  had  lost  only  0.17  per  cent,  of  its  original  value 
and  could  be  used  for  all  ordinary'  analyses.  For  very  accurate 
work,  however,  it  is  advisable  to  standardize  the  solution  fre- 
quently. 

USES  OF  PERMANGANATE  SOLUTION. 
i.  Determination  of  Iron  (Margueritte  1846). 

0.005585  gm.  Fe 


X 


rU.UUo; 
i  c.c.  ^  KMnO4  corresponds  to  -<  0.007185  gm.  FeO 

(  0.007985  gm.  Fe2O3 
In  this  determination  the  iron  is  oxidized  from  the  ferrous 
to  the  ferric  condition: 

2KMnO4-f  10FeSO4+  SH2SO4 = K2SO4+  2MnSO4+  5Fe2(SO4)3+  8H2O 

The  solution  of  the  ferrous  salt  is  strongly  acidified  with  sul- 
phuric acid  (about  5  c.c.  of  concentrated  sulphuric  acid  should 
be  present  for  each  100  c.c.  of  the  solution),  diluted  with  boiled 
water  to  a  volume  of  400  to  500  c.c.,  and  titrated  in  the  cold  by 

*  The  solution  was  already  three  months  old. 

fin  June,  1899,  1  c.c.  of  the  KMnO<  solution  =0.0054853  gm.  Fe;  in 
March,  1900,  1  c.c  of  the  KMnO4  solution  =0.0054761  gm.  Fe.  See  also 
Morse,  Hopkins  and  Walker,  Am.  Chem.  Jour.,  18,  401 . 


6 04  VOLUMETRIC  A NA LYSIS. 

the  addition  of  potassium  permanganate  from  a  glass-stoppered 
burette  until  a  permanent  pink  color  is  obtained.  If  the  perman- 
ganate solution  is  tenth-normal,  the  number  of  cubic  centimeters 
used  multiplied  by  0.005585,  0.007185,  or  0.007985  will  give 
respectively  the  amounts  of  iron,  ferrous  or  ferric  oxide. 

This  determination  affords  very  accurate  results  and  is  un- 
questionably one  of  the  best  methods  for  determining  iron. 

Remark. — The  titration  of  iron  in  hydrochloric  acid  solution 
gives  high  results  unless  particular  precautions  are  taken.  If 
dilute  permanganate  solution  is  allowed  to  run  into  a  cold  dilute 
solution  of  ferrous  chloride  containing  hydrochloric  acid,  the 
former  is  decolorized  and  the  iron  is  oxidized,  but  there  is  a 
noticeable  evolution  of  chlorine.*  More  permanganate  is  used  up 
than  is  necessary  to  oxidize  the  ferrous  salt  to  the  ferric  condition. 

If,  however,  permanganate  is  run  into  cold,  dilute  hydrochloric 
acid,  in  the  absence  of  ferrous  salt,  there  is  no  evolution  of  chlorine. 
Furthermore,  tne  presence  of  a  ferric  salt  does  not  cause  evolution 
of  chlorine.  The  chlorine,  therefore,  is  not  a  result  of  the  direct 
action  of  the  permanganate  upon  the  hydrochloric  acid,  but  is 
due  to  the  intermediate  formation  of  a  peroxide. 

When  permanganate  is  run  into  a  dilute  hydrochloric  acid 
solution  containing  ferrous  chloride  and  considerable  manganous 
salt,  the  ferrous  iron  io  quantitatively  oxidized  to  ferric  iron 
and  there  is  no  evolution  of  chlorine.  This  was  shown  by  Kessler  f 
in  1863  and  by  Zimmermann  {  in  1881.  It  has  since  been  con- 
firmed by  many  other  chemist s.§ 

This  can  be  explained  as  follows:  Permanganate  reacts  with 
manganous  salt  and  forms,  as  Volhard  ||  found,  MnO2.  This 
MnC>2  oxidizes  the  ferrous  iron  to  ferric  iron. 

2FeO  +  MnO2  =  Fe2O3  +  MnO 

more  quickly  than  it  is  able  to  react  with  hydrochloric  acid. 

*  Lowenthal  and  Lenssen,  Z.  anal.  Chem.,  1863,  329. 
f  Pogg.  Ann.,  118,  779,  and  119,  225. 
%  Ber.,  14,  779,  and  Ann.  Chem.  Pharm.,  213,  302. 

§  For  example,  J.  A.  Friend,  J.  Chem.  Soc.,  95,  1228  (1909).     C.C.  Jones 
and  J.  H.  Jeffery.     The  Analyst,  34,  306  (1909). 
i  I  Ann.  Chem.  Pharm.,  198,  337, 


STANDARDIZATION   OF  PERMANGANATE  SOLUTION.         605 

Zimmermann  *  suspected  that,  in  the  presence  of  manganous 
salt:?,  ferrous  iron  is  converted  into  a  peroxide  which  immediately 
breaks  clown  into  ferric  iron  and  oxygen,  and  the  latter  acts  upon 
the  hydrochloric  acid.  This  has  more  recently  been  confirmed 
by  the  interesting  work  of  Manchot.f 

According  to  Manchot,  there  is  formed  in  all  oxidation 
processes  a  "  primary  oxide "  which  has  the  character  of  a 
peroxide.  These  primary  oxides  are  as  a  rule  unstable  compounds 
which  cannot  be  isolated  and  which  constantly  tend  to  give  up 
oxygen  and  pass  to  a  more  stable  lower  state  of  oxidation.  When 
an  acceptor  %  is  present,  it  will  take  up  the  oxygen  which  is  lost  by 
the  primary  oxide  as  it  passes  to  a  lower  and  more  stable  state  of 
oxidation;  in  the  absence  of  an  acceptor  this  oxygen  is  evolved  as 
gas. 

According  to  the  method  of  oxidation,  iron  tends  to  form 
different  primary  oxides.  Thus,  in  the  direct  oxidation  by  means 
of  oxygen,  the  primary  oxide  is  FeO2 ;  in  the  oxidation  by  means 
of  permanganate,  chromic  acid,  or  hydrogen  peroxide,  it  is  Fe205 ; 
whereas  FeOs  is  probably  formed  in  the  oxidation  by  means  of 
hypochlorous  acid. 

The  oxidation  of  ferrous  oxide  to  ferric  oxide,  therefore,  does 
not  take  place  directly,  as  has  been  usually  assumed,  but  the 
primary  oxide  FeO2  is  first  formed,  and  this  reacts  with  more  of 
the  ferrous  oxide,  which  therefore  plays  the  part  of  an  acceptor, 
to  form  ferric  oxide: 

2FeO  +  O2  =  2FeO2, 

2FeO  +  2FeO2  =  2Fe2O3. 

Potassium  permanganate  causes  the  formation  of  Fe2O5  as 
primary  oxide: 

2FeO  4-  Mn2O7  ->Fe2O5  +  2MnO2. 

*  Ber..  14.  779  and  Ann.  Chem.  Phann.,  213,  302. 

f  Ann.  Chem.  Phann.,  325,  105  (1902). 

J  An  acceptor  is  a  substance  which  is  not  oxidized  by  oxygen  alone,  but  can 
be  thus  oxidized  by  the  aid  of  some  other  substance  present  called  an  auto- 
oxydator.  A  substance  which  tends  to  be  peroxidized  may  play  the  part  of 
an  acceptor.  Cf.  Engler,  Ber.,  33,  1097  (1900). 


606  VOLUMETRIC  AKA LYSIS. 

The  MnO2  at  once  oxidizes  more  ferrous  oxide  to  ferric  oxide 
2FeO  +  Mn02  =  Fe203  +  MnO 

and  the  Fe205  converts  the  MnO  to  MnO2  again. 

If,  however,  the  concentration  of  the  manganous  salt  in  the 
solution  is  too  low,  hydrochloric  acid  begins  to  play  the  part  of  an 
acceptor,  so^that  a  part  of  the  Fe20s  is  used  up  in  the  oxidation  of 
hydrochloric  acid: 

Fe2O5  +  10HC1  =  2FeCl3  +  5H20  +  2C12 

The  action  of  the  manganese  sulphate  is  twofold.  On  the  one 
hand  it  regulates  the  velocity  of  the  reaction  between  ferrous 
oxide  and  permanganic  aicd,  for,  according  to  Volhard,  the 
HMn04  acts  upon  the  manganous  salt  with  the  formation  of 
manganese  peroxide,  which  then  reacts  with  the  ferrous  salt; 
on  the  other  hand  it  takes  up  the  oxygen  from  the  iron  peroxide 
and  carries  it  to  the  unoxidized  ferrous  salt.  In  both  cases  it  is 
essential  that  manganese  peroxide  does  not  react  with  hydro- 
chloric acid  very  rapidly,  and  it  is  necessary,  too,  that  the  amount 
of  manganous  salt  shall  greatly  exceed  the  amount  of  iron 
present. 

Zimmermann  suggested  a  similar  explanation,  but  it  seemed 
to  meet  with  but  little  approval,  so  that  the  hypothesis  of  Wagner  * 
was  quite  generally  adopted.  The  latter  claimed  that  the  excess 
of  permanganate  required  for  the  titration  of  ferrous  chloride  in 
the  absence  of  manganous  sulphate  was  due  to  the  intermediate 
formation  and  rapid  oxidation  of  a  ferrous-hydrochloric  acid, 
FeCl2-2HCl. 

Manchot's  explanation,  however,  seems  to  be  the  better  one. 

Although  it  is  possible,  then,  to  titrate  iron  in  hydrochloric 
acid  solutions  in  the  presence  of  manganous  sulphate,  the  method 
possesses  the  disadvantage  that  the  end-point  cannot  be  seen  so 
distinctly  as  when  no  chloride  is  present,  since  ferric  chloride 
forms  a  much  more  yellow  solution  than  does  ferric  sulphate. 

*  Zeitschr.  f.  physikal.  Chem.,  28,  33. 


STANDARDIZATION  OF  PERMANGANATE  SOLUTION.        607 

This   difficulty  can  be   overcome  by  the  addition  of  phosphoric 
acid,  as  suggested  by  C.  Reinhardt.* 

TITRATION    OF    FERROUS   SALTS    IN    HYDROCHLORIC    ACID    SOLUTION. 
METHOD    OF    ZIMMERMANN-REINHARDT. 

From  20  to  25  c.c.  of  the  manganese  sulphate  solution  f  pre- 
pared as  described  below  are  added  to  the  solution,  and  after 
diluting  with  boiled  water  to  a  volume  of  500  c.c.  it  is  titrated 
with  potassium  permanganate  which  is  added  so  slowly  that  the 
drops  can  be  counted.  Care  is  taken  toward  the  last  not  to  add 
a  drop  of  permanganate  until  the  color  of  the  preceding  one  has 
disappeared. 

The  manganous  sulphate  solution  is  prepared  as  follows:  67  gms. 
of  crystallized  manganous  sulphate  (MnSO4+4H2O)  are  dissolved 
in  500  to  600  c.c.  of  water,  138  c.c.  of  phosphoric  acid  (of  specific 
gravity  1.7)  and  130  c.c.  of  concentrated  sulphuric  acid  (sp.  gr. 
1.82)  are  added,  and  the  mixture  is  diluted  to  1  liter. 

If  the  iron  is  present  as  ferric  salt,  it  must  be  reduced  com- 
pletely to  the  ferrous  condition  before  it  can  be  titrated  with 
potassium  permanganate. 

THE  REDUCTION  OF  FERRIC  SALTS  TO  FERROUS  SALTS 

can  be  accomplished  in  a  number  of  different  ways. 

1.  By  Hydrogen  Sulphide. 

This  reduction  has  already  been  described  on  page  99. 

2.  By  Sulphur  Dioxide. 

The  solution  containing  the  ferric  salt  is  neutralized  with 
sodium  carbonate,  J  an  excess  of  sulphurous  acid  is  added, 
the  solution  boiled,  and  a  current  of  carbon  dioxide  is  passed 

*  Stahl  und  Eisen,  1884,  p.  709,  and  Chem.  Ztg.,  13,  323. 

f  It  is  well  to  add  one  cubic  centimeter  of  manganese  sulphate  for  each 
cubic  centimeter  of  HC1  (sp.  gr.  1.12)  present.  Cf.  J.  A.  Friend  or  Jones 
and  Jeffery,  loc.  cit. 

t  Ferric  salts  are  not  completely  reduced  by  sulphurous  acid  in  the  pres- 
ence of  considerable  hydrochloric  or  sulphuric  acid. 


608  VOLUMETRIC  ANALYSIS. 

through  it  until  the  excess  of  the  reagent  is  completely  removed.* 
The  reduced  solution  is  then  cooled  in  an  atmosphere  of  carbon 
dioxide  and  titrated. 

3.  By  Metals. 

The  acid  solution  of  the  ferric  salt,  contained  in  a  small  flask 
fitted  with  a  Bunsen  valve,  is  reduced  by  heating  on  the  water- 
bath  with  the  addition  of  small  pieces  of  chemically-pure  zinc  until 
the  solution  is  completely  colorless  and  a  drop  of  it,  removed  by 
means  of  a  piece  of  capillary  tubing,  will  no  longer  give  any  color 
with  potassium  sulphocyanate  solution.  After  cooling,  the  solu- 
tion is  poured  through  a  funnel  containing  a  platinum  cone  (no 
paper) ,  and  the  undissolved  zinc  remaining  in  the  funnel  is  washed 
several  times  with  boiled  water,  f 

Remark. — Since  zinc  often  contains  iron,  a  blank  experiment 
must  be  made  by  dissolving  3  to  5  gms.  in  the  same  way  and  titra- 
ting the  solution  with  permanganate.  If  iron  is  present,  as  shown 
by  the  fact  that  a  measurable  amount  of  potassium  permanga- 
nate is  decolorized,  the  reduction  of  the  ferric  salt  must  be  effected 
by  means  of  a  weighed  amount  of  zinc  and  a  correction  made  for 
the  iron.  It  is  self-evident  that  in  this  case  the  titration  must 
not  take  place  until  all  of  the  zinc  has  dissolved.  Instead  of  zinc, 
cadmium  and  aluminium  are  frequently  used. 

Remark. — Against  this  method  objections  can  be  raised.  In 
che  first  place,  the  fact  that  a  foreign  metal  is  introduced  into 
the  solution  is  in  many  cases  unfortunate.  Furthermore,  by 
means  of  zinc,  titanic  acid  is  reduced  to  Ti2O3,  only  to  be  oxidized 
again  by  the  permanganate  solution,  so  that  more  permanganate 
solution  will  then  be  required  than  corresponds  to  the  amount  of 
iron  present.  By  means  of  H2S  or  SO2,  titanic  acid  is  not  reduced 
and  there  is  no  foreign  metal  introduced  into  the  solution.  Con- 

*  It  is  not  advisable  to  depend  upon  the  sense  of  smell.  The  escaping 
gas  is  tested  by  passing  it  through  dilute  sulphuric  acid  containing  a  few 

N 
drops  of  —  KMnO4  solution.     If  the  latter  is  not  decolorized  at  the  end  of 

two  or  three  minutes,  the  excess  of  sulphurous  acid  has  been  removed. 

t  The  reduction  by  means  of  zinc  may  be  satisfactorily  accomplished  with 
a  «'  Jones  reductor."  Cf .  Fig.  91,  p.  637. 


REDUCTION  OF  FEXR1C  SALTS   TO  FERROUS   SALTS.         609 

sequently,  for  accurate  mineral  analyses,  it  is  necessary  to  use 
one  of  these  methods,  and  in  fact  the  reduction  by  means  of 
hydrogen  sulphide  is  to  be  preferred.  By  means  of  the  latter  the 
ferric  salt  is  completely  reduced,  independent  of  how  little  or  how 
much  free  acid  is  present  in  the  solution;  again,  any  metals  of 
the  hydrogen  sulphide  group  are  precipitated  at  the  same  time; 
while  finally  it  is  easy  to  recognize  the  fact  that  the  excess 
of  the  gas  has  been  removed  by  the  use  of  the  sensitive  lead 
acetate  paper  test. 

4.  By  Stannous  Chloride. 

This  method  proposed  by  Zimmermann  and  Reinhardt  *  is 
especially  suited  for  metallurgical  purposes,  because  it  can  be 
accomplished  most  rapidly. 

Principle. — The  method  depends  upon  the  fact  that  ferric 
chloride  in  hot  solution  is  easily  reduced  by  stannous  chloride: 

SnCl2  +  2FeCl3  =  SnCl4  +  2FeCl2. 

The  complete  decolorization  of  the  solution  shows  the  end- 
point  of  reduction.  The  excess  of  stannous  chloride  is  afterwards 
oxidized  by  means  of  mercuric  chloride: 

SnCl2  +  2HgCl2  =  SnCl4  +  Hg2Cl2. 

After  this  treatment,  which  consumes  but  a  few  minutes,  some 
manganese  sulphate  solution  is  added  and  the  solution  imme- 
diately titrated  with  potassium  permanganate,  which  is  added 
slowly. 

Requirements. 

(a)  Stannous  chloride  solution.  50  gms.  of  stannous  chloride 
are  dissolved  in  100  c.c.  of  concentrated  hydrochloric  acid  and 
diluted  with  water  to  a  volume  of  one  liter. 

(6)   Hydrochloric  acid,  sp.  gr.  1.12. 

*  Loc.  cit. 


6io  VOLUMETRIC  ANALYSIS. 

(c)  Mercuric  chloride  solution.     A  saturated  solution  of  the 
pure  commercial  salt  in  water  is  used. 

(d)  Manganese  sulphate  solution.     See  p.  607. 

Procedure. — The  ferric  salt  is  dissolved  in  20  to  25  c.c.  of  the 
hydrochloric  acid  (6)  heated  to  boiling,  the  flame  removed,  and  the 
stannous  chloride  solution  (a)  is  added  drop  by  drop  until  the 
iron  solution  just  becomes  colorless.  The  solution  is  cooled  to  at 
least  the  room  temperature  and  10  c.c.  of  mercuric  chloride  (c)  are 
quickly  added,  whereby  a  slight  silky  precipitate  of  Hg2Cl2*  is 
formed.  After  ten  minutes  the  solution  is  diluted  to  about  500 
c.c.,  20  to  25  c.c.  of  the  manganese  sulphate  solution  (d)  are  added, 
and  the  mixture  is  titrated  (very  slowly)  with  potassium  per- 
manganate until  a  pink  color  permanent  for  one  minute  is 
obtained. 

Example:  Determination  of  Iron  in  Hematite,  Fe2O3. — About 
0.25  to  0.3  gm.  of  the  finely-powdered  mineral  is  weighed  out  into 
a  beaker,  3  c.c.  of  the  stannous  chloride  solution  (a)  f  are  added 
and  25  c.c.  of  the  acid  (6).  The  beaker  is  covered  with  a  watch- 
glass  and  its  contents  heated  nearly  to  boiling  until  all  of  the  iron 
oxide  has  dissolved  and  a  white  sandy  residue  is  obtained.  This 
operation  seldom  requires  more  than  ten  minutes.  The  slightly 
yellow  colored  solution  thus  obtained  is  carefully  treated  with 
stannous  chloride  drop  by  drop  until  it  becomes  colorless  and  the 
reduced  solution  is  analyzed  as  above. 


*  If  the  precipitate  produced  by  mercuric  chloride  is  at  all  grayish  in 
color,  the  portion  must  be  thrown  away;  too  large  an  excess  of  stannous 
chloride  was  used.  Moreover,  the  end  point  with  permanganate  is  difficult 
to  see  if  the  solution  contains  much  precipitate. 

t  The  stannous  chloride  greatly  facilitates  the  solution  of  the  hematite. 
If  too  much  is  used,  strong  permanganate  should  be  added  drop  by  drop 
until  the  yellow  color  of  ferric  chloride  appears,  and  the  solution  then  care- 
fully decolorized  again. 


DETERMINATION  OF  METALLIC  IRON.  611 

Determination  of  Metallic  Iron  in  the  Presence  of  Iron  Oxide. 

This  method  is  useful  for  testing  ferrum  reductum  which  is  ob- 
tained by  the  reduction  of  Fe203  in  a  stream  of  hydrogen.  Usually 
the  reduction  is  not  complete  and  the  preparation  contains,  besides 
the  metallic  iron,  some  oxide,  usually  assumed  to  be  Fe3O4.  The 
value  of  the  preparation  depends  upon  the  free  iron  content. 


(a)  Method  of  Wilner  *-Merckj 

Principle.  —  The  method  is  based  upon  the  fact  that  a  neutral 
solution  of  mercuric  chloride  dissolves  iron  according  to  the  equa- 
tion 

Hg+FeCl2 


while  the  Fe304  is  not  attacked.     The  solution  of  ferrous  chloride 
is  titrated  with  permanganate  solution. 

Procedure.  —  About  0.5  g.  of  ferrum  reductum,  in  the  form  of  a 
fine  powder,J  is  placed  in  a  100  c.c.  graduated  flask,  from  which 
the  air  is  replaced  by  CO2,  3  gms.  of  solid  mercuric  chloride  are 
added  and  50  c.c.  of  water.  The  contents  of  the  flask  are  then 
heated  to  boiling,  by  means  of  a  small  flame,  and  the  liquid  boiled 
for  a  minute.  The  flask  is  then  filled  up  to  the  mark  with  boiled 
water.  After  cooling  to  15°  the  solution  is  again  carefully  brought 
to  the  mark,  well  shaken,  and  then  allowed  to  stand  in  the  stop- 
pered flask  until  the  precipitate  has  settled.  The  liquid  is  then 
poured  through  a  dry  filter  and  the  filtrate  caught  in  a  flask  filled 
with  carbon  dioxide.  Of  this  filtrate,  20  c.c.  are  taken,  acidified 
with  20  c.c.  of  sulphuric  acid  (1:4),  treated  with  10  c.c.  of  man- 
ganese sulphate  solution,  §  diluted  to  200  c.c.,  and  treated  with 
tenth-normal  permanganate  solution. 

*  Farm.  Tidskrift,  1880,  225. 

t  Z.  anal.  Chem.,  41,  710  (1902). 

£  A  coarse  powder  is  not  decomposed  quantitatively. 

§  See  page  607. 


6i2  1/OLUMETRIC  A NA LYSIS. 

The  Ferric  Chloride  Method* 

Principle. — A  neutral  solution  of  ferric  chloride  dissolves 
metallic  iron  with  the  formation  of  ferrous  chloride: 

Fe  +  2FeCl3  =  3FeCl2 

and  the  ferrous  chloride  formed  is  titrated  with  permanganate 
solution.  One-third  of  the  iron  thus  found  corresponds  to  the 
weight  of  metallic  iron  present  in  the  sample. 

Procedure. — About  0.5  g.  of  ferrum  reductum  are  placed  in  a 
100  c.c.  graduated  flask,  which  has  been  filled  with  CO2,  and  50  c.c. 
of  ferric  chloride  are  added  (1  gm.  anhydrous  ferric  chloride  in 
20  c.c.  water)  .f  The  flask  is  stoppered  and  its  contents 
frequently  shaken  during  the  next  fifteen  or  twenty  minutes. 
The  solution  is  then  brought  to  the  mark  with  cold,  boiled  water, 
mixed,  the  flask  stoppered,  and  allowed  to  stand  over  night.  Of 
the  clear  supernatant  liquid,  20  c.c.  are  removed  by  a  pipette  and 
titrated,  as  in  the  previous  method,  with  tenth-normal  perman- 
ganate solution.  J 


2.  Determination  of  Manganese.     Method  of  Volhard.§ 
1000  c.c,  N.  KMnO4t  =  ^  =  16.48  gms.  Mn. 

If  an  almost  boiling,  slightly  acid  solution  of  manganese  sul- 
phate is  slowly  treated  with  a  solution  of  potassium  permanga- 
nate, each  drop  will  cause  the  formation  of  manganous  acid 

*  A.  Christensen,  Z.  anal.  Chem.,  44,  535  (1905). 

t  The  ferric  chloride  must  give  a  clear  solution  in  cold  water.  As  it 
often  contains  a  little  ferrous  chloride,  a  blank  test  must  be  made  and  a 
correction,  corresponding  to  the  amount  of  iron  found,  applied  to  the  analysis 
proper. 

J  For  other  methods  of  analyzing  ferrum  reductum,  see  E.  Schmidt, 
Chem.  Ztg  ,  21,  700  (1897).  A.  Marquardt,  Ibid.,  45,  743  (1901).  L.  Wolfram, 
Inaug.  Dissert.,  Erlanger,  1896.  F.  Forster  and  V.  Herold,  z.  Elektrochem. 
1910,  461. 

§  Ann.  d.  Chem.  und  Pharm.,  198,  318. 


DETERMINATION  OF  MANGANESE.  613 

,  which  is  formed  under  certain  conditions,  as  described 
below,  according  to  the  following  scheme: 


K2O-Mn2O7+3MnO  =K2O  +  5MnO2 


2KMnO4 

According  to  this  equation,  therefore,  2KMnO4  will  oxidize 
3  "gm. -atoms  of  manganese,  and  as  1000  c.c.  of  N.  KMnO4  con- 
tain \  gm.-mol.  KMnO4,  evidently  this  amount  of  permanganate 

corresponds  to  —^-  =  16.48  gms.  Mn. 

A.  Guyard,  who  first  determined  manganese  by  this  method, 
assumed  that  the  oxidation  took  place  according  to  the  following 
equation : 

2KMn04+  3MnS04+  7H2O  =  2KHSO4+  H2SO4+  5H2MnO3. 

In  reality,  however,  the  reaction  does  not  take  place  in  this 
way,  but  instead  of  pure  manganous  acid  being  precipitated, 
different  acid  manganites  of  varying  composition  are  formed;  e.g., 

/OH 
Mn=0 

4KMn04+  HMnSO4+  14H2O  =  4KHSO4+7H2SO4+5          Q>Mn. 

Mn^O 

\OH 

Volhard  has  shown  that  if  calcium,  barium,  or,  better  still,  zinc 
salts,  are  present,  manganites  of  these  metals  are  precipitated 


*  Strictly  speaking,  the  normality  of  the  permanganate  is  different  when 
used  to  oxidize  manganese  in  slightly  acid  or  neutral  solution.  In  this  case 
the  manganese  of  the  permanganate  is  reduced  to  the  quadrivalent  form  instead 
of  to  bivalent  manganese,  so  that  a  normal  solution  would  now  contain 

KMnO4  f  KMnO4 

-  gms.  instead  of — -  gms.    Inasmuch  as  it  is  customary  to  stand- 

3  o 

ardize  permanganate  as  outlined  on  pages  555-561,  we  shall  understand  by 
normal  KMnO4,  a  solution  which  is  normal  with  respect  to  these  standards. 
— [Translator.] 


6i4  VOLUMETRIC  ANALYSIS. 

The  precipitate,   although  varying  in  composition,   contains  all 
of  the  manganese  in  the  quadrivalent  form;  e.g., 


4KMn04+  5ZnS04+  6MnSO4+  14H20  = 

=  4KHS04+  7H2S04+  5          Q>  Zn. 

Mn^O 
\OH 

In  case  iron  is  present,  the  reaction  does  not  take  place  quan- 
titatively in  the  direction  from  left  to  right,  so  that  a  different 
procedure  is  then  necessary. 

(A)  PROCEDURE    WHEN    IRON  IS   ABSENT. 

N 
Requirements.  —  1.  A  —  potassium  permanganate  solution. 

2.  A  manganese  sulphate  solution,  obtained  by  dissolving  4.530 
.  of  anhydrous  manganous  sulphate  in  one  liter  of  solution: 


N 
1  c.c.  of  this  solution=l  c.c.  of  —  KMnO4.* 

3.  A  zinc  sulphate  solution  obtained  by  dissolving  200  gms. 
zinc  sulphate  in  one  liter  of  water. 

4.  Zinc  oxide  suspended  in  water,  obtained  by  precipitating 
pure  zinc  sulphate  by  means  of  caustic  potash  solution  in  such  a 
way  that  the  solution  does  not  react  alkaline.     The  residue  is 
washed  several  times  with  hot  water,  then  transferred  to  a  tightly- 
stoppered  bottle,  and  kept  suspended  in  water. 

Standardization  of  the  Permanganate  Solution. 

20  c.c.  of  the  manganese  sulphate  solution  are  placed  in  an 
Erlenmeyer  flask,  40  c.c.  of  zinc  sulphate  solution  and  2  or  3  drops 
of  nitric  acid  f  are  added,  after  which  the  mixture  is  diluted  to 

*  Strictly  speaking,  this  solution  is  5%  normal.  By  definition,  a  ^  solu- 
of  manganese  sulphate  contains  4  =  7.550  gms.  MnSO4  in  one  liter.  Such 

a  solution,  however,  would  not  be  equivalent  to  a  KMnO4  solution  which  is 
tenth  normal  in  acid  solution.     Cf.  foot-note  to  page  527  (Translator). 

f  The  addition  of  the  nitric  acid  causes  the  precipitate  to  settle  much 
more  quickly. 


DETERMINATION  OF  MANGANESE  ///  STEEL  615 

200  c.c.,  heated  to  boiling,  and  treated  with  potassium  permanga- 
nate solution,  added  with  constant  shaking,  until  the  supernatant 
liquid  remains  a  permanent  pink. 

Titration  of  Manganese. 

If  a  neutral  solution  of  manganese  sulphate  is  to  be  analyzed, 
the  same  procedure  is  used  as  in  the  above  standardization.  If 
the  solution  contains  manganous  chloride,  it  should  be  freed  from 
hydrochloric  acid  by  evaporation  with  an  excess  of  sulphuric 
acid.  The  acid  solution  thus  obtained  is  neutralized  with  the 
zinc  oxide  until  a  little  of  the  latter  remains  suspended  in  the 
liquid.  From  this  point  the  procedure  is  the  same  as  before. 

(B)   PROCEDURE   WHEN  IRON  IS  PRESENT. 

If  a  hydrochloric  acid  solution  is  to  be  analyzed  containing  all 
of  the  iron  in  the  ferric  form,  it  is  evaporated  to  dryness  with  the 
addition  of  sulphuric  acid,  the  dry  mass  is  moistened  with  nitric 
acid  and  warmed  until  complete  solution  is  effected.  The  greater 
part  of  the  acid  is  neutralized  with  sodium  hydroxide  solution, 
the  solution  placed  in  a  measuring-flask,  and  an  excess  of  the  zinc 
oxide  is  added  whereby  all  of  the  iron  is  precipitated  as  hydroxide. 
The  liquid  is  diluted  up  to  the  mark  with  water,  filtered  through  a 
dry  filter,  and  an  aliquot  part  of  the  filtrate  is  titrated  as  before 
with  potassium  permanganate  solution.* 

Determination  of  Manganese  in  Steel. 

(a)    Volhard  Method. 

The  solution  is  prepared  for  titration  by  dissolving  the  steel 
borings  f  in  nitric  acid  (sp.  gr.  1.2),  evaporating  the  solution,  after 
the  addition  of  20  c.c.  of  50  per  cent,  sulphuric  acid,  allowing  the 
residue  to  cool,  and  then  adding  150  c.c.  of  cold  water.  The 
water  is  boiled  until  the  ferric  sulphate  is  all  dissolved,  the  solu- 
tion filtered,  the  filtrate  nearly  neutralized  with  sodium  carbonate 
and  the  zinc  oxide  added  exactly  as  described  above. 

*  The  first  few  cubic  centimeters  of  the  filtrate  should  be  discarded,  for 
the  dry  filter  absorbs  some  of  the  dissolved  substance. 

t  One  gm.  of  steel  is  used  when  the  manganese  content  is  about  1  per 
cent,  less  when  the  content  is  higher.  The  process  is  not  well  suited  for  low 
manganese  steels. 


616  VOLUMETRIC  ANALYSIS. 


(6)    The  Bismuthate  Method. 

This  method  originated  with  Schneider, f  who  used  bismuth 
tetroxide  as  the  oxidizing  agent,  but  as  the  oxide  is  difficult  to 
prepare  free  from  chlorides  and  traces  of  chloride  interfere  with  the 
end  point  of  the  titration,  it  was  abandoned  by  Reddrop  and 
Ramage,t  who  proposed  the  use  of  sodium  bismuthate,  NaBiOs. 
The  product  sold  under  this  name  is  of  more  or  less  indefinite 
composition.  It  may  be  prepared  by  heating  20  parts  of  caustic 
soda  nearly  to  redness  in  an  iron  or  nickel  crucible,  adding  in 
small  quantities  from  time  to  time  ten  parts  of  dry  basic  bismuth 
nitrate,  followed  by  two  parts  of  sodium  peroxide,  pouring  the 
yellow  fused  mass  on  an  iron  plate  to  cool.  When  cold,  the 
fusion  is  extracted  with  water,  collected  on  an  asbestos  filter, 
washed  five  times  by  decantation  with  water,  and  dried  in  the 
hot  closet  at  110°.  After  grinding  and  sifting  the  product  is  ready 
for  use. 

The  process  is  based  on  the  fact  that  a  manganous  salt  in 
the  presence  of  an  excess  of  nitric  acid  is  oxidized  to  perman- 
ganic acid  by  sodium  bismuthate.  The  permanganic  acid 
formed  is  very  stable  in  nitric  acid  of  1.135  sp.  gr.  when  the 
solution  is  cold,  but  in  hot  solutions  the  excess  of  bismuthate 
is  rapidly  decomposed  and  then  the  nitric  acid  reacts  with  the 
permanganic  acid;  as  soon  as  a  small  amount  of  manganous 
salt  is  formed  the  remainder  of  the  permanganic  acid  is  decom- 
posed, manganous  nitrate  dissolves  and  manganese  dioxide 
precipitates. 

In  the  cold,  however,  the  excess  of  the  bismuth  salt  may 
be  filtered  off  and  to  the  clear  filtrate  an  excess  of  ferrous  sul- 
phate added;  the  excess  of  the  latter  is  determined  by  titrating 
with  permanganate.  The  end-reactions  are  very  sharp  and  the 
method  is  extremely  accurate. 

1000  c.c.  N.  KMn04  =  10.99  gms.  Mn. 

*  A.  A.  Blair,  J.  Am.  Chem.  Soc.  26,  793. 

fDing.  poly.  J.  269,  224. 

j  Trans.  Chem.  Soc.  1895,  268. 


DETERMINATION  OF  MANGANESE  IN  STEEL  617 

Procedure  for  Steels. — Dissolve  1  gm.  of  drillings  in  50  c.c. 
of  nitric  acid  (sp.  gr.  1.135)  in  an  Erlenmeyer  flask  of  200  c.c. 
capacity,  cool,  and  add  about  0.5  gm.  of  bismuthate.  The 
bismuthate  may  be  measured  in  a  small  spoon  and  experience 
will  soon  enable  the  operator  to  judge  of  the  amount  with  sufficient 
accuracy.  Heat  for  a  few  minutes,  or  until  the  pink  color  has 
disappeared,  with  or  without  the  precipitation  of  manganese 
dioxide.  If  the  solution  now  shows  precipitated  manganese 
dioxide,  add  crystals  of  ferrous  sulphate  free  from  manganese, 
sulphurous  acid  or  sodium  thiosulphate  until  it  becomes  clear. 
Heat  for  two  minutes  to  remove  oxides  of  nitrogen  and  cool  to 
about  15°.  Now  add  2  or  3  gms.  more  of  sodium  bismuthate 
and  agitate  the  contents  of  the  flask  for  several  minutes.  Dilute 
with  50  c.c.  of  3  per  cent,  nitric  acid  and  filter  through  asbestos 
into  a  300-c.c.  Erlenmeyer  flask.  Wash  the  asbestos  with  50  to 
100  c.c.  of  cold  3  per  cent,  nitric  acid.*  Run  into  this  solution 
50  c.c.  of  standardized  ferrous  sulphate  solution  and  titrate 
back  to  pink  color  with  potassium  permanganate. 

The  value  of  the  ferrous  sulphate  solution  in  terms  of  potas^ 
shim  permanganate  must  be  determined  in  the  following 
manner: 

Measure  into  a  250-c.c.  Erlenmeyer  flask  50  c.c.  of  cold  nitric 
acid  (sp.  gr.  1.13),  add  about  0.5  gm.  of  sodium  bismuthate, 
agitate,  and  filter  through  asbestos.  Filter  with  50  c.c.  of  cold 
3  per  cent,  nitric  acid,  introduce  50  c.c.  of  ferrous  sulphate 
solution  and  titrate  with  permanganate  solution  f  to  pink 
color. 

Having  determined  the  value  of  the  permanganate  solution 
in  terms  of  ferrous  sulphate,  the  manganese  in  the  sample  is 
represented  by  the  difference  between  the  amounts  of  perman- 
ganate solution  actually  used  in  the  determination  and  in  the 
titration  of  a  volume  of  ferrous  sulphate  equivalent  to  that  used 
in  the  determination. 

Pig  Iron. — Dissolve  1  gm.  in  25  c.c.  of  nitric  acid  (sp.  gr. 
1.135)  in  a  small  beaker  and  as  soon  as  the  action  has  ceased 

*  30  c.c.  HNO3,  sp.  gr.  1.42,  in  one  liter  of  water. 
f  1  gm.  KMnO4  to  the  liter. 


618  VOLUMETRIC  ANALYSIS. 

filter  on  a  7-cm.  filter  into  a  200-e.c.  Erlenmeyer  flask,  wash 
with  30  c.c.  of  the  same  acid  and  proceed  as  in  the  case  of  steels. 

In  the  analysis  of  white  irons  it  may  be  necessary  to  treat 
the  solution  several  times  with  bismuthate  to  destroy  the 
combined  carbon.  The  solution,  when  cold,  should  be  nearly 
colorless;  if  not,  another  treatment  with  bismuthate  is  neces- 
sary. 

Iron  Ores  Containing  Less  than  Two  Per  Cent,  of  Manganese. — 
Treat  1  gm.  in  a  platinum  dish  or  crucible  with  4  c.c.  of  strong 
sulphuric  acid,  10  c.c.  of  water  and  10  to  20  c.c.  of  hydrofluoric 
acid.  Evaporate  until  the  sulphuric  acid  fumes  freely.  Cool 
and  dissolve  in  25  c.c.  of  1.135  nitric  acid.  If  no  appreciable 
residue  remains,  transfer  to  a  200-c.c.  Erlenmeyer  flask,  using 
25  c.c.  of  1.135  nitric  acid  to  rinse  the  dish  or  crucible  and  proceed 
as  usual.  If  there  is  an  appreciable  residue,  filter  on  a  small 
filter  into  a  beaker,  wash  with  water,  burn  the  filter  and  residue 
in  a  crucible  and  fuse  with  a  small  amount  of  potassium  bisul- 
phate.  Dissolve  in  water  with  the  addition  of  a  little  nitric  acid, 
add  to  the  main  filtrate,  evaporate  nearly  to  dryness,  take  up 
in  1.135  nitric  acid  and  transfer  to  the  flask  as  before. 

Manganese  Ores  and  Iron  Ores  High  in  Manganese. — Treat 
1  gm.  as  in  the  case  of  iron  ores,  using  a  little  sulphurous  acid, 
if  necessary.  Transfer  the  solution  to  a  500-c.c.  flask,  dilute  to 
the  mark,  mix  thoroughly  and  measure  into  a  flask  from  a  care- 
fully calibrated  pipette  such  a  volume  of  the  solution  as  will  give 
from  1  to  2  per  cent,  of  manganese  and  enough  strong  nitric 
acid  (sp.  gr.  1.4)  to  yield  a  mixture  of  1.135  acid  in  a  volume 
of  50  to  60  c.c. 

Ferro-manganese. — Treat  1  gm.  exactly  like  steel.  Dilute 
to  500  or  1000  c.c.  and  proceed  as  in  manganese  ores. 

Ferro-silicon. — Treat  1  gm.  with  sulphuric  and  hydrofluoric 
acids  and  proceed  as  with  iron  ores. 

Special  Steels. — Steels  containing  chromium  offer  no  special 
difficulties,  except  that  it.  must  be  noted  that  while  in  hot  solu- 
tions the  chromium  is  oxidized  to  chromic  acid,  which  is  reduced 
by  the  addition  of  sulphurous  acid,  the  oxidation  proceeds  so 
slowly  in  cold  solutions  that  if  there  is  no  delay  in  the  filtration 
and  titration  the  results  are  not  affected.  Steels  containing 


DETERMINATION  GF  MANGANESE  IN  STEEL.  619 

tungsten  are  sometimes  troublesome  on  account  of  the  necessity 
for  getting  rid  of  the  tungstic  acid.  Those  that  decompose 
readily  in  nitric  acid  may  be  filtered  and  the  filtrate  treated  like 
pig  iron,  but  when  it  is  necessary  to  use  hydrochloric  acid  it  is 
best  to  treat  with  aqua  regia,  evaporate  to  dryness,  redissolve  in 
hydrochloric  acid,  add  a  few  drops  of  nitric  acid,  dilute,  boil, 
and  filter.  Get  rid  of  every  trace  of  hydrochloric  acid  by  repeated 
evaporations  with  nitric  acid  and  proceed  as  with  an  ordinary 
steel. 

(c)   Williams  Method.* 


1000  c.c.        KMn04  =       =  =  2.747  gms.  Mn. 

Principle.  —  If  a  nitric  acid  solution  of  a  manganous  salt  is 
heated  with  potassium  chlorate,  all  of  the  manganese  is  pre- 
cipitated as  the  dioxide: 

Mn(N03)  2  +  2KC103  +  H20  =  MnO2  •  H20  +  2KN03  +  2C102. 

The  Mn02  is  dissolved  in  a  measured  volume  of  acid  ferrous 
sulphate,  and  the  excess  is  titrated  with  tenth-normal  permanga- 
nate. 

Procedure.  —  Of  ferro-manganese  from  0.3  to  0.5  gin.,  of 
spiegel  iron  about  1  gm.,  and  of  ordinary  steel  from  2  to  3  gms.,  are 
placed  in  a  600  c.c.  Erlenmeyer  flask  and  dissolved  in  60  c.c.  of 
nitric  acid,  sp.  gr.  1.2.  To  prevent  loss  by  spattering,  a  small 
funnel  is  placed  in  the  neck  of  the  flask.  After  evaporating  the 
solution  to  a  volume  of  about  15  c.c.,  50  c.c.  of  concentrated  nitric 
acid,  sp.  gr.  1.42,  and  3  gms.  of  solid  potassium  chlorate  are  added, 
and  the  solution  is  boiled  for  fifteen  minutes.  It  is  then  removed 
from  the  source  of  heat  and  the  treatment  with  50  c.c.  concentrated 
nitric  acid  and  3  gms.  of  potassium  chlorate  is  repeated,  after 
which  the  solution  is  boiled  for  fifteen  minutes  longer.  The 
solution  is  cooled  quickly  by  placing  the  flask  in  cold  water,  and 
the  precipitated  manganese  dioxide  is  filtered  on  asbestos,f 

*  Trans.  Inst.  Min.  Eng.,  10,  100.  See  also  W.  Hampe,  Chem.  Ztg.,  7, 
73  (1883),  9,  1478  (1885),  and  Ukena,  Stahl  und  Eisen,  11,  373  (1891). 

t  A  satisfactory  filter  is  obtained  by  placing  a  little  glass  wool  in  a  funnel, 
and  on  this  a  little  asbestos,  such  as  is  used  for  Gooch  crucibles. 


620  yOLU  METRIC  ANALYSIS. 

washed  with  concentrated  nitric  acid  till  free  from  iron,  and  with 
water  till  free  from  acid.  The  asbestos  pad  and  precipitate  is 
transferred  to  the  original  flask,  covered  with  50  c.c.  of  standard- 
ized ferrous  sulphate  solution,*  and  diluted  with  water  to  a 
volume  of  200  c.c.  The  contents  of  the  flask  are  shaken  with 
glass  beads  until  all  the  precipitate  is  dissolved,  and  the  solution 
is  then  titrated  with  tenth  -normal  permanganate.  . 

The  amount  of  manganese  present  is  computed  as  follows  : 

N 
50  c.c.  of  ferrous  sulphate  solution  require  T  c.c.  —^  KMnO4 

50  c.c.  of  ferrous  sulphate  solution  +a  g.  of  iron  require  t  c.c. 


N 
Consequently  a  g.   of  the  substance  =  (77—  t)   c.c.  —  KMnO4 


and 

0.002747(7T-OX100 


Mn. 


(d)  G.  v.  Knorre's  Persulphate  Method.-f 

Principle.  —  If  a  solution  of  manganous  sulphate  containing 
a  little  free  sulphuric  acid  is  treated  with  ammonium  persulphate, 
the  manganese  is  precipitated  quantitatively  as  hydrated  man- 
ganese dioxide. 

MnSO4  +  (NH4)  2S208  +  3H20  =  MnO2  •  H2O  +  (NH4)  2SO4  +  2H2SO4 

and  the  latter  can  be  estimated  as  in  the  above  determination. 

Procedure.  —  In  the  case  of  the  harder  alloys  of  iron  and 
manganese,  the  sample  is  pulverized  as  much  as  possible  in  a  steel 
mortar.  The  weights  of  sample  taken  correspond  to  those  recom- 
mended for  the  previous  determination.  The  weighed  substance 
is  treated  in  a  beaker  with  sulphuric  acid  (1:10)  at  the  boiling 
temperature,  using  50  c.c.  of  the  dilute  acid  for  the  harder  alloys 
and  60  c.c.  for  the  softer  ones.  As  soon  as  the  evolution  of 

*  10  g.  FeSO4-7H2O,    100  c.c.    concentrated  HaSO4,  and  900  c.c.  water. 
t  Z.  angew.  Chem.,  14,  1149  (1901). 


ADDITIONAL  METHODS  FOR  DETERMINING  MANGANESE.     621 

hydrogen  ceases,  the  solution  is  filtered  through  a  small  filter 
which  is  washed  with  cold  water  until  the  washings  give  no  test 
for  iron  with  potassium  ferricyanide.  Frequently,  especially  in 
the  case  of  ferro-manganese  rich  in  silicon,  the  insoluble  residue 
still  contains  a  little  manganese,  so  that  for  an  accurate  analysis 
Ledebur  ignites  the  filter  and  precipitate  in  a  platinum  crucible, 
treats  the  residue  with  hydrofluoric  acid  and  about  0.5  c.c.  of 
concentrated  sulphuric  acid,  and  evaporates  in  an  air  bath  until 
sulphuric  anhydride  vapors  are  evolved.  After  cooling  the 
contents  of  the  crucible  are  added  to  the  main  solution.  Then 
from  150  to  250  c.c.  of  ammonium  persulphate  solution  are  added 
(60  gms.  per  liter)  the  solution  diluted  to  about  300  c.c.  and  heated 
to  boiling.  After  boiling  for  fifteen  minutes,  the  precipitate  is 
allowed  to  settle  and  is  filtered,  washed,  treated  with  an  excess  of 
ferrous  sulphate  solution  and  titrated  exactly  as  in  the  previous 
Williams  method. 

3.  Determination  of  Uranium.     Method  of  Belhoubek,* 

Zimmermann,t  Hillebrand.j: 

1000  c.c.  N.  KMnO4=^  =?^  =119.3  gms.  U. 

—         & 

This  method  is  especially  suited  for  testing  the  purity  of  a 
precipitate  of  U3O8  obtained  in  the  analysis  of  uranium  minerals. 
It  is  based  upon  the  fact  that  when  U308  is  heated  in  a  closed  tube 
with  dilute  sulphuric  acid  at  150°  to  175°  C.  it  is  readily  decom- 
posed according  to  the  equation 

U308  +  4H2S04=2U02S04  +  U(S04)2+4H20, 

forming  uranyl  and  uranous  sulphates.  The  latter  compound 
is  oxidized  to  the  former  by  means  of  potassium  permanganate, 

2KMnO4+5U(SO4)2+2H2O  = 

=  2KHS04+  2MnS04+  H2SO4+  5UO2SO4. 

*  Journ.  f.  prakt.  Chem.,  99,  231. 

t  Ann.  der  Chem.  u.  Pharm.,  232,  285. 

J  U.  S.  Geol.  Survey,  No.  78,  90  (1889). 


622  VOLUMETRIC  ANALYSIS. 

From  this  equation  it  follows  that  2  gm.-mols.  of  KMnO4  are  equiva- 
lent to  5  gm.-atoms  of  uranium,  and  1000  c.c.  N.  KMnO4  solution 


TT 

(=-j-KMn04)  =  J  gm.-atom  of  uranium  =  —  =  —   -  =119.25  gms.  U. 

Procedure.  —  The  weighed  amount  of  U3O8  is  placed  in  a  tube 
closed  at  one  end,  10  to  15  c.c.  of  dilute  sulphuric  acid  (1:6)  are 
added,  and  the  open  end  of  the  tube  is  made  narrower  by  heating 
in  a  blast-lamp  and  drawing  it  out  somewhat.  The  air  in  the 
tube  is  removed  by  inserting  a  long  capillary  so  that  it  reaches  to 
the  bottom  of  the  tube  containing  the  substance,  and  conducting 
a  current  of  carbon  dioxide  through  it;  the  larger  tube  is  finally 
sealed  without  removing  the  capillary.  The  tube  is  then  heated  in 
a  "bomb  furnace"  at  150-175°  C.  until  everything  has  dissolved 
to  a  clear  green  liquid.  After  cooling,  the  tube  is  opened  by 
making  a  scratch  with  a  file  and  touching  it  with  a  hot  glass  rod, 
The  contents  are  poured  into  a  large  porcelain  dish,  diluted  with 

N 
distilled  water  to  500-700  c.c.,  and  titrated  with  —  KMnO4  solu- 

tion until  a  permanent  pink  color  is  obtained. 

1  c.c.  ~  KMnO4=  0.01  1925  g:n.  U  =  0.013525  gm.  UO2  oxidized.* 
Remark.  —  The  above  method  gives  very  exact  results. 


4.  Determination  of  Oxalic  Acid. 
1000  c.c.  N.  KMn04=  H«C«O.-2H«O=  126.05  =  63Q2  g 

The  procedure  is  exactly  the  same  as  was  described  under 
the  standardization  of  permanganate  by  means  of  oxalic  acid 
(page  598) 


*  It  must  be  remembered,  however,  that  only  one-third  of  the  total  uranium 
in  U3O8  has  been  oxidized  by  the  KMnO4  (U3O8  -  2UO3  +  UO2) .    Consequently, 

with  regard  to  the  total  uranium,  1  c.c.  ^-  KMnO4  =  0.03578  gm.  U  =  0.0421:* 
-m.  U3O8.— [Translator]. 


ANALYSIS  OF  RED  LEAD.  623 

5.  Determination  of  Calcium. 


1000  c.c.  N.  KMnO4  =  —  =-  =  20.05  gms.  Ca. 

The  calcium  is  precipitated  as  described  on  p.  70  in  the  form 
of  its  oxalate,  filtered,  and  washed  with  hot  water.  The  still  moist 
precipitate  is  transferred  to  a  beaker  by  means  of  a  stream  of 
water  from  the  wash-bottle,  and  the  part  remaining  on  the  filter  is 
removed  by  allowing  warm  dilute  sulphuric  acid  to  pass  through 
it  several  times.  To  the  turbid  solution  in  the  beaker,  20  c.c.  of 
sulphuric  acid  (1:1)  are  added,  and  after  dilution  with  hot  water 
to  a  volume  of  from  300  to  400  c.c.  the  oxalic  acid  is  titrated  with 

~  KMn04  solution. 

1  c.c.^r  KMnO4  =0.002005  gm.  Ca. 

6.  Determination  of  Pb02  in  Minium  [Red  Lead,  Pb30  J  .   Method 

of  Lux.* 


°39  1 
1000  c.c.  N.  KMn04=-2  =  ^^  =119.55  gms.  PbOr 

Principle.  —  If  lead  peroxide  (PbO2)  is  treated  with  oxalic  acid 
in  acid  solution,  the  latter  is  oxidized  according  to  the  following 
equation: 

PbO2+  H2C2O4  =  PbO+  H2O  +  2C02. 

If  the  decomposition  takes  place  with  a  measured  amount  of 
titrated  oxalic  acid  solution  and  the  excess  of  the  latter  is  titrated 
by  means  of  potassium  permanganate  solution,  the  difference 
shows  the  amount  of  oxalic  acid  necessary  to  effect  the  reduction 
of  the  lead  peroxide. 

Procedure.  —  About  0.25  gm.  of  minium  (red  lead)  is  weighed 
into  a  porcelain  dish,  20  to  30  c.c.  of  double-normal  nitric  acid 
are  added,  and  the  lead  oxide  is  dissolved  by  heating: 

Pb3O4+  4HNO3  =  2Pb(NO3)2+  H2O+  H2Pb03. 

X 

After  solution  is  effected,  50  c.c.  ^-  oxalic  acid  are  added,  the 

o 

N 

solution  is  heated  to  boiling,   and  titrated  hot  with  —  KMn04. 

5 

*  Z.  anal.  Chem.,  19,  p.  153. 


624  yOLU METRIC  ANALYSIS. 

N  N 

If  t  c.c.  -=•  KMn04  solution  were  used,  then  50 -t  c.c.  -=-  H2C2O4 

o  o 

were  necessary  for  the  reduction  of  the  amount  of  Pb02  contained 
in  the  minium  (a  gm.)  taken  for  analysis. 

Since  1000  c.c.  N.  H2C2O4=  119.55  gms.  PbO2,  then  1000  c.c.  - 

5 

=23.91  gms.  PbO2  and  1  c.c.  =  0.02391  gm.  Pb02. 


-*"••  £  ^-*  £t  ^-^  rr  p* 

Consequently 

N 
(50-0  c-c-  -F  oxalic  acid  correspond  to  (50-0x0.02391  gm. 

Pb02. 

The  per  cent,  of  the  latter  is 

a :  (50-  0X0.02391  =  100 :x 

(50-0X2.391 
z  =  —  per  cent.  Pb02.* 

7.  Determination  of  Mn02  in  Pyrolusite. 
1000  c.c.  N.  KMnO4=     "   *=— ^—  =43.47  gms.MnO3. 


Method  of  Levol  and  Poggiale,  Modified  by  G.  Lunge.^ 

After  drying  at  100°  to  constant  weight,  1.0866  gms.  of  the 
finely  powdered  pyrolusite  are  placed  in  a  250  c.c.  flask  which  is 
provided  with  a  Contat  valve  (see  page  602) .  The  air  is  expelled 
by  conducting  CO2  into  the  flask,  and  then  75  c.c.  of  the  ferrous 
sulphate  solution,  prepared  as  described  below,  are  added,  the 
flask  closed,  and  its  contents  heated  over  a  small  flame  until  there 
is  no  longer  any  dark-colored  residue.  The  flask  is  cooled  quickly, 
the  contents  diluted  with  200  c.c.  water,  and  the  excess  of  ferrous 
sulphate  titrated  with  0.5N  KMnO4  solution.  Immediately  be- 
fore the  analysis,  the  titer  of  the  ferro-sulphate  solution  is  deter- 
mined by  taking  25  c.c.  of  it,  diluting  to  200  c.c.  and  titrating  with 
permanganate. 

*  To  express  the  results  in  per  cent.  Pb3O4,  the  number  2.391  should  be 
replaced  by  6.853.— [Translator.] 

t  Chem-techn.  Untersuchungsmethoden.     Edition  6,  Vol.  I.,  p.  569. 


DETERMINATION  OF  MnO2    IN  PYROLUSITE.  625 

By  the  treatment  of  the  pyrolusite  with  ferrous  sulphate,  the 
following  reaction  takes  place: 

MnO2  +  2FeSO4 + 2H2SO4  =  Fe2  (SO4)3 + MnSO4 + 2H2O . 
The  computation  of  the  percentage  of  MnO2  is  as  follows: 

75  c.c.  FeSO4  solution require  T  c.c.  0.5N  KMnO4 

75  c.c  FeSO4  + 1.0866 gms.  pyrolusite. .       "      t  c.c.  0.5N  KMn04 

.'.  1.0866  gms.  pyrolusite  require  T  -t  c.c.  0.5N  KMn04 
corresponding  to  (T  —  t)  X  0.02173  gm.  MnO2  and  in  percentage 
(T-t)X  0.02173X100 


1.0866 


=  2(T-t)%  Mn02. 


The  ferrous  sulphate  solution  is  prepared  as  follows:  200  c.c. 
of  concentrated  sulphuric  acid  are  slowly  poured,  with  stirring, 
into  500  c.c.  of  water  and  while  the  mixture  is  still  hot  100  gms. 
of  powdered  FeSO4-6H2O  crystals  are  added;  on  stirring,  solution 
should  take  place  within  a  few  minutes.  The  solution  is  finally 
diluted  to  one  liter,  and  when  cold  is  ready  for  use. 

(b)  The  Oxalic  Add  Method  of  Fresenius-Will,  Modified  by  Mohr* 

About  0.4  gm.  of  finely  powdered  pyrolusite,  which  has  been 

N 

dried  at  100°,  is  heated  on  the  water-bath  with  50  c.c.  —  oxalic 

o 

acid  and  20  c.c.  sulphuric  acid  (1 : 4)  until  no  more  black  particles 
remain  undissolved.     The  solution  is  diluted  with  200  c.e.  of  hot 

N 

water  and  titrated  with  ^-  KMn04  solution.     The  reaction  which 

o 

takes  place  between  the  manganese  dioxide  and  the  oxalic  acid  is 
expressed  by  the  following  equation : 

MnO2+  H2SO4 + H2C2O4=  MnSO4+ 2CO2-f  2H.A 

1  c.c.  -r  KMn04=0.0087  gm.  MnO2. 
o 

*  Fresenius-Will  carried  out  the  analysis  in  an  alkalimeter  and  determined 
the  CO,  evolved  by  loss  in  weight. 


626  VOLUMETRIC  ANALYSIS. 

8.  Determination  of  Formic  Acid  (Lieben).* 
1000  c.c.  N.  KMnO<-8xHff°H..8Xff02-  13.80  gm3.  HCOOH.f 

In  cold  acid  solutions  permanganate  reacts  only  slowly  with 
formic  acid,  while  in  a  hot  solution  the  latter  is  lost  by  volatiliza- 
tion, so  that  the  titration  in  open  vessels  is  impossible;  in  alkaline 
solutions,  on  the  other  hand,  the  oxidation  takes  place  readily  and 
quantitatively  in  the  cold : 

KMnO4 + 3HCOOK  =  K2CO3  +  KHCO3  +  2MnO2  +  H2O. 

Procedure. — The  formic  acid  is  neutralized  by  an  excess  of 
sodium  carbonate,  and  permanganate  is  run  into  the  hot  {  sodium 
formate  solution  until  the  clear  liquid  above  the  precipitate  is' 
colored  reddish. 

9.  Analysis  of  Nitrous  Acid  (Lunge). 
1000  c.c.  N.  KMnO^^I^J^i  ;  =23.51  gms.  HNO2. 

On  account  of  the  volatility  of  nitrous  acid,  the  aqueous  solu- 
tion of  the  nitrite,  or  the  solution  of  nitrous  acid  in  concentrated 
sulphuric  acid  (nitrose) ,  is  measured  from  a  burette  into  a  known 
amount  of  permanganate  solution,  which  has  been  made  acid  with 
sulphuric  acid,  diluted  to  a  volume  of  about  400  c.c.  and  warmed  to 
40°  C.  The  nitrous  acid  is  thereby  oxidized  to  nitric  acid : 

2KMn04+  5HN02+  3H2SO4  =  K2S04+  2MnSO4+  3H2O+  5HN03, 

and  the  decolorization  of  the  solution  shows  the  end-point. 
Toward  the  end  the  nitrous  acid  must  be  added  slowly,  for  the 
change  from  red  to  colorless  requires  some  time. 

10.  Analysis  of  Hydrogen  Peroxide. 
1000  c.c.  N.  KMn04=^'=^?=  17.01  gms.  H2Or 

Ten  cubic  centimeters  of  commercial  3  per  cent,  hydrogen 
peroxide  are  placed  in  a  100-c.c.  measuring-flask,  diluted  up 

*  Monatshefte,  XIV,  p  746,  and  XVI,  p.  219. 

t  In  reality,  the  normal  solution  of  formic  acid  would  contain  *  (not  -fa) 
the  molecular  weight.  See  foot-note  to  page  612. 

t  The  titration  is  made  in  hot  solution  because  the  manganous  acid 
formed  does  not  settle  well  from  a  cold  solution. 


ANALYSIS   OF  HYDROGEN  PEROXIDE.  627 

to  the  mark  with  water,  and,  after  thoroughly  mixing,  10  c.c. 
(  =  1  c.c.  of  the  original  solution)  are  placed  in  a  beaker,  and  diluted 
with  water  to  a  volume  of  300  to  400  c.c.  After  adding  20  to  30 

N 
c.c.  of  sulphuric  acid  (1:4),  the  solution  is  titrated  with  —  KMnO4 

until  a  permanent  pink  color  is  obtained.  The  following  reac- 
tion takes  place: 

2KMnO4+  5H202+  4H2SO4  =  2KHSO4+  2MnS04+  8H2O+  5O2. 

Frequently  it  happens  that  the  first  drop  of  the  permanganate 
causes  a  permanent  coloration  of  the  solution.  This  shows  that 
either  not  enough  sulphuric  acid  is  present,  or  else  there  is  no  more 
hydrogen  peroxide  left  in  the  solution.  In  this  case  a  little  more 
sulphuric  acid  is  added,  when  if  the  coloration  still  remains  the 
preparation  is  surely  spoiled,  as  can  be  shown  by  the  titanic  or 
chromic  acid  tests  (cf.  Vol.  I,  p.  42). 

The  amount  of  hydrogen  peroxide  is  expressed  either  as  per 
cent,  by  weight  or  as  per  cent,  by  volume. 

Example. — 10  c.c.  of  the  above-mentioned  dilute  solution  of 
hydrogen  peroxide  (  =  1  c.c.  of  the  original  solution)  required 

N 
17.86  c.c.  —  KMn04  solution,  corresponding  to 

17.86X0.001701  =  0.03038  gm.  H2O2. 

As  the  specific  gravity  of  the  original  hydrogen  peroxide  solution 
can  be  assumed  to  be  1,  it  therefore  contains  3.04  per  cent.  H2O2. 

When  expressed  in  "per  cent,  by  volume"  the  result  shows 
how  many  cubic  centimeters  of  oxygen  can  be  obtained  from 
100  c.c.  of  the  solution. 

In  this  case  100  c.c.  of  the  hydrogen  peroxide  solution  con- 
tain 3.04  gms.  of  H2O2  and,  on  being  decomposed,  1  gm.-mol.  H2O2 
sets  free  1  gm.-at.  O: 

H202=H20+0 
34.02  =  18.02+16, 

or  11195  c.c.  of  oxygen  at  0°  C.  and  760  mm.  pressure;  conse- 
quently 3.04  gms.  H2O2  will  evolve 

34.02: 11195  =  3.04:  x 

3.04X11200 

x== <>A  r>o =  1000  c.c.  oxygen  measured  under  standard  con 

o4.Uz 

ditions  of  temperature  and  pressure. 


628  VOLUMETRIC  ANALYSIS. 

100  c.c.  of  the  commercial  hydrogen  peroxide,  therefore,  will 
evolve  1000  c.c.  of  oxygen,  i.e.,  ten  times  its  own  volume.  This 
is  somewhat  anomalously  designated  as  hydrogen  peroxide  of  10 
per  cent,  by  volume. 

100  c.c.  3  per  cent,  hydrogen  peroxide  =  10  per  cent,  by  volume. 
100  c.c.  6     "      "  "  "       =20     "      "      " 

100  c.c.  9     "       "  "  "       =30     "      "      « 


ii.  Analysis  of  Barium  Peroxide. 


1000  c.c.  N.  KMnO<  =  -        =  -—^  =  84.70  gms.  BaO2. 


About  0.2  gm.  of  the  substance  is  weighed  into  a  400  c.c. 
beaker,  covered  with  300  c.c.  of  cold  water,  and  treated,  under 
constant  stirring,  with  20-30  c.c.  of  hydrochloric  acid  (1:5). 
When  all  the  Ba(>2  has  dissolved,  the  solution  is  titrated  with 
0.1  N.  KMnO4.  The  addition  of  H2SO4  is  not  advisable,  as  the 
precipitated  BaSCU  is  likely  to  enclose  some  Ba(>2  which  will 
then  escape  the  titration. 

Another  method  for  the  analysis  of  BaCb  has  been  proposed 
by  Kassner.* 

12.  Analysis  of  Potassium  Percarbonate. 

TC  f1  O       1  Q8  y 
1000  c.c.  N.  KMn04  =  ^6  =          =  99.10  gms.  K2C206. 


0.25  gm.  potassium  percarbonate  is  weighed  out  into  300  c.c. 
of  cold,  dilute  sulphuric  acid  (1:30),  in  which  it  dissolves  with 


*  Arch.  Pharm.,  228, 432. 


ANALYSIS   OF  PERSULPHATES.  629 

violent  evolution  of  carbon  dioxide  and  formation  of  an  equiva- 
lent amount  of  hydrogen  peroxide  : 


and  the  latter  is  titrated  with  potassium  permanganate. 


13.  Analysis  of  Persulphates  (Persulphuric  Acid,  H2S208). 


(  97.08  gms. 

1000  c.c.  N.  KMn04  =  -  —  \  111.4     "     (NHO.SA 
2      (  135.2     " 


A  solution  of  persulphuric  acid  does  not  reduce  permanga- 
nate, nor  does  it  react  with  titanic  acid;  on  the  other  hand  it  oxi- 
dizes ferrous  salts  immediately  in  the  cold  to  ferric  salts,  and  by 
means  of  this  behavior  it  can  be  easily  determined.  The  ammo- 
nium and  potassium  salts  are  now  commercial  products,  and  are 
analyzed  as  follows  :  About  0.3  gm.  of  the  salt  is  weighed  out  into 
a  flask  fitted  with  a  Bunsen  valve,  the  air  is  replaced  by  carbon 
dioxide,  30  c.c.  of  a  freshly  titrated  solution  of  ferrous  sulphate 
are  added  and  then  200  c.c.  of  hot  water;  the  flask  is  closed  and 
its  contents  rotated.  The  salt  dissolves  without  difficulty,  and 
the  ferrous  sulphate  is  oxidized: 

H2S2O8-f-2FeSO4  =  Fe2(S04)  3  +  H2S04. 

After  all  of  the  salt  has  dissolved,  the  contents  of  the  flask 
are  cooled  by  placing  the  flask  in  cold  water,  and  the  excess  of 

N 

ferrous  salt  is  titrated  with  —  KMnO4.* 


*  The  ferrous  sulphate  must  be  added  to  the  persulphate,  and  then  the 
hot  water.  If  the  hot  water  is  added  first,  the  persulphate  is  decomposed 
somewhat  and  the  results  obtained  will  be  low. 


630  VOLUMETRIC  A  NA  LYSIS. 

In  this  way  it  is  found  that: 

N 
30  c.c.  ferrous  sulphate  solution  require  T  c.c.  —  KMnO4  solution. 

N 
30  c.c.  ferrous  sulphate  +  a  gm.  persulphate  require  t  c.c.  —  KMnO4 

solution. 

Consequently  a  gm.  of  persulphate  correspond  to  (T  —  t)  c.c. 


In  the  case  of  the  potassium  salt,  since  1000  c.c.  N.  KMn04 
135.2  gms.  K2S2O8,  and  1  c.c.      KMnO4  =  0.01352  gm.  K2S208, 


we  have:    (T-t)  X0.01352  gm.  K2S208  in  a  gm.  of  the  commer- 
cial salt,  or  in  per  cent.  : 


1.352(^-0 
x  =  —  -=per  cent.  K2S208. 


With  the  ammonium  salt  the  factor  becomes  0.01141  instead  of 
0.01352. 

The  ferrous  sulphate  necessary  for  this  determination  is  pre- 
pared by  roughly  weighing  out  30  gms.  of  crystallized  ferrous 
sulphate  (FeSO4  +  7H20),  dissolving  it  in  900  c.c.  of  water, 
and  diluting  to  1000  c.c.  with  pure  concentrated  sulphuric 
acid. 

Persulphates  may  also  be  analyzed  very  satisfactorily  by  means 
of  oxalic  acid.*  When  a  sulphuric  acid  solution  of  a  persulphate 
is  treated  with  oxalic  acid  alone,  there  is  no  perceptible  reaction. 
On  adding  a  small  amount  of  silver  sulphate  as  catalyzer,  however, 
a  lively  evolution  of  carbon  dioxide  takes  place,  and  at  the  water- 
bath  temperature  the  reaction  is  soon  completed. 

H2S208  +  H2C204  =  2H2SO4  +  2C02. 
*  R.  Kempf,  Ber.,  38,  3965  (1905). 


DETERMINATION   OF  HYDROXYLAMINE.  031 

The    excess   of   the   oxalic   acid   can  be  titrated  with  perman- 
ganate. 

Procedure. — About  0.5  gm.  of  the  persulphate  is  placed  in  a 
400-c.c.  Erlenmeyer  flask,  50  c.c.  of  tenth-normal  oxalic  acid  solu- 
tion, and  a  solution  of  0.2  gm.  silver  sulphate  in  20  c.c.  of  10  per 
cent,  sulphuric  acid  are  added,  and  the  mixture  is  heated  on  the 
water  bath  until  the  evolution  of  carbon  dioxide  ceases;  this 
requires  not  more  than  15  or  20  minutes.  The  solution  is  then 
dilated  to  about  100  c.c.  with  water  at  about  40°  and  titrated  with 
tenth-normal  permanganate. 


14.  Determination  of  Hydroxylamine  (Raschig).* 

•VTT  /~\TT        oo  no 

1000  c.c.  N.  KMnO4  =  ^  —  '  -  =—f—  =  16.52  gms.  NH2OH. 


Principle.  —  Hydroxylamine  is  oxidized  in  hot  acid  solutions 
by  means  of  ferric  salts  to  nitrous  oxide  and  water: 

2NH2OH  +  02  =  N20  +  3H20, 

and  an  equivalent  amount  of  ferrous  salt  is  formed: 

2NH2OH  +  2Fe2  (SO4)  3  =  4FeSO4  +  2H2S04  +  H20  +  N2O. 

The  amount  of   ferrous  salt  is  determined  by  titration  with 

N 

—  potassium  permanganate. 

Procedure.  —  About  0.1  gm,  of  the  hydroxylamine  salt  is  placed 
in  a  500-c.c.  flask  and  dissolved  in  a  little  water,  30  c.c.  of  a  cold 
saturated  solution  of  ferric-ammonium  alum  are  added,  and  10  c.c.  of 
dilute  sulphuric  acid  (1:4).  The  contents  of  the  flask  are  heated 

*  Ann.  d.  Chem.  und  Pharm.,  241,  p.  190. 


632  VOLUMETRIC  ANALYSIS. 

to  boiling  and  kept  at  this  temperature  for  five  minutes,  after 
which  the  solution  is  diluted  with  distilled  water  to  a  volume  of 
about  300  c.c.  and  immediately  titrated  with  permanganate 
solution. 

Remark. — If  only  slightly  more  than  the  theoretical  amount 
of  the  ferric  salt  is  added,  the  oxidation  of  the  hydroxylamine 
does  not  take  place  entirely  in  accordance  with  the  above  equa- 
tion, but  part  of  the  substance  is  oxidized  to  nitric  oxide: 

2NH2OH+  3O  =  3H2O+  2NO, 
so  that  it  is  then  impossible  to  obtain  exact  results. 

15.  Determination  of  Hydroferrocyanic  Acid  (de  Hae'n).* 

1000  c.c.  N.KMnO4  =  l  mol.  K4Fe(CN)6=  368.3  gms.  K4Fe(CN)8. 

Principle. — By  oxidation  in  acid  solution,  hydroferricyanic 
acid  is  formed  from  hydroferrocyanic  acid: 

2H4Fe(CN)6+  O  =  H2O+ 2H3Fe(CN)6. 

This  procedure  is  chiefly  used  for  the  analysis  of  potassium 
fexrocyanide  (yellow  prussiate  of  potash),  so  that  the  concentra- 
tion of  the  permanganate  solution  is  expressed  in  terms  of  this 
salt. 

Procedure. — 0.9  gm.  of  the  salt  to  be  analyzed  is  dissolved  in 
100  c.c.  of  water,  10  c.c.  of  dilute  sulphuric  acid  are  added,  and 
this  solution  is  titrated  in  a  porcelain  dish  with  permanganate 
until  a  permanent  pink  color  is  obtained.  It  is  not  easy 
to  determine  the  end-point.  On  acidifying,  the  solution  of  the 
ferrocyanide  becomes  milky  with  a  bluish  tinge,  and  on  the  addi- 
tion of  permanganate  at  first  a  yellow  shade  is  obtained,  after- 
wards becoming  green,  and  finally  on  the  addition  of  more  perman- 
ganate the  color  changes  to  pink.  On  account  of  the  difficulty 
in  determining  this  point,  de  Hae'n  recommends  that  the  perman- 
ganate be  standardized  against  pure  potassium  ferrocyanide  solu- 
tion (K4Fe(CN)6+3H2O). 

*  Ann.  d.  Chem.  und  Pharm.,  90,  p.  160. 


DETERMINATION  OF  HYDROFERRICYANIC  ACID,  ETC.        633 

1  6.  Determination  of  Hydroferricyanic  Acid. 

1000  c.c.  X.  KMnO4  =  l  mol.  K,Fe(CN),  =  329.2  gms.  K,Fe(CN)v 

Principle.  —  The  potassium  ferricyanide  is  reduced  in  alkaline 
solution  to  potassium  ferrocyanide,  and  the  latter  is  titrated  with 
permanganate. 

Procedure.  —  In  a  300-c.c.  flask,  6.0  gms.  of  the  ferricyanide  are 
dissolved  in  water,  the  solution  made  alkaline  with  potassium 
hydroxide,  heated  to  boiling,  and  an  excess  of  a  concentrated 
ferrous  sulphate  solution  is  added.  At  first  yellowish-brown 
ferric  hydroxide  is  precipitated,  later  black  ferrous-ferric  hydrox- 
ide is  formed,  and  this  shows  the  completion  of  the  reaction. 
After  cooling,  the  contents  of  the  flask  are  diluted  with  water  up 
to  the  mark,  filtered  through  a  dry  filter  (after  thoroughly  mix- 
ing), and  50  c.c.  of  the  filtrate  *  (  =  1  gm.  of  the  substance)  are  taken 

N 
for  the  titration  with  —  KMnO4  solution. 

17.  Determination  of  Chloric  Acid. 


1000  cc  N  KMnO  -552i-  \  20'44  &»*'  KC1°. 
6       '    I  17.74      "     NaCIO, 

About  5  gms.  of  potassium  chlorate,  or  4  gms.  of  the  sodium 
salt,  are  dissolved  in  water,  and  the  solution  diluted  to  1  liter. 
After  thoroughly  mixing,  10  c.c.  are  placed  in  a  flask  fitted  with  a 
Bunsen  valve  and  the  air  expelled  from  the  flask  by  a  current 
of  carbon  dioxide.  After  this  50  c.c.  of  a  freshly-standardized 
solution  of  ferrous  sulphate  (prepared  as  described  on  p.  630) 
are  added,  and  the  solution  boiled  ten  minutes.  The  following 
reaction  takes  place: 

KC103+  6FeSO4+  3H2SO4  =  KC1+  3Fe2(SO4)3+  3H2O. 

After  cooling  the  solution  is  diluted  with  cold  distilled  water, 
10  c.c.  of  manganous  sulphate  solution  are  added  (cf.  p.  607), 
and  the  excess  of  the  ferrous  sulphate  is  titrated  with  potassium 
permanganate.  We  find  that: 

*  The  first  ten  or  fifteen  cubic  centimeters  of  the  filtrate  should  be 
discarded. 


634  VOLUMETRIC  ANALYSIS. 

N 
50  c.c.  ferrous  sulphate  .............  required  T  c.c.  —  KMnO4  sol. 

50  c.c.       "  "  +  10  c.c.  chlorate  sol.  "        t  c.c.  ^      "         " 

10  c.c.  chlorate  solution  =  —  gm.  substance  =  (T  —  t)c.c.  —  KMnO4" 

For  the  analysis  of  potassium  chlorate  a  gm.  of  the  substance 
contain  (T-  t)X  0.2044  gm.  KC1O3,  and  the  per  cent,  present  is 

20.44  X(T-t) 

—  =  per  cent. 

The  calculation  for  sodium  chlorate  is  analogous. 
1  8.  Determination  of  Nitric  Acid  (Pelouze-Fresenius). 


RNO  21.01  gms.  HNO, 

1000  c.c.  N.  KMn04=^^a=      28.34      "     NaNO, 


33.70  KNO3 

This  method  depends  upon  the  fact  that  on  heating  a  nitrate 
in  the  presence  of  considerable  hydrochloric  acid  and  ferrous 
chloride  the  latter  is  oxidized  to  ferric  chloride  and  the  nitric  acid 
is  reduced  to  nitric  oxide: 

2KNO3+  6FeCl2+  8HC1  =  2KC1+  2NO+  4H2O+  6FeCl3. 
As  a  measure  for  the  amount  of  nitrate  reduced  we  have: 

1.  The  excess  of  ferrous  salt. 

2.  The  ferric  salt  produced. 

3.  The  nitric  oxide  formed. 

The  method  of  Schlosing-Grandeau  described  on  p.  456  is 
based  upon  the  measurement  of  the  nitric  oxide  formed.  C.  D. 
Braun  *  estimates  the  amount  of  ferric  salt  formed,  while  Pelouze 
and  Fresenius  determine  the  amount  of  ferrous  salt  not  used  up 
in  the  reduction  of  the  nitric  acid. 

Procedure. — A  weighed  amount  of  iron  wire  (about  1.5  gms.) 
is  placed  in  a  long-necked  flask,  and  the  air  expelled  by  passing 
a  current  of  pure  carbon  dioxide  through  it  for  two  or  three 
minutes.  After  this  30  to  40  c.c.  of  pure,  concentrated  hydro- 
chloric acid  are  added  and  the  flask  is  placed  in  an  inclined  posi- 
tion and  closed  by  means  of  a  rubber  stopper  through  which 
tubes  pass  so  that  a  current  of  carbon  dioxide  can  be  conducted 

*  Journ.  f.  prakt.  Chem.,  81  (1860),  p.  421. 


DETERMINATION   OF  NITRIC  ACID.  635 

through  the  flask.  The  solution  is  heated  on  the  water-bath 
in  this  atmosphere  of  carbon  dioxide  until  the  iron  has  com- 
pletely dissolved,  when  the  solution  is  allowed  to  cool  in  a  cur- 
rent of  the  gas.  Meanwhile  about  0.25  to  0.3  gm.  of  the  nitrate 
is  weighed  out  in  a  small  glass  tube  closed  at  one  end;  this  is 
thrown  into  the  acid  solution  of  the  ferrous  sulphate  and  the  flask 
quickly  closed  again.  The  flask  is  then  once  more  placed  in  its 
inclined  position  upon  the  water-bath  and  heated  for  fifteen 
minutes,  while  the  current  of  carbon  dioxide  is  continually 
passed  through  it.  The  tube  through  which  the  gas  leaves 
the  flask,  during  the  whole  operation,  dips  into  a  beaker  filled 
with  water  so  that  there  is  no  chance  of  any  air  getting  back  into 
the  flask.  After  this  the  solution  is  heated  to  boiling  and  kept 
there  until  its  dark  color  disappears  and  the  yellow  color  of  the 
ferric  chloride  becomes  apparent.  In  order  to  make  sure  that  the 
nitric  oxide  is  entirely  removed,  the  contents  of  the  flask  are  boiled 
five  minutes  longer  and  then  allowed  to  cool  in  the  atmosphere 
of  carbon  dioxide.  When  cold  the  solution  is  poured  into  a 
beaker,  the  flask  washed  out  with  a  little  boiled  water,  the  solu- 
tion is  diluted  to  a  volume  of  about  400  to  500  c.c.,  10  c.c.  of 
manganese  sulphate  solution  are  added,  and  the  unoxidized  iron 

N 
is  titrated  with  —  KMnO4  solution. 

The  amount  of  pure  iron  present  in  the  wire  used  is  deter- 
mined under  the  same  conditions  as  prevailed  during  the  pre- 
vious operation,  using  a  smaller  portion  of  wire  but  the  same 
amount  of  acid,  manganese  sulphate,  etc. 

The  calculation  is  as  follows: 

If  a  gm.  of  potassium  nitrate  and  p  gm.  of  the  wire  were  taken  for 

N 
the  analysis,  t  c.c.  of  ^-  KMnO4  were  required  to  oxidize  the  excess 

of  iron,  and  further  p  gm.  of  the  wire  require  T  c.c.  of  —  KMnO4 
solution,  we  have,  then: 

p  gm.  iron require  T  c.c.  —  KMnO4  solution 

N 
p  gm.  iron  -fa  gm.  saltpeter t  c.c.  —  KMnO4 

N 
and  a  gm.  saltpeter      =         (T  —  t}  c.c.  —  KMnO4, 


636  VOLUMETRIC  ANALYSIS. 

so  that  a  gm.  of  saltpeter  contain  (T—  t)  X0.01685  gm.  KNO3, 
and  in  per  cent. 

(T7-  OX  1.685  T.vr.   a 

—  =per  cent.  KJ\O3.* 

Remark.  —  This  method  gives  results  just  as  accurate  as  those 
obtained  by  the  method  of  Devarda,  but  the  latter  determination 
is  much  easier  to  carry  out. 

The  determination  becomes  simpler  if  the  contents  of  the 
iron  wire  is  assumed  to  be  99.7  per  cent.  Fe  and  the  second  titra- 
tion  thus  done  away  with.  It  does  not  take  long  to  make  the 
analysis  of  the  wire,  however,  and  it  is  advisable  to  do  it.  Instead 
of  titrating  the  excess  of  the  ferrous  salt  with  potassium  perman- 
ganate solution,  a  solution  of  potassium  dichromate  may  be  used. 
For  the  determination  of  the  ferric  salt  formed,  cf.  p.  681. 

19.  Determination  of  Vanadium. 


1000  c.c.     .  KMn04  =        -6  =  ~=9.12  gms.  V2OS- 

Sulphur  dioxide  is  conducted  into  the  boiling  solution  of  an 
alkali  vanadate  containing  sulphuric  acid  until  the  solution 
appears  a  pure  blue;  by  this  means  the  vanadic  acid  is  reduced 
to  vanadyl  salt: 


The  boiling  is  continued  and  a  current  of  carbon  dioxide*  is 
passed  through  the  solution  until  the  escaping  gas  will  no  longer 
decolorize  a  solution  of  potassium  permanganate,  showing  that 
the  excess  of  the  sulphur  dioxide  has  been  expelled.  The  hot 
solution  is  then  titrated  with  potassium  permanganate  until  a 
permanent  pink  color  is  obtained.  The  end-point  is  easily  recog- 
nized only  when  the  solution  is  hot.  This  accurate  determina- 
tion is  used  for  the  analysis  of  vanadium  in  iron  and  steel,  or  in 
ores.  (Cf.  p.  310.) 

*  Of  course  the  calculation  can  be  made  from  the  amount  of  iron  oxi- 
ized.  In  that  case: 

Fe  :£KNO3  =  (p  —tX 0.02793)  :z 

(p-tX 0.02793)-  KNO, 
a;— gms.  KNOj  in  a  gm.  of  substance, 

and  in  per  cent. 

100(p-«XO.Q2793)KNOJ 


3Fe-a 


=  per  cent  KNO3. 


DETERMINING  PHOSPHORUS  IN  IRON  AND  STEEL. 


637 


20.  Blair  Method  for  Determining  Phosphorus  in  Iron  and  Steel.* 

Principle. — The  substance  is  dissolved  in  nitric  acid,  all 
carbonaceous  matter  is  destroyed  by  the  action  of  strong  per- 
manganate solution,  any  precipitated  manganese  dioxide  is 


FIG.  91. 

redisssolved,  and  the  phosphorus  is  precipitated  in  slightly  acid 
solution  as  ammonium  phosphomolybdate.  The  precipitate  is 
dissolved  in  ammonia,  the  solution  acidified  with  sulphuric  acid 

*  Andrew  Blair.     The  Chemical  Analysis  of  Iron. 


638  VOLUMETRIC  ANALYSIS. 

and  the  molybdenum  reduced  by  means  of  a  so-called  Jones 
reductor.  The  reduced  solution  is  titrated  with  perman- 
ganate. 

The  Jones  reductor  (Fig.  91)  is  made  by  placing  a  platinum 
spiral  (or  glass  beads)  in  the  bottom  of  a  glass-stoppered  tube 
which  is  30  cm.  long  and  has  an  inside  diameter  of  18  mm.  Upon 
the  spiral,  or  beads,  is  placed  a  plug  of  glass  wool  and  then  a  thin 
layer  of  asbestos  such  as  is  used  for  Gooch  crucibles.  The  tube  is 
then  filled  with  amalgamated  zinc  to  within  5  cm.  of  the  top. 
This  zinc  can  be  prepared  by  taking  some  20-  to  30-mesh  zinc, 
cleaning  it  with  a  little  hydrochloric  acid,  and  adding  mercuric 
chloride  until  hydrogen  ceases  to  be  evolved.  In  this  condition 
the  zinc  is  scarcely  acted  upon  at  all  by  hydrochloric  acid,  but  is 
capable  of  reducing  an  iron  or  molybdenum  solution  just  as 
effectively  as  if  it  were  not  amalgamated.  On  top  of  the  column 
of  zinc  is  placed  a  little  glass  wool  to  serve  as  filter. 

Procedure. — A  2  gm. -sample  is  taken  in  the  case  of  steels  and 
1  gm.  in  the  case  of  cast  irons.  The  metal  is  weighed  into  a  250- 
c.c.  Erlenmeyer  flask  and  dissolved  in  100  c.c.  of  nitric  acid  (sp.  gr. 
1.13)  which  is  prepared  by  mixing  one  volume  of  nitric  acid 
(sp.  gr.  1.42)  with  3  volumes  of  water  and  then  testing  the  gravity. 
A  small  funnel  is  placed  in  the  neck  of  the  flask  and  the  solution 
heated  until  all  the  iron  has  dissolved  and  the  nitrous  fumes 
expelled.  Ten  c.c.  of  strong  permanganate  solution  (15  gms.  to 
the  liter)  are  added  and  the  boiling  continued  until  the  pink  color 
of  the  permanganate  disappears.  The  slight  precipitate  of 
manganese  dioxide  is  dissolved  by  the  addition  of  a  little  sodium 
sulphite  solution.  After  filtering,  40  c.c.  of  ammonia  (sp.  gr. 
0.96)  are  added,  the  solution  is  brought  to  a  temperature  of  about 
40°  and  treated  with  40  c.c.  of  a  freshly -prepared  solution  of 
ammonium  molybdate.*  The  flask  is  then  closed  with  a 


*  100  gms.  of  pure  molybdic  acid  (MoO3)  is  stirred  into  400  c.c.  of  cold 
distilled  water,  and  80  c.c.  of  concentrated  ammonia  added.  The  solution  is 
filtered,  and  the  filtrate  slowly  poured,  with  constant  stirring,  into  a  solution 
of  400  c.c.  nitric  acid  (sp.  gr.  1.42)  in  600  c.c.  of  water.  After  the  addition 
of  0.05  gm.  of  microcosmic  salt,  the  solution  is  allowed  to  stand  24  hours 
and  is  then  filtered. 


DETERMINING   PHOSPHORUS  IN  IRON  AND  STEEL.          639 

solid  rubber  stopper  and  shaken  vigorously  for  five  minutes. 
After  allowing  the  precipitate  to  settle  for  a  few  minutes,  it  is 
filtered  and  washed  promptly  with  acid  ammonium  sulphate 
solution  (1000  c.c.  water,  25  c.c.  concentrated  H^SO^  and  15  c.c. 
strong  ammonia) ,  until  the  washings  give  no  test  for  molybdenum 
when  treated  with  a  drop  of  yellow  ammonium  sulphide  solution. 
The  color  obtained  is  compared  with  a  similar  amount  of  the  wash 
water  itself  which  has  been  treated  with  the  same  ammonium 
sulphide. 

The  ammonium  phosphomolybdate  precipitate  is  dissolved 
in  a  mixture  of  5  c.c.  concentrated  ammonia  (sp.  gr.  0.90)  and  20 
c.c.  of  water,  the  filter  washed  with  water  and  the  filtrate  treated 
with  10  c.c.  of  concentrated  sulphuric  acid.  It  is  then  run  through 
the  Jones  reductor.  A  blank  is  run  with  the  reductor  before  each 
series  of  determinations,  using  the  same  quantity  of  reagents. 
After  a  reductor  has  stood  for  some  time,  it  should  be  well 
washed  with  dilute  sulphuric  acid,  before  even  running  a  blank 
test. 

In  making  blanks  and  in  all  determinations,  the  procedure  is 
as  follows:  100  c.c.  of  dilute  sulphuric  acid  (25  c.c.  concentrated 
acid  to  1  liter  of  water)  are  run  into  the  funnel,  B,  and  the  stop- 
cock C  is  opened,  using  a  little  suction.  When  only  a  little  of  the 
dilute  acid  remains  in  the  funnel,  the  hot  solution  to  be  reduced  is 
added  and  when  this  has  nearly  passed  out  of  the  funnel,  it  is 
followed  by  250  c.c.  of  hot  dilute  sulphuric  acid,  washing  out  the 
original  beaker  with  this  acid  and  adding  it  in  small  portions. 
Finally  100  c.c.  of  water  are  passed  through  the  reductor.  At  no 
time,  however,  should  any  air  be  allowed  to  enter,  as  it  forms 
hydrogen  peroxide  and  vitiates  the  result. 

The  reduced  solution  is  titrated  with  tenth-normal  per- 
manganate. 

The  ammonium  phosphomolybdate  precipitate,  (NH4)3P04- 
12MoC>3,  contains  12  molecules  of  MoOs  to  1  atom  of  phosphorus. 
Although  zinc  reduces  MoOa  to  MosOs,  there  is  a  slight  oxidation 
by  the  air  in  the  flask,  so  that  correct  results  are  obtained  by 
assuming  a  reduction  to  Mo240s7  and  subsequent  oxidation  by  the 
permanganate  to  MoOs  again.  Therefore  during  the  titration 
the  following  reaction  takes  place: 


640  VOLUMETRIC  ANALYSIS. 

Mo24O37  +  35O  =  24MoO3 
and 

!P  =  12MoO3  =  35H. 

To  illustrate  the  computation,  let  it  be  assumed  that  the 
ammonium  phosphomolybdate  precipitate  from  2  gms.  of  a  sample 
of  steel  requires  by  the  above  method  12  c.c.  of  permanganate 
solution,  of  which  1  c.c.  =0.00392  gm.  Fe.  The  blank  on  the 
reductor  was  0.18  c.c.  The  phosphorus  present  in  the  steel  is 
then: 

(12 - 0. 18)  X 0.00392 XPX 100 

35Fex2  =per  cent,  phosphorus. 

Remarks. — The  Jones  reductor  may  be  used  to  advantage  for 
reducing  sulphuric  acid  iron  solutions  which  are  to  be  titrated 
with  permanganate.  The  blank  experiment  must  always  be 
made,  as  the  zinc  invariably  contains  a  little  iron.  If  in  the  above 
determination  the  reductor  tube  is  prolonged  so  that  it  reaches 
nearly  to  the  bottom  of  the  flask,  and  dips  into  50  c.c.  of  ferric 
alum  solution  (100  gms.  ferric  alum,  25  c.c.  concentrated  H2SO4 
and  40  c.c.  glacial  H3PO4)  the  molybdenum  comes  in  contact  with 
this  solution  while  it  is  entirely  reduced  to  Mo203.  The  ferric  alum 
at  once  oxidizes  the  Mo2O3  to  MoO3  and  an  equivalent  amount  of 
iron  is  reduced  to  the  ferrous  condition.  The  titration  with 
permanganate  can  then  be  carried  out,  and  in  the  computation 
the  molecular  weight  of  Mo2O3  is  used  instead  of  Mo24O37  as  above 
and  1P  =  36H.  The  blank  determination  should  be  carried  out 
with  the  ferric  alum  solution  in  the  flask. 

Concordant  results  can  be  obtained  by  both  methods,  but  the 
latter  has  the  advantage  that  there  is  no  danger  of  some  of  the 
molybdenum  being  oxidized  while  shaking  the  flask  during  the 
titration. 

In  the  case  of  steels  containing  tungsten  and  vanadium,  the 
phosphorus  may  be  left  in  the  residue  insoluble  in  nitric  acid. 


POTASSIUM  BICHROMATE  METHODS.  641 

B.  Potassium  Bichromate  Methods. 

Determination  of  Iron  according  to  the  Method  of  Penny. 
1000  c.c.  N.  KjOjO^l  gm.-at.  Fe  =  55.85  gms  Fe. 

Principle. — If  a  solution  of  a  ferrous  salt,  in  either  hydrochloric 
or  sulphuric  acid,  is  treated  with  an  alkali  chromate  solution,  the 
chromate  is  at  once  reduced  in  the  cold  and  the  ferrous  salt  is 
oxidized  quantitatively : 

K£r2Q7+  6FeSO4+  8H2SO4  =  2KHSO4-f 

+  02(S04)3+  3Fe2(S04)3+  7H2O 
or 

K2O2O7+  6FeCl2+  14HC1  =  2KC1+  2CrCl3-f  6FeCl3+  7H2O. 

On  account  of  the  formation  of  the  chromic  salt  the  solution 
becomes  emerald-green  in  color. 

The  end-point  of  the  reaction  is  determined  by  removing  a 
drop  of  the  solution  and  testing  it  with  a  freshly-prepared  solu- 
tion of  potassium  ferricyanide;  if  no  blue  coloration  is  formed, 
the  ferrous  salt  has  been  completely  oxidized. 

N 
The  —  potassium  dichromate  solution  necessary  for  -this  titra- 

tion  may  be  prepared  by  dissolving      2     2   7  =  4.903  gms.  of  the 

OU 

salt,  purified  as  described  on  p.  36,  and  dried  at  130°  C.  It  is  not 
advisable  to  remove  the  last  traces  of  moisture  by  melting 
the  salt,  for,  either  by  overheating  or  by  means  of  the  dust  of  the 
air,  there  is  some  reduction  of  the  chromate,  so  that  subsequently 
a  turbid  solution  will  be  obtained,  containing  small  amounts  of 
suspended  Cr2O3. 

Method  of  Titration. — To  the  acid  solution  of  the  ferrous  salt 
contained  in  a  beaker  (with  about  0.1  to  0.15  gm.  iron  in  each 

N 
100  c.c.)  the  solution  of  —=  K2Cr2O7  is  added,  preferably  from  a 

glass-stoppered  burette. 

From  time  to  time  a  drop  of  the  solution  is  removed  on  the  end 
of  a  glass  stirring-rod  to  a  white  porcelain  plate,  and  placed  beside 
a  drop  of  a  not  more  than  2  per  cent,  solution  of  potassium  ferri- 


642  VOLUMETRIC  ANALYSIS. 

cyanide.*  By  means  of  a  stirring-rod  one  solution  is  made  to  run 
into  the  other.  If  considerable  ferrous  salt  remains  in  the  solu- 
tion, the  blue  color  will  be  formed  immediately,  but  in  propor- 
tion as  the  ferrous  salt  is  replaced  by  ferric  salt,  a  bluish-green 
color  is  obtained,  perceptible  at  the  junction  of  the  two  solutions. 
As  soon  as  no  more  bluish-green  coloration  is  to  be  detected  the 
reaction  is  complete.  In  all  cases  the  analysis  is  made  in  dupli- 
cate, and,  other  things  being  equal,  the  second  determination 
should  be  the  more  accurate.  This  time  it  is  possible  to  add 
almost  the  whole  of  the  required  amount  of  bichromate  at  once, 
and  for  the  testing  not  more  than  two  or  three  drops  of  the  dilute 
solution  of  ferrous  salt.  The  loss  of  ferrous  solution  will  then  be 
inappreciable. 

Remark. — The  dichromate  method  is  slightly  less  accurate 
than  the  permanganate  method,  but  it  possesses  the  advantage 
that  a  solution  of  a  ferrous  salt  containing  hydrochloric  acid  can 
be  titrated  without  the  addition  of  manganese  sulphate,  even 
when  the  solution  is  turbid  with  suspended  insoluble  salts,  fibres 
of  filter-paper,  etc.  In  turbid  solutions  it  is  difficult  to  recognize 
the  permanganate  end-point.  A  further  advantage  lies  in  the 
fact  that  the  normal  dichromate  solution  can  be  readily  pre- 
pared by  simply  weighing  out  the  required  amount  of  the  pure, 
dry  salt,  and  diluting  the  aqueous  solution  to  a  volume  of  1  liter. 
It  is  then  unnecessary  to  test  the  concentration  in  any  other  way. 

Determination   of   Manganese   in   Iron   and   Steel.     Method   of 

J.  Pattinson.f 

Principle. — If  a  solution  containing  iron,  manganese,  and  cal- 
cium salts  is  treated  with  "chloride  of  lime"  solution,  all  of  the  iron 
and  manganese  are  precipitated,  the  latter  in  the  form  of  its 
hydrated  dioxide.  The  whole  precipitate  is  dissolved  in  an  acid 
ferrous  sulphate  solution  of  known  strength,  and  the  excess  of  the 
latter  is  titrated  with  dichromate  solution : 

*The  potassium  ferricyanide  must  be  absolutely  free  from  f errocyanide ; 
and  as  the  former  is  readily  reduced  by  the  dust  of  the  air,  the  surface  of 
the  salt  should  be  washed  off  several  times  with  water  before  dissolving  it 
for  the  test  solution. 

t  Journ.  of  the  Chem.  Soc.  (1879),  p.  365. 


DETERMINATION  OF  MANGANESE  IN    IRON  AND   STEEL.     643 

MnO2+  2FeO  =  Fe203+  MnO. 
1000  c.c.  N.  K2Cr207  =  IFe  =  ^  =  ^j^  =  27  .47  gms.  Mn. 

Procedure. — 5  gms.  of  the  iron  or  steel  (or  1  gm.  of  ferromanga- 
nese)  are  dissolved  in  hydrochloric  acid,  the  solution  oxidized  with 
nitric  acid,  evaporated  to  a  small  volume,  poured  in  a  100-c.c. 
measuring-flask,  and  diluted  up  to  the  mark  with  water.  After 
thoroughly  mixing,  20  c.c.  of  the  solution  are  placed  in  a  large 
beaker  (of  about  1  liter  capacity)  and  neutralized  with  pure  cal- 
cium carbonate.  The  carbonate  is  added  in  small  portions  until 
the  solution  finally  becomes  a  dark  brown  but  still  remains  clear. 
After  this  50  c.c.  of  "chloride  of  Lme"  solution*  are  added,  and 
more  calcium  carbonate  with  constant  stirring  until  finally  a  little 
of  the  latter  remains  undissolved.  To  the  slimy  contents  of  the 
beaker  700  c.c.  of  hot  water  are  added,  and  after  stirring,  the  in- 
soluble residue  is  allowed  to  settle,  which  requires  but  two  or  three 
minutes.  If  the  supernatant  liquid  is  violet,  on  account  of  the 
formation  of  calcium  permanganate,  one  or  two  drops  of  alco- 
hol are  added,  the  liquid  boiled,  and  the  precipitate  again  allowed 
to  settle;  in  this  case  the  upper  liquid  should  be  colorless.  If,  per- 
chance, it  should  be  still  colored,  the  treatment  with  the  alcohol 
must  be  repeated.  The  clear  solution  is  then  decanted  through 
a  filter  which  is  placed  in  a  funnel  containing  a  platinum  cone  and 
connected  with  a  suction  flask.  The  precipitate  is  decanted  with 
300  c.c.  of  hot  water  four  times,  then  transferred  to  the  filter 
without  making  any  attempt  to  remove  the  last  portions  of  the 
precipitate  from  the  sides  of  the  beaker,  and  washed  with  the 
aid  of  suction  until  the  filtrate  will  no  longer  turn  iodo-starch 
paper  blue.  The  precipitate  together  with  the  filter  is  then  placed 
in  the  original  beaker  in  which  the  precipitation  took  place,  50  c.c. 
of  a  freshly-standardized  ferrous  sulphate  solution  containing  sul- 
phuric acid  are  added,  and  the  liquid  is  stirred  until  the  precipitate 
has  entirely  dissolved,!  leaving  behind  the  filter-paper  and  some- 
times small  amounts  of  undissolved  calcium  sulphate.  The  ex- 

*  Prepared  by  shaking  15  gms.  of  fresh  bleaching  powder  with  1  liter  of  water 
and  allowing  the  mixture  to  stand  until  the  supernatant  solution  is  clear. 

t  If  the  precipitate  should  not  completely  dissolve,  a  little  sulphuric 
acid  (1 : 1)  is  added  until  the  brown  color  entirely  disappears. 


644  VOLUMETRIC  ANALYSIS. 

cess  of  the  ferrous  sulphate  is  titrated  with  potassium  dichro- 
mate  solution.  In  order  to  compensate  any  error  that  may  arise 
from  the  presence  of  the  filter-paper,  an  equally  large  filter  is 
placed  in  the  ferrous  sulphate  solution,  when  it  is  standardized. 

The  calculation  is  simple: 

Assume  that  a  gms.  of  steel  are  dissolved  in  100  c.c.  of  the 

solution  and  of  this  20  c.c.  (  =—  gms.  steel)  were  taken  for  the 

\     5  / 

analysis;   further,   50   c.c.    of   ferrous   sulphate   solution  =  T   c.o. 

—  K2O207  and  50  c  c.  ferrous  sulphate+f-  gms.  substance  =  t  c.o. 
10  o 

— K2Cr2O7.  Consequently—  gms.  substance  =  (T—  t)  c.c. — K2Cr2O7. 
10  5 

Since  1000  c.c.  N.  K2Cr207  solution  =27.47  gms.  Mn,  then  1  c.c. 

N 

— K2O2O7  will  correspond  to  0.002747  gm.  Mn,  and  we  have 

(T—  t)  X 0.002747  gm.  Mn  in  -=-  gms.  steel  and  in  per  cent. 

(T7- OX  1.374 

-  =per  cent.  Mn. 
a 

Remark. — According  to  the  author's  experience,  this  method 
is  one  of  the  best  for  the  determination  of  manganese  in  iron  and 
steel.  As  regards  the  time  required,  four  determinations  can  be 
carried  out  together  within  four  hours.  It  is  not  particularly 
suited  to  the  analysis  of  alloys  rich  in  manganese. 

C.  lodimetry. 

The  fundamental  reaction  of  iodimetry  is  the  following: 
2Xa2S2O3  + 12  = 2NaI  +  Na2S4O6. 

If  to  a  solution  containing  an  unknown  amount  of  iodine  a 
little  starch  solution  is  added,  and  sodium  thiosulphate  solution 
is  run  in  from  a  burette,  the  blue  color  will  disappear  from  the 
solution  as  soon  as  the  iodine  has  all  been  reduced  to  hydriodic 
acid  (sodium  iodide)  in  accordance  with  the  above  equation.  This 
reaction  is  ore  rf  the  most  sensitive  reactions  used  in  analytical 
chemistry.  If,  therefore,  a  sodium  thiosulphate  solution  of  known 
strength  is  at  hand,  we  have  a  means  of  determining  not  only  iodine 
itself,  but  all  of  those  substances  (oxidizing  agents)  which  when 
treated  with  potassium  iodide  set  free  iodine,  or  evolve  chlorine 
when  acted  upon  by  hydrochloric  acid.  Consequently,  iodimetric 
processes  are  not  only  accurate  but  capable  of  most  general  appli- 


PREPARATION  OF  SODIUM  THIOSULPHATE  SOLUTION.        645 

cation.     For  most  analyses  a  —  sodium  thiosulphate  solution  and 

N 
a  —  iodine  solution  are  required,  and  starch  solution  as  indica- 

N 
tor.     In  some  few  cases  -      solutions  are  used. 


Preparation  of  Sodium  Thiosulphate  Solution. 

From  the  above  equation  it  is  evident  that  1  gm.-at.  1  =  1  gm.- 
mol.  Na2S2O3=l  gm.-at.  H.  Hence,  exactly  -fa  gm.-mol.  of  crys- 
tallized sodium  thiosulphate  (Na2S2O3+  5H2O)  must  be  taken  for  1 
liter  of  tenth-normal  solution.  Such  a  solution,  however,  would 
.rapidly  change  in  concentration,  some  of  the  salt  being  decomposed 
by  the  action  of  the  carbon  dioxide  in  the  distilled  water: 

1  .  Na2S2O3+  2H2CO3  =  2NaHC03+  H2S2O3, 
2.  H2S2O3  =  H2SO3+S, 

and  the  solution  would  become  stronger,  fdt  the  sulphurous  acid 
formed  reacts  with  more  iodine  than  the  corresponding  amount 
of  thiosulphate: 

H2S03+  21+  H20  =2HI+  H2S04. 

After  all  the  carbonic  acid  in  the  distilled  water  has  been  used  up, 
the  solution  can  be  kept  for  months  without  suffering  an  appreciable 
change  in  concentration  (see  p.  649). 

A  large  amount  of  the  thiosulphate  solution  (about  5  liters) 
is  prepared  by  roughly  weighing  out  the  required  amount  of  the 
commercial  salt  *  and  after  standing  for  from  eight  to  fourteen 
days,  the  solution  is  standardized  by  one  of  the  following  methods. 

Standardization  of  Sodium  Thiosulphate  Solution. 
1.  With  Pure  Iodine. 

Commercial  iodine  is  contaminated  with  chlorine,  bromine 
water,  and  sometimes  cyanogen;  it  must  be  purified.  For  this 


*  The  molecular  weight  of  Na2S2O3  +  5H2O  is  248.32.     To  prepare  1  liter 
—  solution  24.832  gms.  of  the  salt  are  necessary, 
25  gms.     For  5  liters,  125  gms.  should  be  weighed  out. 


of  —  solution  24.832  gms.  of  the  salt  are  necessary,  or,  in  round  numbers, 


mm 


646  VOLUMETRIC  ANALYSIS. 

purpose  5  or  6  gms.  of  the  commercial  product  are  ground  up  with 
2  gms.  potassium  iodide,  and  any  chlorine  or  bromine  present 
forms  with  this  potassium  chloride  or  bromide,  setting  free  an 
equivalent  amount  of  iodine.  The  mixture  is  placed  in  a  dry 
beaker,  B  (Fig.  92),  of  about  300  c.c.  capacity  and  upon  the  beaker 
is  placed  the  bulb-tube  K,  which  is  closed  at  one 
end.  The  latter  is  filled  with  water  at  the  room 
temperature  and  the  glass  is  surrounded  with  an 
asbestos  cylinder  (not  shown  in  the  illustration). 
The  beaker  is  then  placed  on  wire  gauze  and  heated 
over  a  small  flame.  The  iodine  sublimes  rapidly 
and  collects  as  a  crystalline  crust  on  the  bottom 
of  the  bulb-tube,  and  practically  none  of  it  is  lost. 
As  soon  as  the  evolution  of  violet  vapors  from  the 
bottom  of  the  beaker  has  practically  ceased,  the  sublimation  is 
complete.  The  flame  is  removed,  and  after  allowing  to  cool,  the  bulb- 
tube  K  is  removed  with  the  iodine  adhering  to  it.  Tn  order  to  remove 
the  latter,  a  current  of  cold  water  is  conducted  through  the  tube  a 
into  the  bulb  and  out  at  6.  This  causes  the  glass  to  contract 
somewhat  and  the  whole  of  the  iodine  crust  can  be  removed  by 
lightly  pushing  it  with  a  clean  glass  rod.  It  is  caught  upon  a 
watch-glass,  broken  up  into  large  pieces,  and  the  sublimation  is 
repeated  without  the  addition  of  potassium  iodide  at  as  low  a  tem- 
perature as  possible;  in  this  way  a  product  free  from  potassium 
iodide  is  obtained.  The  iodine  thus  prepared  is  ground  somewhat 
in  an  agate  mortar  and  dried  in  a  desiccator  containing  calcium 
chloride.  If  dried  over  sulphuric  acid,  some  of  the  latter  is  likely 
to  be  present  in  the  iodine.  Furthermore,  the  cover  of  the  desicca- 
tor must  not  be  greased,  for  grease  is  attacked  by  iodine  vapors, 
forming  hydriodic  acid,  which  might  cause  contamination. 
'  The  Weighing  Out  of  the  Iodine. — In  each  of  two  or  three 
small  weighing-tubes  with  tightly-fitting  glass  stoppers  are  placed 
2  to  2J  gms.  of  pure  potassium  iodide  free  from  iodate  and  J  c.c. 
of  water  (not  more),  the  tubes  are  stoppered  and  accurately 
weighed  by  the  method  of  swings.  The  tubes  are  then  opened, 
0.4-0.5  gm.  of  pure  iodine  is  added  to  each,  the  tubes  are  quickly 
stoppered  and  again  weighed;  the  difference  shows  the  amount 
of  iodine.  The  iodine  dissolves  almost  instantly  in  the  concen- 


- 


STANDARDIZATION  OF  SODIUM  THIOSULPHATE  SOLUTION.     647 

trated  potassium  iodide  solution.  One  of  the  tubes  is  then  placed 
in  the  neck  of  a  500-c.c.  Erlenmeyer  flask  which  is  held  in  an 
inclined  position  and  contains  200  c.c.  of  water  and  about  1  gm. 
of  potassium  iodide.  The  tube  is  dropped  to  the  bottom  of  the 
flask,  but  just  as  it  begins  to  fall  the  stopper  is  removed  and  allowed 
to  follow  it.  In  this  way  there  is  no  iodine  lost,  which  will  be 
the  case  if  the  contents  of  a  tube  are  washed  into  the  water.*  A 
solution  is  thus  prepared  containing  a  known  amount  of  iodine 
and  to  it  the  sodium  thiosulphate  solution  to  be  standardized 
is  added  from  a  Mohr  burette  until  the  liquid  is  pale  yellow 
in  color.  Now,  2  or  3  cc.  of  starch  solution  are  added  and  the 
solution  carefully  titrated  until  it  becomes  colorless.  From  the 
mean  of  two  or  three  determinations,  the  strength  of  the  thio- 
sulphate solution  is  calculated.  For  example,  it  was  found  that 

(a)  0.5839  gm.  iodine  required  50.07  c.c.  Na^Og  solution, 

or  1  c.c.  =  0.01  1661  gin.  iodine. 
(6)  0.5774  gm.      "  "       49.42  c.c.  Na2S203  solution, 

or  1  c.c.  =  0.01  1683  gm.  iodine. 

The  mean  value  is  1  c.c.  =  0.011672  gm.  iodine. 

If  this  number  is  divided  by  the  amount  of  iodine  which  would 
be  contained  in  1  c.c.  of  normal  iodine  solution,  the  normality 
of  the  sodium  thiosulphate  solution  will  be  obtained.  Thus,  in 

this  case  the  solution  is    '  ,,  =  0.09201  normal. 

2.  W'ith  Potassium  Biiodate  (C.  Thon)J 

If  a  solution  of  potassium  biiodate  is  added  to  a  solution  of 
potassium  iodide  containing  hydrochloric  acid,  the  following 
reaction  takes  place: 

121 


38995  15231) 

/  389  9")\ 
If,  therefore,  3.2496  gms.  f        '  —  j  of  pure  potassium  biiodate 

are  contained  in  one  liter  of  the  aqueous  solution,  10  c.c.  of  such  a 

*  Wagner  first  called  attention  to  this  fact,  and  it  has  been  confirmed  in 
the  author  's  laboratory. 

fZeitschr,  f.  anal/Chem.,  XVI  (1877),  p.  477. 


648  VOLUMETRIC  ANALYSIS. 

solution  on  being  treated  with  an  excess  of  potassium  iodide  and  hy- 
drochloric acid  will  set  free  exactly  as  much  iodine  as  would  be  con- 

N  . 
tained  in  10  c.c.  of  —  iodine  solution.     By  means  of  such  a  solution 

a  known  amount  of  iodine  may  be  obtained  at  any  time  and  in 
this  way  the  solution  of  sodium  thiosulphate  may  be  standardized. 
At  present  it  is  possible  to  obtain  commercially  very  pure  potas- 
sium biiodate,  but  the  product  is  seldom  pure  enough  for  the 

N 
preparation  of  a  —  solution.     It  is  better  to  prepare  a  solution 

by  weighing  out  3.2496  gms.  for  1  liter  and  determining  the 
concentration  accurately  by  titrating  it  against  a  solution  of 
thiosulphate  which  has  been  freshly  standardized  against  pure 
iodine.  In  this  way  a  solution  is  obtained  which  can  be  con- 
veniently used  from  time  to  time  for  testing  the  concentration 
of  the  thiosulphate  solution. 

Method  of  Titrating. — One  or  two  grams  of  pure  potassium 
iodide  are  placed  in  a  beaker,  dissolved  in  as  little  water  as  possible, 
and  to  this  5  c.c.  of  hydrochloric  acid  (1  :  5),  and  then  20-25  c.c. 
of  the  biiodate  solution  are  added- ^ever  in  the  reverse  order). 
Iodine  is  liberated,  immediately  and  quantitatively.  After  dilut- 
ing with  about  200  c.c.  of  distilled  water,  the  iodine  is  titrated  as 

under  1. 

•• 

3.  With  Potassium  Permanganate  (Volhard)  * 

On  adding  potassium  permanganate  solution  to  an  acid  solu- 
tion containing  potassium  iodide,  the  permanganate  is  reduced 
to  manganous  salt,  while  an  equivalent  amount  of  iodine  is  set 
free  from  the  iodide: 

2KMnO4  +  10KI  +  16HC1  =  12KC1  +  2MnCl2 + 8H2O  + 101. 

If  an  accurately-standardized  solution  of  potassium  perman- 
ganate is  at  hand,  it  can,  therefore,  be  used  advantageously  for 
the  standardization  of  the  sodium  thiosulphate  solution.  The  pro- 
cedure is  the  same  as  was  described  with  the  potassium  biiodate 
solution. 

*  Ann.  d.  Chem.  u.  Pharm.,  242,  p.  98. 


PERMANENCE  OF  A  SODIUM  1HIOSULPHATE  SOLUTION.      649 

4.  With  Potassium  Dichr ornate. 

Similarly,  an  acid  solution  of  potassium  iodide  (1:10)  will, 
in  the  cold,  quantitatively  reduce  chromic  acid  to  green  chromic 
salt,  setting  free  an  equivalent  amount  of  iodine:* 

K2Cr207 + 6KI  +  14HC1  =  8KC1  +  2CrCl3  +  7H2O  +  61. 

By  weighing  out  4.903  gms.  of  pure,  dry  potassium  dichro- 
mate  a  tenth-normal  solution  is  prepared  and  a  measured  amount 
of  it  is  added  to  the  acid  solution  containing  about  3  gms.  of 
potassium  iodide  and  10  c.c.  strong  hydrochloric  acid.  In 
this  case,  however,  the  solution  is  diluted  with  500-600  c.c.  of 
water,  for  here  the  color  change  is  not  from  blue  to  colorless  but 
from  blue  to  light  green.f  With  too  concentrated  solutions  the  end- 
point  is  indistinct,  so  that  a  considerable  dilution  is  necessary. 

Permanence  of  —  Sodium  Thiosulphate  Solutions. 

A  two-months-old  sodium  thiosulphate  solution  was  stand- 
ardized against  pure  iodine  in  June,  1899,  and  its  concentration 
found  to  be 

1  c.c.  =  0.011672gm.  I. 

In  March,  1900,  or  about  eight  months  later,  the  same  solution 
of  thiosulphate  was  again  standardized  and  found  to  be 

1  c.c.  =  0.011667. 

At  the  end  of  eight  months,  therefore,  the  concentration  of 
the  solution  was  practically  unchanged.  Frequently  the  addition 
of  ammonium  carbonate  is  recommended  in  order  to  obtain  a  more 
permanent  solution;  it  has  the  opposite  effect. 

Preparation  of  —  Iodine  Solution. 
10 

There  is  no  advantage  to  be  obtained  by  dissolving  the  theo- 
retical amount  of  sublimed  iodine  in  a  definite  volume  of  solution, 

*  The  solution  should  be  quite  acid  with  hydrochloric  acid.  In  very  dilute 
sulphuric  acid  solutions  the  chromic  acid  is  reduced  very  slowly  if  at  all. 

f  In  all  these  methods  starch  solution  is  added  toward  the  end  of  the  reac- 
tion. See  p.  647. 


650  VOLUMETRIC  ANALYSIS. 

for  the  latter  cannot  be  kept  very-  long  unchanged.  It  is  more 
practical  to  prepare  the  iodine  solution  by  placing  20-25  gm*.  of 
pure  potassium  iodide  in  a  liter  flask  dissolving  it  in  as  little  water 
as  possible  and  then  adding  about  12.7  gms.  of  commercial  iodine, 
weighed  out  roughly  on  a  watch-glass.  The  contents  of  the  flask 
are  shaken  until  the  iodine  is  all  dissolved.  When  this  is  accom- 
plished, the  solution  is  diluted  up  to  the  mark  with  water  and 
standardized  according  to  one  of  the  following  methods. 

V 
1.  With  ~—  Sodium  Thiosulphate  Solution. 

Of  the  thoroughly  mixed   iodine  solution,  25  c.c.  are  titrated 
with  the  standard  sodium  thiosulphate  solution. 

N 
If  25  c.c.  of  iodine  solution  require  25.16  c.c.  of 


N 
solution,  1  c.c.  of  the  former  =1.0064  c.c.  of  —  solution,  or,   in 

other  words,  the  solution  is  0.10064  normal. 

A7" 
2.  With  —  Arsenious  Acid. 

If  iodine  is  allowed  to  act  upon  a  solution  of  arsenious  acid 
the  reaction  which  takes  place  may  be  expressed  as  follows: 

H3  As03  +  12  +  H2O  <=±  H3AsO4  +  2HI. 

This  reaction  can  be  made  to  go  completely  in  either  directions 
according  to  the  conditions.* 

If  the  hydriodic  acid  is  immediately  removed  from  the  solu- 
tion as  fast  as  it  is  formed,  the  reaction  will  proceed  quantita- 
tively in  the  direction  from  left  to  right,  In  the  presence  of 
sufficient  hydrochloric  acid,  however,  the  reaction  will  take 
place  completely  in  the  opposite  direction.  When  it  is  desired 
to  make  the  oxidation  of  the  arsenious  acid  quantitative,  there- 
fore, there  should  be  but  very  little  hydrogen  ions  in  solution 
at  any  time;  in  other  words,  the  solution  must  remain  as  nearly 
neutral  as  possible.  The  presence  of  free  alkali  is  not  permis- 
sible because  any  appreciable  concentration  of  the  hydroxyl 
ion  reacts  with  iodine  to  form  iodide,  hypoiodite,  and  eventually 
iodate. 

Alkali  bicarbonates  are  without  action  upon  iodine,  so  that 

*  E.  W.  Washburn,  J.  Am.  Chem.  Soc.,  30,  21  (1908). 


PREPARATION  OF  A   STANDARD  IODINE  SOLUTION         651 

sodium  bicarbonate  is  used  for  the  neutralization  of  the  hydriodic 
acid  formed  by  the  above  reaction. 

From  the  equation,  it  is  evident  that  1  gm.-at.  1=    '*   3=='~T:  = 

49.5  gms.  As2O3,  and  —  gm.-at.  I  corresponds,  therefore,  to  -1.95 

N 
gms.  As-P^the  amount  necessary  for  1000  c.c.  of  —  solution. 

N 
For  the  preparation  of  the   rr  arsenious   acid   solution,   the 

vitreous  form  of  commercial  As203  is  sublimed  from  a  porcelain  dish 
upon  a  watch-glass.  If  arsenic  trisulphide  is  present  (shown  by  a 
yellow  sublimate  being  first  formed)  the  preparation  must  be  previ- 
ously purified.  For  this  purpose  it  is  dissolved  in  hot  hydrochloric 
acid  (1:2),  the  insoluble  sulphide  filtered  off,  and  the  arsenic 
trioxide  caused  to  deposit  by  cooling  the  filtrate.  After  pour- 
ing off  the  mother-liquor,  the  crystals  are  washed  several  times 
with  water,  dried  on  the  water-bath,  and  the  pure  substance 
obtained  by  sublimation.  After  standing  for  twelve  hours  in 
a  desiccator  over  calcium  chloride,  4.95  gms.  of  the  oxide  are 
accurately  weighed  out  into  a  porcelain  dish  and  dissolved  by 
warming  \vith  a  little  concentrated  sodium  hydroxide  solution. 
After  two  or  three  minutes  all  will  be  dissolved.  The  solution  is 
now  poured  through  a  funnel  into  a  graduated  liter  flask,  and 
the  dish  carefully  washed  out  with  water.  A  drop  of  phenol- 
phthalem  is  added  to  the  contents  of  the  flask  and  pure  dilute 
sulphuric  acid  until  the  solution  is  decolorized.  About  20  gms. 
of  sodium  bicarbonate  are  dissolved  in  500  c.c.  of  water  and  the 
filtered  solution  is  added  to  the  barely-acid  contents  of  the  flask. 
If  the  mixture  reacts  alkaline  (shown  by  the  red  color  of  the  phe- 
nolphthalein) ,  a  few  drops  of  sulphuric  acid  are  added  until  it  be- 
comes colorless,  after  which  the  solution  is  diluted  up  to  the  mark 
with  water.  After  thoroughly  mixing,  a  burette  is  filled  with  it  and 
titrated  against  a  measured  amount  of  iodine  solution  as  under  1. 

3.   With  Anhydrous  Sodium  Thiosulphate* 
Anhydrous  sodium  thiosulphate  may  be  prepared  in  a  state  of 
sufficient  purity  to  permit  its  use  for  standardizing  iodine  solutions. 
A  saturated  solution  of  the  commercial  salt  is  prepared  at  30°  to 

*  S.  W.  Young,  J.  Am.  Chem.  Soc.,  26,  1028  (1904). 


652  VOLUMETRIC  ANALYSIS. 

35°  and  then  cooled  while  stirring  constantly.  The  salt  thus 
obtained  is  dehydrated  over  sulphuric  acid  until  it  has  fallen  to  a 
powder,  and  a  little  of  it  in  a  test-tube  shows  no  sign  of  fusion 
when  heated  to  50°.  The  final  dehydration  is  effected  by 
heating  at  80°  with  repeated  stirring  of  the  powder. 

Young  standardized  a  solution  of  iodine  by  this  method  and 
obtained  the  same  value  as  by  titrating  against  a  thiosulphate 
solution  which  had  been  standardized  against  pure  iodine. 

The  Starch  Solution. 

About  5  gms.  of  powdered  starch  are  nibbed  into  a  paste  with 
a  little  cold  water,  and  the  paste  is  slo\\ly  added  to  a  liter  of  boiling 
water  contained  in  a  porcelain  dish.  The  boiling  is  continued 
for  one  or  two  minutes  so  that  an  almost  clear  solution  is  obtained 
The  liquid  is  cooled  by  placing  the  dish  in  cold  water,  and  after 
standing  overnight  the  clear  liquid  is  filtered  into  small  50-c.c. 
medicine  bottles.  These  are  placed  in  a  water-bath  and  filled 
up  to  the  neck  with  the  starch  solution,  heated  two  hours,  and 
closed  by  means  of  soft  stoppers  before  removing  from  the  hot- 
water  bath.  The  solution  thus  sterilized  can  be  kept  almost 
indefinitely  without  the  slightest  trace  of  mould  formation.  Such 
a  solution  prepared  according  to  the  above  directions  by  H.  N. 
Stokes  remained  perfectly  clear  after  standing  H  years  and  was 
as  sensitive  then  as  when  first  made  up.  After  opening  the  bottle, 
mould  begins  to  form  within  one  week,  which  explains  why  the 
solution  is  poured  into  small  bottles;  it  may  then  be  used  before 
it  becomes  spoiled. 

It  is  nowadays  much  more  convenient  to  use  the  Zulkowsky 
"soluble  starch,"  which  is  obtained  commercially  in  the  form 
of  a  paste.  The  reagent  is  prepared  by  dissolving  a  little  of  the 
paste  in  cold  water. 

Sensitiveness  of  the  lodo-Starch  Reaction. 

As  already  mentioned  in  Vol.  I,  p.  267,  iodine  produces  a 
blue  color  with  starch  only  when  hydriodic  acid  or  a  soluble  iodide 
is  present,  and  further  the  formation  of  the  blue  color  depends 
not  only  upon  the  presence  of  iodide  but  is  largely  influenced 
by  the  concentration  of  the  iodide  solution.  With  the  same 
amount  of  iodide  and  different  volumes  of  liquid  quite  different 


SENSITIVENESS   OF   THE  1ODO-STARCH  REACTION.          653 

amounts  of  iodine  are  necessary  to  produce  the  blue  color.  From 
this  it  is  evident  that  in  any  iodimetric  analysis  about  the  same 
concentration  should  be  maintained  as  in  the  case  of  the  star*- 

N 
ardization  of  the  solutions  used  for  the  analysis.    When  —  solutions 

are  used,  the  error  produced  by  not  following  this  rule  is  a  small 
one  and  for  most  purposes  can  be  neglected.  On  the  other  hand, 

N 
when  an  analysis  is  made  with  —^  solutions,  a  large  error  may 

be  introduced. 

To  show  what  the  error  can  amount  to,  the  following  results 
will  be  given.  To  each  of  the  following  amounts  of  water,  1.5  c.c. 

N 
of  starch  solution  were  added  and  then  -^  iodine  solution  until 

1UU 

a  barely-visible  coloration  was  obtained. 

N 

c.c.  Water.  --—  Iodine  Solution. 

1UU 

50 0.15  c.c. 

100 0.30   " 

150 0.47   " 

200 0.64   " 

These  experiments  were  repeated  using  3  c.c.  of  the  starch 
solution  with  almost  the  same  results.  But  when  to  each  1 
gm.  of  potassium  iodide  was  added,  the  following  results  were 
obtained : 

Water.  -^-  Iodine  Solution. 

1UU 

50c.c.  +  lgm.  KI 0.04  c.c. 

100  "  +1    "     "  O.C4  " 

150  "  +1    "     "  0.04  " 

200  "  +1    "     " 0.14  " 

500  "  +1    "     "  0.32  " 

500  "  +3gms."  0.32  " 

620  "  +3    "     "  0.32  " 

The  results  show  that  the  amount  of  iodine  solution  necessary 


654  VOLUMETRIC  ANALYSIS. 

to  produce  the  blue  color  in  the  absence  of  potassium  iodide  *  is 
directly  proportioned  to  the  dilution.  If  the  solution  contains 
1  gm.  of  potassium  iodide,  a  blue  color  will  be  produced  by  the 
same  amount  of  iodine  solution  as  long  as  not  more  than  150  c.c. 
of  solution  are  present,  but  with  a  greater  volume  than  that,  more 
iodine  is  necessary  independent  of  whether  the  solution  contains 
1  gm.  or  more  of  potassium  iodide. 

In  order  to  show  the  action  of  the  iodide  more  distinctly,  a 
very  dilute  iodine  solution  was  added  to  50  c.c.  of  water  containing 
starch  solution  and  in  the  absence  of  iodide,  15  c.c.  were  added 
before  the  blue  color  was  permanent.  After  adding  1  gm.  of 
potassium  iodide,  it  was  only  necessary  to  add  1.5  c.c.  of  the 
dilute  iodine. 

When  solutions  were  used  without  the  addition  of  potassium 
iodide,  the  same  amount  of  iodine  solution  (0.03  c.c.f)  was 
necessary  when  not  more  than  300  c.c.  of  water  were  present. 
With  600  c.c.  of  water,  0.06  c.c.  of  iodine  was  necessary,  and  with 
1000  c.c.  it  was  found  that  0.15  c.c.  of  iodine  solution  was  re- 
quired. On  the  other  hand,  when  the  solution  contained  1  gm. 
of  potassium  iodide,  only  0.06  c.c.  of  iodine  was  necessary  in 
1000  c.c.  of  liquid. t 

ANALYSES  BY  IODIMETRIC  PROCESSES. 

i.  Determination  of  Free  Iodine. 

N 
1000  c.c.  —  iodine  solution  =  12.692  gm.  I. 

The  iodine  is  dissolved  in  a  solution  of  potassium  iodide.  The 
solution  is  titrated  either  with  sodium  thiosulphate  or  with  arse- 
nious  acid  exactly  as  described  under  the  standardization  of  an 
iodine  solution. 

2.  Determination  of  Chlorine  in  Chlorine  Water. 

N 
1000  c.c.  —  iodine  solution  =  3. 546  gm.  Cl. 

*  With  the  exception  of  the  potassium  iodide  contained  in  the  iodine 
solution  itself. 

t  0.03  c.c.  =  l  drop. 

%  The  temperature  of  the  solution  also  exerts  an  influence.  Other  things 
being  equal,  the  end-point  is  best  obtained  in  a  cold  solution. — [Translator.] 


DETERMINATION  OF  HYPOCHLOROUS  ACID.  655 

A  measured  amount  of  chlorine  water  is  added  to  a  solution 
containing  an  excess  of  potassium  iodide.  The  point  of  the 
pipette  should  be  held  just  above  the  surface  of  the  iodide  solution 
and  the  latter  should  be  contained  in  a  glass-stoppered  bottle. 
After  the  chlorine  water  has  been  added,  the  contents  of  the 
bottle  are  vigorously  shaken,  and  the  iodine  set  free  is  titrated 
with  sodium  thiosuiphate  as  above: 

KI  +  C1=KC1+I. 
3.  Determination  of  Bromine  in  Bromine  Water. 

1000  c.c.  T-Q  iodine  solution  =7. 992  gm.  Br. 

The  procedure  is  the  same  as  under  2: 
KI  +  Br=KBr+I. 

4.  Determination  of  Hypochlorous  Acid  in  the  Presence  of 

Chlorine. 

The  determination  is  based  upon  the  following  reactions: 

HOC!  +  2KI  =  KC1 + KOH  + 12 ; 
C12+2KI  =  2KC1+I2. 

1  gm.-mol.  of  hypochlorous  acid  sets  free  1  gm.-mol.  of  iodine, 
but  produces  at  the  same  time  1  gm.-mol.  of  potassium  hydroxide, 
while  the  chlorine  simply  sets  free  an  equivalent  amount  of  iodine. 
After  neutralizing  the  alkali  by  means  of  an  excess  of  hydrochloric 
acid  and  determining  the  iodine  by  titration  with  sodium  thio- 
suiphate, the  excess  of  hydrochloric  acid  is  titrated  wdth  standard 
alkali  solution. 

N 
Procedure.— A.  measured  volume  of  —  hydrochloric    acid    is 

added  to  a  potassium  iodide  solution,  to  this  a  known  amount 
of  the  mixture  of  chlorine  and  hypochlorous  acid  is  added,  and  the 

N 
iodine  set  free  is  titrated  with  -^  thiosuiphate  solution.     The  now 

colorless  solution  is  treated  with  methyl  crange  and  the  excess  of 

N 
hydrochloric  acid  is  titrated  with  —  NaOH.    The  KOH  produced 

by  the  action  of  the  hypochlorous  acid  upon  the  iodide  requires 


656  VOLUMETRIC  ANALYSIS. 

N  N 

half  as  much  —  acid  for  neutralization  as  are  required  of  — 

solution  to  react  with  the  iodine  set  free  by  the  action  of  the  hypo- 
chlorous  acid. 

Example. — If  V  c.c.  of  chlorine -fhypochlorous  acid  were  taken 

N  N 

for  analysis,  t  c.c.  ~  HC1  present  at  the  start,  T  c.c.  —  Na2S2O3 

N 
used  for  titrating  the  iodine,  and  ^  c.c.  ^-  NaOH  for  titrating  the 

N 
excess  of  acid,  then  t—t^  c.c.  T^  acid  were  required  to  neutralize 

N 
the   potassium   hydroxide  and    2(t  —  tj  c.c.  JQ  Na2S2O3  to    react 

vvith  the  iodine  formed  from  the  hypochlorite. 
Hence  (t-ti)  0.005247  *=gm.  HOC1  in  V  c.c.  solution 

and 

T-2(t-tl)  0.003546  =  gm.  Cl  in  V  c.c.  solution. 

5.  Determination  of  Iodine  in  Soluble  lodides.f 

(a)  By  Decomposition  with  Ferric  Salts. 

If  a  solution  of  a  soluble  iodide  is  treated  with  an  excess  of 
iron-ammonium  alum  and  acidified  with  sulphuric  acid,  the 
ferric  salt  will  be  reduced  to  ferrous  salt  with  separation  of  iodine: 

Fe2(S04)  3  +  2HI  =  H2S04  +  2FeSO<  + 12. 

If  the  solution  is  heated  to  boiling,  the  iodine  escapes  with 
the  steam  and  can  be  collected  in  a  solution  of  potassium  iodide 
and  then  titrated  with  sodium  thiosulphate  or  arsenious  acid. 
This  method  is  suited  for  separating  iodine  from  bromine,  for 
bromides  do  not  reduce  ferric  salts.  The  bromide  will  be  found 
in  the  residue  obtained  after  the  distillation,  and  is  best  deter- 
mined gravimetrically. 

*  HOC1  =  52.47;  1  c.c.  ^  solution  =  0.005247  gin.  HOC1  (against  XaOH). 

f  In  the  case  of  insoluble  iodides,  the  metal  must  first  be  removed  if  the 
iodine  is  to  be  determined  volumetrically.  This  can  be  accomplished  by 
the  method  of  Mensel  (Z.  anal.  Chem.,  12,  137).  It  may  be  said,  however, 
that  the  volumetric  method  offers  no  advantages  over  the  gravimetric  one. 


DETERMINATION  OF  IODINE  IN  SOLUBLE  IODIDES.         657 

(b)  By  Decomposition  with  Nitrous  Add  (Fresenius). 

This  excellent  method,  which  is  especially  suited  for  deter- 
mining small  amounts  of  iodine  in  the  presence  of  bromine  and 
chlorine  in  mineral  waters,  depends  upon  the  easy  oxidation  of 
hydriodic  acid  by  means  of  nitrous  acid: 

2HI  +  2HNO2=2H2O  +  2NO  +  2I. 

Hydrochloric    and    hydrobromic    acids    are   not   attacked   by 

nitrous  acid. 

Procedure. — In   the   small    apparatus   shown   in   Fig.    93   the 

neutral  or  slightly  alkaline  solution  of  the  iodide  is  placed;  it  is 
slightly  acidified  with  dilute  sulphuric  acid,  and 
a  little  freshly-distilled,  colorless  carbon  bisulphide 
(or  chloroform)  is  added,  so  that  it  does  not  quite 
reach  to  the  stop-cock,  near  the  bottom  of  the  tube. 
Then  two,  or  at  the  most  three,  drops  of  "nitrose"* 
are  added,  the  tube  stoppered  and  vigorously 
shaken,  after  which  the  carbon  bisulphide  is 
allowed  to  settle  once  more.  The  small  amount 
of  the  latter  which  at  first  adheres  to  the  glass 
sides  is  made  to  run  to  the  bottom  by  revolving 
and  inclining  the  tube.  On  the  upper  surface  of 
the  liquid  there  will  still  remain  a  few  tiny  drops 
of  carbon  bisulphide.  To  obtain  these  a  funnel 
containing  a  filter  moistened  with  water  is  placed 
under  the  glass  stop-cock,  the  stopper  is  removed 
from  the  tube  and  the  aqueous  solution  is 
allowed  to  run  through  the  filter,  but  the  carbon 
bisulphide  will  remain  behind  on  the  paper. 
The  carbon  bisulphide  remaining  in  the  tube  is 
shaken  three  times  writh  successive  portions  of 
distilled  water,  and  each  time  the  latter  is  allowed 
to  run  off  through  the  same  filter.  The  funnel  is 
then  placed  at  the  top  of  the  tube,  punctured  with 
FIG  93.  a  pointed  glass  rod,  and  the  carbon  bisulphide 

washed  into  the  tube  by  means  of  about  0.5  c.c.  of  water.     After 
*  Cf.  Vol.  I,  p.  285. 


658  VOLUMETRIC  ANALYSIS. 

this  one  or  two  drops  of  sodium  bicarbonate  solution  are  added 
and  thoroughly  shaken  with  the  carbon  bisulphide,  then  standard 
sodium  thiosulphate  solution  is  added  until  the  reddish-violet 
carbon  bisulphide  solution  becomes  colorless. 

The  value  of  the  sodium  thiosulphate  solution  is  not  determined 
as  ordinarily,  but  by  means  of  a  potassium  iodide  solution  treated 
as  above  described. 

Remark.  —  This  method  is  useful  for  determining  small  amounts 
of  iodine  in  the  presence  of  relatively  large  amounts  of  chlorine 
and  bromine,  as  in  the  analysis  of  mineral  waters.  For  the  stand- 
ardization of  the  sodium  thiosulphate  solution,  as  nearly  as  possible 
the  same  amount  of  potassium  iodide  is  used  as  is  present  in  the 
unknown  solution;  this  is  determined  by  the  color  of  the  carbon 
bisulphide.  Pure  potassium  iodide  must  be  used  for  this  purpose, 
and  its  purity  tested  by  means  of  a  gravimetric  determination  of 
the  iodine  present  in  the  salt  after  it  has  been  dried  at  170°-180°  C. 

The  reason  the  sodium  thiosulphate  solution  must  be  stand- 
ardized in  this  way  is  as  follows: 

When  an  aqueous  solution  containing  iodine  is  shaken  with 
carbon  bisulphide,  not  all  of  the  iodine  but  the  greater  part  of 
it  will  pass  into  the  latter  solvent.*  The  error  is  compensated, 
however,  by  standardizing  the  solution  in  the  same  way. 

*  If  the  solution  of  a  substance  is  shaken  with  another  solvent  in.  which 
the  former  does  not  mix,  the  original  amount  of  the  substance  divides  itself 
between  the  two  solvents,  and  in  fact  the  concentration  of  one  solution 
(amount  of  the  dissolved  substance  present  per  cubic  centimeter)  always 
bears  a  constant  relation  to  that  of  the  other. 

Thu3  if  xQ  gms.  of  iodine  are  dissolved  in  V  c.c.  of  water,  and  (he  solution 
is  shaken  with  Fx  c.c.  of  carbon  bisulphide,  then  xl  gms.  of  iodine  will  remain 
in  ths  aqueous  solution  and  XO—XL  gms.  will  pass  into  the  carbon  bisul- 
phide. 

The  amount  x  is  found  by  the  following  equation: 


—  and    °       *  are  the  concentrations  in  each  of  the  solutions  and  k  is  the  dis- 

tribution coefficient,  which  is  T^jy  for  iodine.1     If  the  aqueous  solution  is  now 

shaken  with  the  same  amount  of  fresh  carbon  bisulphide,  then  x%  gms.  of 

^Berthelot  and  Jungfleisch,  Comptes  rend.,  69.  P  338. 


DETERMINATION  OF  BROMINE  IN  SOLUBLE  BROMIDES.       659 

If,  after  shaking  with  carbon  bisulphide,  the  aqueous  solution 
still  appears  yellow,  it  must  be  treated  a  second,  and  perhaps 
a  third,  time  with  fresh  amounts  of  carbon  bisulphide. 

6.  Determination  of  Bromine   in   Soluble  Bromides  (Bunsen). 

If  chlorine  water  is  added  to  a  colorless  bromide  solution  in 
a  porcelain  dish,  the  solution  becomes  yellow: 

KBr+Cl  =  KCl  +  Br. 

If  it  is  heated  to  boiling,  the  bromine  is  expelled  arid  the  solu- 
tion becomes  colorless  again.  The  addition  of  the  chlorine  water 
is  continued  until  finally  no  yellow  coloration  is  produced. 

Preparation  and  Standardization  of  the  Chlorine  Water. 

100  c.c.  of  a  saturated  chlorine  water  are  diluted  to  500  c.c. 
and  titrated  against  a  weighed  amount  of  pure  potassium  bromide 
which  has  been  dried  at  170°  C.,  the  same  amount  of  bromide  being 
taken  for  the  standardization  as  is  supposed  to  be  present  in 
the  solution  to  be  analyzed.  During  the  titration,  the  burette 
containing  the  chlorine  water  is  enveloped  in  black  paper  to  pro- 
tect its  contents  from  the  light,  and  the  tip  of  the  burette  is  held 

iodine  will  remain  in  the  water  and  xl—x2  will  be  extracted  by  the  carbon 
bisulphide.     In  this  case,  however, 


(kV    \  2 
y  +VkJ    gms.  iodine, 


so  that  after  shaking  n  times  with  fresh  portions  of  carbon  bisulphide,  the 
amount  of  iodine  remaining  in  the  water  would  be : 

(3)    xn  =x0  [          ,     )     gms.  iodine. 


Assuming  that  in  the  analysis  0.005  gm.  of  iodine  was  dissolved  in 
10  c.c.  of  water  and  that  this  solution  was  shaken  once  with  1  c.c.  of  carbon 
bisulphide,  then  according  to  equation  (1) 

400X1°  1 

^ -0.005— ^-=0.005-^=0.0001  gm.  iodine 

1+400 
would  remain  dissolved  in  the  water,  or  an  amount  that  can  be  neglected. 


660  VOLUMETRIC  ANALYSIS. 

just  above  the  surface  of  the  hot  bromide  solution,  so  that  as 
little  chlorine  as  possible  is  lost  by  evaporation. 

7.  Determination  of  Iodine  and  Bromine  in  Mineral  Waters. 

According  to  the  amount  of  halogen  present,  from  5  to  60  liters 
of  water  are  taken  for  the  analysis. 

The  amount  of  bromine  and  iodine  present  is  usually  small 
compared  with  the  chlorine,  so  that  the  residue  obtained  by  the 
evaporation  of  a  large  amount  of  water  cannot  be  used  directly 
for  the  analysis,  but  by  partial  crystallization  a  mother-liquor 
rich  in  bromide  and  iodide  must  first  be  obtained. 

Procedure. — The  water  is  placed  in  a  large  porcelain  evaporating- 
dish,  a  liter  at  a  time,  and  if  not  already  alkaline,*  enough  pure 
sodium  carbonate  solution  is  added  to  make  it  distinctly  so,  and 
the  water  is  evaporated  to  about  one-fourth  of  its  original  volume. 
This  causes  the  separation  of  some  calcium  and  magnesium  car- 
bonates in  the  presence  of  hydroxides  of  iron  and  manganese, 
while  all  of  the  halogen  salts  remain  in  solution.  The  residue 
is  filtered  off  and  thoroughly  washed  with  water.  The  filtrate 
is  further  concentrated  until  salts  begin  to  crystallize  out,  and 
the  hot  solution  is  then  poured  into  three  times  its  volume  of 
absolute  alcohol ;  this  causes  the  greater  part  of  the  sodium  chloride 
and  other  undesired  salts  to  precipitate.  After  standing  twelve 
hours,  the  alcoholic  liquid  is  filtered  and  the  residue  washed  five 
or  six  times  with  95  per  cent,  alcohol. 

The  alcoholic  solution,  which  contains  all  of  the  iodine  and 
bromine  with  considerable  chlorine  in  the  form  of  the  alkaline 
salts,  is  treated  with  five  drops  of  concentrated  potassium  hydroxide 
solution  and  almost  all  of  the  alcohol  distilled  off,  while  a  current  of 
air  is  passed  through  the  solution  by  means  of  a  capillary  tube 
reaching  to  the  bottom  of  the  liquid  in  the  distilling-flask. 

The  residue  from  the  distillation  is  further  concentrated  until 
salts  again  begin  to  crystallize  out  and  the  precipitation  with  alco- 
hol is  repeated.  The  alcohol  is  again  distilled  off,  but  this  time 
with  the  addition  of  only  one  or  two  drops  of  potassium  hydroxide 

*  The  solution  is  alkaline  if  after  the  addition  of  phenolphthalei'n  the 
solution  turns  red  on  boiling. 


DETERMINATION   OF  IODINE  AND  BROMINE.  661 

solution.  According  to  the  amount  of  salts  present  in  solution 
this  operation  is  repeated  from  three  to  six  times.  The  final 
filtrate,  after  the  alcohol  has  been  distilled  off,  is  placed  in  a  plati- 
num dish,  evaporated  to  dryness,  the  dish  covered  with  a  watch- 
glass,  and  the  residue  gently  ignited  to  destroy  organic  matter. 
The  residue  from  the  ignition  is  dissolved  in  a  little  water,  the 
carbonaceous  material  filtered  off,*  the  solution  slightly  acidified 
with  dilute  sulphuric  acid,  the  iodine  liberated  by  the  addition  of 
one  or  two  drops  of  "nitrose,"  and  titrated  with  sodium  thio- 
sulphate,  after  shaking  with  chloroform,  as  described  on  p.  657. t 
The  bromine  is  determined  in  the  aqueous  solution  obtained  after 
the  extraction  of  the  iodine  with  chloroform.  The  acid  solution  is 
made  alkaline  by  the  addition  of  sodium  carbonate  solution,  two 
drops  of  a  saturated  sugar  solution  are  added,  and  the  solution 
evaporated  to  dryness  in  a  platinum  dish.  With  a  watch-glass 
upon  the  dish,  the  residue  is  gently  ignited  in  order  to  destroy  the 
sugar  and  the  excess  of  nitrite. t  After  this  has  been  accomplished 
the  residue  is  dissolved  in  water,  filtered,  acidified  slightly  with 
sulphuric  acid,  and  the  bromine  titrated  with  chlorine  water  as 
described  on  p.  659. 

Remark. — If  sufficient  mineral  water  is  available  it  is  better  to 
divide  the  mother-liquor  containing  the  bromide  and  iodide  into 
two  portions;  in  one  portion  the  iodine  io  determined  as  before, 
while  in  the  other  the  bromine  and  iodine  are  determined  by 
titration  with  chlorine  water. § 

8.  Analysis  of  Peroxides  (Bunsen). 

All  peroxides  of  the  heavy  metals,  which  evolve  chlorine  on 
treatment  with  hydrochloric  acid,  can  be  determined  with  great 

*  If  the  filtrate  is  not  completely  colorless,  it  is  evaporated  and  again 
ignited. 

t  Lecco  determines  the  iodine  colori metrically  (Zeitschr.  f.  anal.  Chem., 
XXXV,  p.  318). 

t  The  addition  of  the  sugar  causes  the  nitrite  to  be  destroyed  at  a  lower 
temperature  than  would  otherwise  be  the  case,  and  the  danger  of  losing 
bromine  by  volatilization  is  avoided. 

§  As  the  chlorine  water  was  standardized  against  bromine,  an  amount 
of  the  latter  equivalent  to  the  iodine  present  is  deducted  from  the  amount 
represented  by  the  chlorine  water  used;  the  difference  shows  the  bromina 
present. 


662  I/O LU METRIC  ANALYSIS. 

accuracy  by  conducting  the  chlorine  into  potassium  iodide  solu- 
tion and  titrating  the  deposited  iodine  with  sodium  thiosulphate 
or  arsenious  acid  solution.  It  is  only  necessary  to  make  sure 
that  the  chlorine  is  allowed  to  act  upon  the  potassium  iodide 
without  loss.  For  all  such  determinations,  Bunsen  employed  the 
apparatus  shown  in  Fig.  94.  The  small  decomposition-flask  of  about 
40  c.c.  capacity  has  a  ground-glass  connection  with  the  delivery- 


B 


FIG.  94. 


tube  *  and  is  held  firmly  in  place  by  means  of  rubber  rings,  as  at  a. 
The  lower  end  of  the  bent  delivery-tube  is  drawn  out  into  a  not- 
too-small  capillary. 

Procedure. — The  finely-powdered  substance  is  placed  in  the 
small  glass-stoppered  weighing-tube  (Fig.  94  B),  which  has  a 
small  piece  of  glass  fused  on  the  end,  and  weighed.  The  tube  is 
then  taken  hold  of  by  means  of  the  glass  at  the  bottom, f  intro- 
duced into  the  neck  of  an  absolutely  dry  decomposition-flask, 
and  the  required  amount  of  the  substance  is  allowed  to  fall  into 
it  by  carefully  revolving  the  weighing-tube.  On  again  weighing 
the  tube,  the  amount  of  substance  taken  is  determined.  Hydro- 
chloric acid  is  now  added  (its  concentration  depends  upon  the 
nature  of  the  substance),  the  delivery  tubing  is  at  once  connected 
with  the  flask  and  introduced  into  the  retort  containing  potassium 
iodide  solution.  By  means  of  a  tiny  flame,  the  contents  of  the 
flask  are  heated  to  boiling  and  from  half  to  two-thirds  of  the  liquid 

*  Instead  of  the  ground-glass  connection,  Bunsen  used  a  tube  of  the 
same  size  as  the  neck  of  the  flask  and  connected  them  with  rubber  tubing, 
the  two  glass  tubes  being  against  one  another. 

t  By  holding  the  tube  in  this  way,  deviations  of  weight,  due  to  unequal 
warming,  are  avoided. 


DETERMINATION  OF  MANGANESE  DIOXIDE.  665 

is  distilled  over  into  the  retort.  In  order  to  prevent  the  iodide 
solution  from  sucking  back  into  the  flask,  the  delivery-tube  is 
taken  out  of  the  retort  before  removing  the  flame;  the  contents 
of  the  tube  are  then  washed  into  the  retort. 

The  potassium  iodide  solution  is  poured  into  a  large  beaker, 
the  retort  washed  out  several  times  with  a  little  water,  and  then 
\vith  potassium  iodide  solution  in  order  to  remove  any  iodine 
which  may  remain  adhering  to  the  glass.  The  iodine  is  titrated  with 

N 

—  sodium  thiosulphate  solution.     In  this  way  pyrolusite,  chro- 

mates,  lead  peroxide,  minium,  eerie  oxide,  selenic,  telluric,  and 
molybdic  acids  may  be  analyzed. 

(a)  Determination  of  Manganese  Dioxide  in  Pyrolusite. 


1000  c.c.  ^  Na2S2O3  solution  =~p»  =          =  4.347  gms.  MnO2. 

How  much  pyrolusite  shall  be  taken  for  the  analysis?  * 

If  possible,  an  amount  should  be  taken  for  analysis  which  will 

N 
not  require  more  than  one  buretteful  of  the        Na^Og  solution. 


We  assume  that  the  sample  contains  100  per  cent,  of  MnO2,  and 
calculate  how  much  of  the  latter  would  correspond  to  50  c.c.  of 


N" 
1  c.c.  —  solution:  0.004347  gm.    MnO2  =  50:s; 

x  =  50X0.004347  =  0.2173  gm.  MnO2. 

Consequently  for  the  analysis  about  0.2  gm.  of  the  substance 
is  taken,  which  has  been  dried  at  100°  C.  To  this  25  c.c.  of  hydro- 
chloric acid  (1:2)  are  added  and  the  analysis  is  made  as  described 
above. 

*  This  is  applicable  to  almost  every  volumetric  analysis.  To  insure  the 
most  accurate  results,  the  concentration  of  the  standard  solution  and  the 
weight  of  substance  taken  for  analysis  should  be  so  chosen  that  between  35 
and  50  c.c.  of  the  reagent  are  used  in  the  final  titration.  In  this  way  the  errors 
in  determining  the  end-point,  reading  the  burette,  etc.,  will  not  influence  the 
result  appreciably.  —  [Translator.] 


664  VOLUMETRIC  ANALYSIS. 

The  calculation  is  based  upon  the  following  equations; 
MnO, + 4HC1  =  2H20  +  MnCl, + Cl* 

2Cl=2I=lMn02, 

lCl=lI  =  iMn02=43.47  gms. 

The  amount  of  substance  taken  for   analysis = a  gms.,  and   the 

N 

^-  Na^Oa  solution  used  for  the  titration  of  the  iodine =t  c.c.    Then 

a:ZX0.004347=100:z; 

0.4347  •* 
x  = =per  cent.  Mn02. 

The  determination  of  chromates,  lead  peroxide,  and  selenic 
acid  is  carried  out  in  the  same  way,  except  that  concentrated 
hydrochloric  acid  is  used  for  the  decomposition. 

(b)  Determination  of  Telluric  Acid. 

If  the  telluric  acid  is  present  as  the  hydrous  acid  (H2Te04+2H20) 
or  as  tellurate,  the  analysis  is  performed  in  the  same  way  as  with 
selenic  and  chromic  acids.  If,  however,  the  tellurium  is  present 
as  the  anhydrous  acid  or  as  the  anhydride,  the  method  must  be 
modified,  for  these  substances  are  scarcely  attacked  by  concen- 
trated hydrochloric  acid.  They  are  placed  in  the  decomposition- 
flask,  dissolved  in  a  little  concentrated  potassium  hydroxide,*  and  to 
the  tellurate  solution  thus  obtained  the  concentrated  hydrochloric 
acid  is  added ;  the  reduction  then  is  accomplished  without  difficulty : 

K2Te04  +  4HC1  =  2KC1 + H2TeO, + H20  +  C12. 
According  to  this  equation 

To       1 97  ^ 

101=11  =  ^  =  ^  =  63.75  gms.  Te. 
*  The  solution  could  not  be  effected  by  using  sodium  hydroxide. 


DETERMINATION  OF  CERIC  OXIDE  AND   VANADIC  ACID.    665 


c)  Determination  of  Ceric  Oxide. 


1000  c.c.  jj  iodine  solution  =2  =        p  =  17.  225  gms.  CeO,. 

Ceric  oxide  when  mixed  with  considerable  lanthanum  and  di- 
clymium  oxides  is  reduced  by  distillation  with  concentrated  hydro- 
chloric acid: 

2CeO2  +  8HC1  =  4H20  +  2CeCl3  +  C12. 

If,  however,  the  mixture  contains  but  little  of  the  two  last 
substances,  or  if  it  is  pure  eerie  oxide,  the  heating  with  concen- 
trated hydrochloric  acid  is  of  no  avail;  the  eerie  oxide  will  not 
dissolve. 

In  the  presence  of  hydriodic  acid,  however,  the  reduction 
takes  place  readily,  so  that  it  is  only  necessary  to  add  2  gms. 
of  potassium  iodide  to  a  weighed  amount  of  the  substance 
(0.67-0.68  gm.)  in  the  decomposition-flask,  and  then,  after  the 
addition  of  hydrochloric  acid,  violet  vapors  of  iodine  can  be  dis- 
tilled from  the  solution: 

2CeO2  +  2KI  +  8HC1  =  2KC1  +  2CeCl3  +  4H2O  +  21. 

Often  there  will  be  so  much  iodine  given  off  that  the  solid  is 
likely  to  stop  up  the  tube  and  the  flask  will  often  explode.  To 
prevent  this,  the  end  of  the  delivery-tube  is  not  drawn  out  into 
a  capillary,  but  at  the  bottom  an  opening  of  about  4  mm.  in  diame- 
ter is  left.  During  the  operation,  the  flame  must  be  protected 
from  air-currents,  for  otherwise  there  is  danger  of  liquid  sucking 
back  from  the  retort. 


(W)  Determination  of  Vanadic  Acid.f 
1000  c.c.  ^  iodine  solution  =^^  =  ^^=9.12  gms.  V2O6. 

By  boiling  vanadic  acid,  or  one  of  its  salts,  with  concentrated 
hydrochloric  acid,  the  vanadium  is  reduced  with  evolution  of 
chlorine.  Unfortunately,  this  reaction  cannot  be  used  for  the 
determination  of  vanadic  acid,  for  the  amount  of  chlorine  evolved 


666  VOLUMETRIC  A  'N  'A  LYSIS. 

depends  upon  the  concentration  of  the  vanadium  solution;  the 
vanadium  is  not  reduced  to  a  definite  oxide.  On  the  other  hand, 
by  means  of  hydrobromic  acid,*  vanadic  acid  is  reduced  to  a  blue 
vanadyl  salt: 

V205+2HBr  =  V204+H2 


If  the  free  bromine  is  absorbed  in  potassium  iodide,  and  the 
liberated  iodine  titrated  with  sodium  thiosulphate,  a  sharp  deter- 
mination of  the  vanadium  will  be  obtained.  To  carry  out  this 
analysis,  about  0.3-0.5  gm.  of  the  vanadate,  together  with  1.5 
to  2  gms.  of  potassium  bromide,  is  placed  in  the  decomposition- 
flask  of  the  Bunsen  apparatus  (Fig.  94,  p.  662),  30  c.c.  of  con- 
centrated hydrochloric  acid  are  added,  and  distillation  is  effected 
as  before.  The  decomposition  is  always  complete  when  the 
liquid  in  the  flask  is  a  pure  blue. 

If  hydriodic  acid  is  used  instead  of  hydrobromic  acid,  the 
vanadic  acid  is  reduced  still  further,  almost  to  V^03.t  In  fact, 
a  complete  reduction  to  the  latter  oxide  can  be  accomplished  if 
potassium  iodide,  concentrated  hydrochloric  acid,  and  1  or  2  c.c. 
of  syrupy  phosphoric  acid  are  added  and  the  liquid  distilled 
until  no  more  vapors  of  iodine  are  evolved.  According  to 
Steffan,  this  will  always  be  the  case  when  the  liquid  is  reduced  to 
one-third  of  its  original  volume. 

(e)  Determination  of  Molybdic  Acid. 
1000  c.c.  ^  Na2S203=^-'=  ^  =  14.4  gms.  MoO3. 

The  determination  depends  upon  the  fact  that  molybdic  acid 
is  reduced  to  molybdenum  pentoxide  by  means  of  hydriodic  acid 
with  liberation  of  iodine: 

2MoO3  +  2HI  -  H2O  +  Mo2O5  +  12. 

Remark.  —  This  method  finds  no  practical  application  on  account 
of  the  fact  that  it  is  difficult  to  obtain  a  quantitative  reduction  in 

*  Holverscheidt,  Dissertation,  Berlin,  1890. 

t  Friedheim  and  Euler,  Berichte,  28  (1895),  2067. 

j  Ibid.,  28  (1895),  2067,  and  29  (1896),  2981. 


DETERMINATION  OF   V 'SNA DIG  AND  MOLYBDIC  ACID.        667 

accordance  with  the  above  equation.  Gooch  and  Fairbanks  *  found 
that  if  a  solution  containing  molybdic  acid  is  distilled  in  the 
Bunseri  apparatus  with  potassium  iodide  arid  hydrochloric  acid, 
until  iodine  vapors  are  no  longer  visible  and  'the  solution  is  a  light 
green,  too  little  iodine  is  obtained.  On  the  other  hand,  if  the 
distillation  is  continued  still  further,  they  found  that  the  reduction 
goes  on  and  more  iodine  is  obtained  than  corresponds  to  the  above 
equation.  Steffan,f  who  tested  the  method  in  the  author's  labo- 
ratory, obtained  results  agreeing  with  those  published  by  Gooch 
and  Fairbanks.  By  means  of  hydrobromic  acid,  molybdic  acid 
is  not  reduced. 


(/)  Determination  of  Vanadic  and  Molybdic  Acids  in  the  Presence 
of  One  Another. 


According  to  Steffan,  these  two  acids  may  be  determined 
very  accurately  when  present  together.  The  vanadic  acid  is 
determined,  according  to  Holverscheidt,  by  distillation  with 
potassium  bromide  and  concentrated  hydrochloric  acid,  absorp- 
tion of  the  bromine  in  potassium  iodide  solution,  and  titration 
of  the  liberated  iodine  (cf.  p.  666).  The  contents  of  the  distilla- 
tion flask,  in  which  the  vanadium  is  present  as  vanadyl  salt  and 
the  molybdenum  as  molybdic  acid,  are  treated  with  hydrogen 
sulphide  in  a  pressure-flask,  and  the  precipitated  molybdenum 
sulphide  is  filtered  through  a  Gooch  crucible,  and  weighed  as  Mo03, 
as  described  on  p.  286,  The  results  obtained  by  this  method  are 
perfectly  satisfactory. 

As  molybdic  acid  is  unattacked  by  hydrobromic  acid,  but 
is  reduced  to  Mo2O5  with  separation  of  iodine  by  means  of  hydri- 
odic  acid,  Friedheim  and  Euler  proposed  the  following  method  for 
the  determination  of  vanadic  and  molybdic  acids  when  present 
together : 

The  mixture  of  the  two  acids  is  distilled  as  before  with  potassium 

*  Gooch  and  Fairbanks,  Zeitschr.  f.  anorg.  Chem.,  XIII  (1897),  101,  and 
XIV,  317. 

t  Steffan,  Inaug.  Dissertation,  Zurich,  1902. 


668  VOLUMETRIC  ANA  LYSIS. 

bromide  and  hydrochloric  acid  and  the  vanadium  thereby  reduced 
to  the  tetroxide  compound 

V2O5-+  2HBr  =  H2O  +  V2O4  +  Br2, 

with  separation  of  two  atoms  of  bromine  which  are  determined 
iodimetrically.  To  the  cold  solution  remaining  in  the  distilling- 
flask,  potassium  iodide,  hydrochloric  acid,  and  syrupy  phosphoric 
acid  are  added,  and  the  distillation  continued  until  no  more  iodine 
is  given  off  and  the  solution  is  a  light  green. 

By  means  of  this  second  reduction  the  vanadium  tetroxide  is 
supposed  to  be  reduced  to  V2O3, 


and  consequently  more  iodine  is  liberated  by  the  vanadium. 
Furthermore,  according  to  Friedheim  and  Euler,  the  molybdenum 
is  reduced  to  Mo2O5: 

2Mo03  +  2HI  -  H2O  +  Mo,06+  1,,. 

If,  therefore,  the  amount  of  iodine  corresponding  to  the  first 
titration  is  deducted  from  the  amount  obtained  in  the  second, 
the  difference  should  correspond  to  the  amount  of  molybdenum 
present.  But  Gooch  and  Fairbanks  have  shown  that  this  is 
not  the  case.* 

The  error  in  the  method  lies  in  the  fact  that  the  vanadic  acid 
is  only  reduced  completely  to  V2O3  when  the  solution  is  distilled 
to  one-third  of  its  original  volume.  In  this  case,  however,  the 
molybdenum  is  reduced  further  than  corresponds  to  the  formation 
of  Mo2O5;  too  much  iodine  is  liberated  and  too  high  a  value  is 
obtained  for  the  molybdic  acid  present.  On  the  other  hand, 
if  after  the  addition  of  the  potassium  iodide  the  liquid  is 
only  distilled  until  the  iodine  vapors  cease  to  appear  and  the 
solution  is  a  light  green,  the  vanadium  is  not  completely 
reduced  to  V2O3,  and  then  a  too  low  value  for  the  molybdenum 
is  obtained. 

*  The  results  of  Gooch  and  Fairbanks  have  been  confirmed  in  every  re» 
spect  by  Steffan. 


DETERMINATION  OF  HYPOCHLOROUS  ACID.  669 

9.  Analysis  of  Chlorates. 

This  is  carried  out  the  same  way  as  the  analysis  of  pyrolusite 
(cf.  p.  663) : 

KC1O3  +  6HC1  =  KC1  +  3  H2O  +  3C12 

1  N 

— -  gm.  =  at.  iodine  =  1000  c.c.  ^  Na2S2O3  solution 

KC1O,     122.6 
=  "60"^  =  ~60~~  =  2<°  3  gms<  KC1°3' 

Many  oxidizing  agents  can  be  determined  iodimetrically  with- 
out previous  distillation  with  hydrochloric  acid. 

For  other  methods  of  analyzing  chlorates  iodimetrically, 
consult  H.  Ditz,  Chem.-Ztg.  1901,  727  and  Luther  and  Rutter, 
Z.  anal.  Chem.  46,  521  (1907). 

10.  Determination  of  Hypochlorous  Acid. 

This  determination  is  made  use  of  in  the  analysis  of  chloride 
of  lime. 

Procedure. — Into  a  tared  weighing-tube  about  5  gms.  of  "  chloride 
of  lime  "  are  introduced,  and  the  stoppered  tube  is  weighed.  Its 
contents  are  washed  into  a  porcelain  dish,  rubbed  to  a  paste  by 
means  of  a  pestle,  and  then  transferred  without  loss  to  a  500-c.c. 
measuring-flask,  diluted  up  to  the  mark  with  water  and  well  shaken. 
Of  this  turbid  solution,  20  c.c.  are  run  into  10  c.c.  of  10  per  cent, 
potassium  iodide  solution,  and  after  acidifying  with  hydrochloric 

N 
acid  the  iodine  set  free  is  titrated  with  —  Na2S2O3.     The  result 

is  expressed  in  per  cent,  of  chlorine. 

Remark. — If  the  "  chloride  of  lime  "  contained  calcium  chlo- 
rate it  will  be  partially  reduced  by  hydrochloric  acid  and 
potassium  iodide  with  liberation  of  iodine,  and  consequently 
the  results  obtained  for  hypochlorite  chlorine  (bleaching  chlorine) 
will  l)e  too  high.  In  this  case  the  hypochlorite  is  best  determined 
by  a  chlorimetric  process  with  arsenious  acid  (see  p.  701). 


670  VOLUMETRIC  ANALYSIS. 

ii.  The  Analysis  of  lodates. 
1000  c.c.  ^r  Na2S203=  ——3=  —^-  =  2.932  gms.  HI03. 


The  solution  of  the  iodate  is  allowed  to  run  into  an  acid 
solution  containing  an  excess  of  potassium  iodide.  Iodine  is 
set  free  according  to  the  equation 


and  the  iodine  is  titrated  with  thiosulphate  solution  as  described 
on  p.  647. 

12.  The  Analysis  of  Periodates. 

1ST  TTTO        1  Q1   Q^ 

1000  c.c.  |  Na2S203=^-4     ^p-  2.399  gms.  HIO,. 

The  analysis  of  periodates  is  carried  out  exactly  as  with 
iodates;  the  reaction  that  takes  place  is 

KIO4  +  7KI  +  SHC1  =  8KC1  +  4H20  +  4I2. 

13.  Analysis  of  a  Mixture  of  Iodate  and  Periodate.* 

If  a  neutral  or  slightly  alkaline  solution  of  an  alkali  periodate 
is  treated  with  a  solution  of  potassium  iodide,  the  following 
reaction  takes  place  : 

KIO4  +  2KI  +  H2O  =  2KOH+KIO3+I2. 

The  liberated  iodine  is  titrated  with  tenth-normal  arsenious 
acid  (not  with  sodium  thiosulphate)  ;  in  a  neutral  solution  the 
iodate  does  not  react  with  potassium  iodide.  For  the  analysis 
of  a  mixture  of  iodate  and  periodate,  the  following  procedure  is 
used: 

In  one  sample  the  iodate  +  periodate  is  determined  by  adding 
the  solution  of  the  substance  to  an  acid  solution  containing  an 
excess  of  potassium  iodide  and  the  liberated  iodine  is  titrated 
with  sodium  thiosulphate  solution. 

A  second  sample  of  the  substance  is  dissolved  in  water,  a  drop 

*  E.  Muller  and  O.  Friedberger,  Berichte,  1902,  2655. 


ANALYSIS   OF  IODIDES.  671 

of  phenolphthalein  added,  and  the  solution  is  made  just  alkaline 
enough  to  give  the  pink  color  with  phenolphthalein,  adding  alkali 
if  the  solution  is  acid  and  hydrochloric  acid  if  the  solution  is 
strongly  alkaline.  To  the  barely  alkaline  solution,  10  c.c.  of  a 
cold,  saturated  solution  of  sodium  bicarbonate  are  added  and  then 
an  excess  of  potassium  iodide;  the  liberated  iodine  is  at  once 
titrated  with  tenth-normal  arsenious  acid.* 

Example.  —  In  a  mixture  of  KIO3  and  KI04  weighing  a  grams, 
the  iodine  liberated  on  treatment  with  an  acid  solution  of  KI 
reacts  with  T  c.c.  of  0.1  N  Na2S2O3  and  the  same  weight  of  sample 
liberates  in  alkaline  solution  only  enough  iodine  to  react  with 
t  c.c.  of  O.lX  As2O3  solution.  By  comparing  the  equations  given 
under  12  and  13,  it  is  evident  that  the  periodate  alone  would 
react  with  4£  c.c.  of  O.lN  Xa2S203  in  acid  solution.  The  amount 
of  KIO4  and  KIO3  present  will  be 

/  v  1  1  ^0 
JX0.01150  gm.=       g      %  KI04, 

(f-40  X0.003567  gm.-  t^-*)  X0.3567% 


14.  Analysis  of  Iodides,  f 

Method  of  H.  Dietz  and  B.  M.  Margosches. 

1000  c.c.        KI03  =      =^^  =  10.58  gm.  iodine. 


The  solution  of  the  iodide  is  treated  with  an  excess  of  tenth- 
normal  potassium  iodate  solution,  acidified  with  hydrochloric 
acid,  a  piece  of  calcite  added,  as  suggested  by  Prince,  J  and  boiled 
until  all  the  iodine  is  expelled.  The  solution  is  allowed  to  cool, 
then  an  excess  of  potassium  iodide  is  added,  and  the  iodine  now 

*The  iodine  cannot  be  titrated  in  the  alkaline  solution  with  sodium 
thiosulphate,  and  the  iodine  in  the  acid  solution  cannot  be  titrated  with  the 
arsenious  acid. 

fChem.  Ztg.,  1904,11,  1191. 

J  Inaug.  Dissert.  Zurich,  1910. 


672  VOLUMETRIC  ANALYSIS. 

liberated,  which  corresponds  to  the  excess  of  potassium  iodate 
used,  is  titrated  with  tenth-normal  Na2S203  solution. 
From  the  equation 

KI03  +  5KI  +  6HC1  =  6KC1  +  3H20  +  3I2, 

it  is  evident  that  five-sixths  of  the  iodine  liberated  comes  from 
the  iodide.  If,  therefore,  T  c.c.  of  O.lN  KIO3  solution  were 
added  and  t  c.c.  of  O.lN  Na2S2O  were  used  for  titrating  the 
excess  of  KIO3,  then  there  is  present 

(T'-OXO.OlOSSgm.  iodine  as  iodide. 

15.  Determination  of  Copper  with  Potassium  Iodate.* 

Potassium    iodate    in    dilute    hydrochloric    acid    solution    is 
reduced  by  potassium  iodide  to  free  iodine  (cf  .  p.  670)  : 

KIO3+5KI  +  6HC1 


but  if  the  solution  is  strongly  acid  with  hydrochloric  acid  and  an 
excess  of  the  iodate  is  added,  the  iodine  is  oxidized  to  IC1: 

2I2  +  KIO3  +  6HC1  =  KC1  +  5IC1  +  3  H2O, 

and  in  this  case  the  whole  reaction  may  be  expressed  by  the 
equation: 

KIO3  +  2KI  +  6HC1  =  3KC1  +  3IC1  +  3H20. 

The  IC1  is  not  very  stable,  and  is  at  once  reduced  to  free  iodine 
in  the  presence  of  any  oxidizable  substance. 

L.  W.  Andrew  f  has  shown  that  quite  a  number  of  reducing 
substances,  such  as  free  iodine,  iodides,  arsenites,  and  antimonites, 
can  be  titrated  with  potassium  iodate  very  exactly,  by  taking 
advantage  of  the  fact  that  when  the  reducing  agent  is  present 
in  excess  free  iodine  is  formed,  which  is  oxidized  quantitatively 
by  more  iodate,  provided  the  proper  amount  of  hydrochloric  acid 
is  present.  Copper  solutions  are  precipitated  quantitatively 

*  Jamieson,  Levy,  and  Wells,  Jour.  Am.  Chem.  Soc.,  30,  750  (1908). 
.,  25,  756  (1903). 


DETERMINATION  OF  COPPER  WITH  POTASSIUM  IODATE      673 

by  potassium  thiocyanate  and  sulphurous  acid  as  cuprous  thio- 
cyanate,  CuSCN,  and  Parr*  has  estimated  copper  quantita- 
tively by  titrating  this  precipitate  with  permanganate.  The 
oxidation  is,  however,  simpler  and  more  accurate  when  the 
titration  is  effected  by  potassium  iodate,  or  biiodate.  The 
reaction  goes  through  the  stage  in  which  iodine  is  set  free,  but 
the  latter  is  oxidized  completely  to  iodine  chloride  upon  the 
addition  of  more  iodate: 

(a)     2CuSCN  +  3KIO,  +  4HC1  =  2CuSO4  +  12  +  IC1  +  2HCN  +  3KC1 
+  H20. 

(6)     2I2  +  KIO3+6HC1=KC1  +  5 


and  the  whole  reaction  is  (multiplying  (a)  by  2  and  adding 

(c)    4CuSCN  +  7KI03  +  14HC1  =  4CuSO4  +  7IC1  +  4HCN  +  7KC1 
+  5H2O. 

The  potassium  iodate  solution  is  very  stable  and  can  be 
preserved  for  years  if  protected  from  evaporation.  The  standard 
solution  used  can  be  prepared  by  weighing  out  a  known  amount 
of  the  pure  salt  and  dissolving  to  a  definite  volume,  or  the  solution 
may  be  standardized  against  pure  copper,  carrying  out  the  process 
as  in  an  analysis.  A  convenient  concentration  is  one-twentieth 
of  the  formula  weight. 

Procedure.  —  To  0.5  gm.  of  the  ore  in  a  200  c.c.  flask,  add  6  to 
10  c.c.  of  strong  nitric  acid,  and  boil  gently,  best  over  a  free  flame, 
keeping  the  flask  in  constant  motion  and  inclined  at  an  angle 
of  about  45°,  until  the  larger  part  of  the  acid  has  been  removed. 
If  this  does  not  completely  decompose  the  ore,  add  5  c.c.  of 
strong  hydrochloric  acid  and  continue  the  boiling  until  the 
volume  of  liquid  is  about  2  c.c.  Now  add  gradually  and  carefully, 
best  after  cooling  somewhat,  12  c.c.  of  sulphuric  acid  (1  :  1), 
and  continue  the  boiling  until  sulphuric  acid  fumes  are  evolved 
copiously.  Allow  to  cool,  add  25  c.c.  of  cold  water,  heat  to 

*  J.  Am.  Chem.  Soc.,  22,  685  (1900). 


674  VOLUMETRIC  ANALYSIS. 

boiling,  and  keep  hot  until  the  soluble  sulphates  have  dissolved. 
Filter  into  a  beaker,  and  wash  the  flask  and  filter  thoroughly 
with  cold  water.*  Nearly  neutralize  the  filtrate  with  ammonia 
and  add  10  to  15  c.c.  of  strong  sulphur  dioxide  water.  Heat 
just  to  boiling  and  add  5  to  10  c.c.  of  a  10  per  cent,  solution  of 
ammonium  thiocyanate,  according  to  the  amount  of  copper 
present.  Stir  thoroughly,  allow  the  precipitate  to  settle  for  5 
or  10  minutes,  filter  on  paper,  and  wash  with  hot  water  until 
the  ammonium  thiocyanate  is  completely  removed. 

Place  the  filter  with  its  contents  in  a  glass-stoppered  bottle 
of  about  250  c.c.  capacity,  and  by  means  of  a  piece  of  moist 
filter-paper  transfer  into  the  bottle  also  any  precipitate  adhering 
to  the  stirring-rod  and  beaker.  Add  to  the  bottle  about  5  c.c. 
of  chloroform,  20  c.c.  of  water  and  30  c.c.  of  concentrated  hydro- 
chloric acid  (the  two  latter  liquids  may  be  previously  mixed). 
Now  run  in  standard  potassium  iodate  solution,  inserting  the 
stopper  and  shaking  vigorously  between  additions.  A  violet 
color  appears  in  the  chloroform,  at  first  increasing  and  then 
diminishing,  until  it  disappears  with  great  sharpness.  The 
rapidity  with  which  the  iodate  solution  may  be  added  can  be 
judged  from  the  color  changes  of  the  chloroform. 

In  order  to  make  another  titration  it  is  not  necessary  to  wash 
the  bottle  or  throw  away  the  chloroform.  Pour  off  two-thirds 
or  three-fourths  of  the  liquid  in  order  to  remove  most  of  the 
pulped  paper,  too  much  of  which  interferes  with  the  settling  of 
the  chloroform  globules  after  agitation,  add  enough  properly 
diluted  acid  to  make  about  50  c.c.  and  proceed  as  before.  In 
this  case,  where  iodine  monochloride  is  present  at  the  outset, 
the  chloroform  becomes  strongly  colored  with  iodine  as  soon  as 
the  cuprous  thiocyanate  is  added,  but  this  makes  no  difference 
with  the  results  of  the  titration. 

*  With  substances  containing  appreciable  amounts  of  silver  a  few  drops  of 
hydrochloric  acid  should  be  added  before  making  this  filtration,  but  not 
enough  to  dissolve  any  considerable  amounts  of  the  lead  sulphate  or  antimonio 
oxide  that  may  be  present. 


DETERMINATION  OF  CHROMIUM  IN   CHROMITE.  675 

1 6.  Analysis  of  Soluble  Chromates. 

A  concentrated,  acid  solution  of  potassium  iodide  is  treated 
with  a  weighed  amount  of  the  chromate,  diluted  with  water, 
and  the  liberated  iodine  titrated.  (Cf.  standardization  of  sodium 
thiosulphate  against  potassium  dichromate,  p.  649.) 

i6a.  Determination  of  Chromium  in  Chromite. 
About  0.2  gm.  of  the  finely  powdered  chromite  is  intimately 
mixed  with  2  gms.  of  sodium  peroxide  in  a  porcelain  crucible. 
This  crucible  is  placed  inside  a  larger  porcelain  crucible  and 
heated  for  fifteen  or  twenty  minutes  over  a  small  flame.*  At  the 
end  of  this  time,  all  the  chromium  will  be  converted  into  soluble 
sodium  chromate.  The  crucible  and  its  contents  are  placed  in 
100-200  c.c.  of  water,  which  is  heated  to  boiling  and  kept  at  this 
temperature  until  the  melt  is  completely  disintegrated.  The 
ferric  oxide  is  then  filtered  off,  the  filtrate  evaporated  in  a  porce- 
lain dish  nearly  to  dryness,f  the  residue  taken  up  in  as  little 
water  as  possible,  one  or  two  grams  of  potassium  iodide  added, 
the  solution  diluted  to  about  400  c.c.,  and  the  free  iodine  titrated 
with  tenth-normal  thiosulphate  solution: 

1  c.c.  ^  Na2S2O3  =  0.001733  gm.  Cr. 

17.  Determination  of  Lead  Peroxide. 

Method  of  Diehl,  modified  by  Topf.% 

The  analysis  depends  upon  the  fact  that  lead  peroxide  is  re- 
duced by  means  of  potassium  iodide  in  acetic  acid  solution  when 
considerable  alkali  acetate  is  present: 

PbO2  +  4HI  =  PbI2  +  2H2O  + 12. 

N 
After  diluting  with  water  the  iodine  is  titrated  with  —  Na2S2O3 

solution. 

*  If  the  crucible  is  heated  too  hot,  it  is  likely  to  be  strongly  attacked  by 
the  sodium  peroxide.  With  care,  a  single  crucible  may  be  used  for  four  or 
six  determinations. 

f  The  evaporation  to  dryness  is  necessary  to  remove  the  last  traces  of  per- 
oxide. 

t  Diehl,  Dingl.  polyt,  Journ.,  246,  p.  196,  and  Topf,  Zeitschr.  f.  analyt. 
Chem,  XXVI  (1887),  p.  296. 


676  VOLUMETRIC  ANALYSIS. 

Procedure. — About  0.5  gms.  of  the  substance  are  dissolved  with 
1.2  gms.  of  potassium  iodide  and  10  gms.  of  sodium  acetate  in 
5  c.c.  of  5  per  cent,  acetic  acid.  The  solution  is  diluted  with 
water  to  a  volume  of  25  c.c.  and  titrated  with  sodium  thiosulphate. 

Remark. — Moist  lead  peroxide  reacts  almost  instantly  on 
undergoing  the  above  treatment;  thoroughly  dried  material,  on 
the  other  hand,  dissolves  after  a  few  minutes  provided  it  is  finely 
ground.  If,  however,  the  dry  peroxide  is  in  the  form  of  coarse 
grains,  it  may  be  several  hours  before  the  reaction  is  finished, 
or  the  decomposition  may  be  incomplete. 

Furthermore,  too  much  potassium  iodide  should  not  be  used, 
as  otherwise  lead  iodide  will  separate  out.  In  that  case  from  3  to 
5  gms.  more  of  sodium  acetate  are  added  and  a  few  cubic  centi- 
meters of  water.  The  mixture  is  shaken  until  the  lead  iodide 
has  dissolved  completely  and  not  till  then  diluted  to  a  volume  of 
25  c.c.  The  solution  must  remain  perfectly  clear  and  there 
should  not  be  a  trace  of  lead  iodide  precipitate. 

This  excellent  method  may  also  be  used  by  the  analysis  of 
minium  (red  lead). 

18.  Determination  of  Ozone  in  Ozonized  Oxygen. 

1000  c.c.  —  Na2S2O3  =  — 3  =  — =  2.4  gms.  O3. 
10  20     20 

(a)  Schonbein's  Method. 

The  most  accurate  method  for  estimating  ozone  consists  in 
allowing  the  ozonized  oxygen  to  act  upon  potassium  iodide  solution 
whereby  free  iodine  is  formed : 

2KI  +  O3  +  H2O  =  2KOH  + 12  +  O2, 

and  the  iodine  may  be  titrated,  after  acidifying  the  solution  with 
dilute  sulphuric  acid,  by  means  of  N/10  sodium  thiosulphate. 

It  is  not,  however,  immaterial  whether  the  ozone  reacts  with 
a  neutral  or  with  an  acid  solution.  In  the  latter  case  far  too  much 
iodine  is  liberated,  although  in  the  former  case  exactly  the  right 
amount  is  set  free.  Sir  B.  C.  Brodie  *  called  attention  to  this 

t  Phil.  Trans.,  162,  435^84  (1872). 


DETERMINATION  OF   OZONE  IN   OZONIZED  OXYGEN.        677 

fact  in  his  classic  researches  on  ozone.  Brodie  confirmed  the 
results  obtained  of  his  titrations  by  weighing  the  amount  of  ozone 
used  in  the  experiments.  This  work  of  Brodie's  appears  to  have 
been  forgotten,*  for  many  other  chemists  have  since  that  time 
attempted  to  work  out  an  iodimetric  method  for  estimating 
ozone,  some  using  acid  solutions  of  potassium  and  iodide  and  some 
neutral  solutions  to  absorb  the  gas,  although  for  a  long  time  it 
occurred  to  no  one  else  that  the  results  could  be  checked  by  weighing 
out  a  definite  amount  of  ozone  for  test  experiments.  In  1901, 
however,  this  was  done  in  a  very  simple  way  by  R.  Ladenburg 
and  R.  Quasig,f  who  were  without  knowledge  of  Brodie's  work. 
Their  method  consisted  in  weighing  a  glass  bulb  of  known  capacity 
which  was  provided  with  glass  stop-cocks,  rilling  it  with  oxygen 
and  then  weighing.  The  oxygen  was  then  replaced  by  ozone,  so 
that  the  gain  in  weight  multiplied  by  three  represented  the  amount 
of  ozone  present. 

In  order,  now,  to  titrate  the  ozone,  Ladenburg  and  Quasig 
expelled  the  gas  from  the  bulb  by  distilled  water,  and  conducted 
it  slowly  through  a  neutral  solution  of  potassium  iodide  which 
was  subsequently  treated  with  an  equivalent  amount  of  sul- 
phuric acid  and  the  liberated  iodine  titrated  with  N  sodium  thio- 
sulphate. 

The  results  of  Ladenburg  and  Quasig  have  been  carefully 
tested  in  the  author's  laboratory  %  and  the  method  improved 
somewhat  by  absorbing  the  ozonized  oxygen  by  potassium  iodide 
solution  in  the  glass  bulb  itself  rather  than  expelling  the  gas  from 
the  bulb  and  passing  it  into  the  iodide  solution. 

The  estimation  of  ozone  by  weighing  is  a  much  too  round- 
about process  to  permit  a  practical  application,  particularly  on 
account  of  the  fact  that  the  measurement  and  weighing  of  the  gas 
must  take  place  in  a  room  at  constant  temperature,  a  condition 
which  cannot  in  many  cases  be  readily  fulfilled.  Consequently 
the  volumetric  titration  of  the  gas  is  far  more  practical. 

Procedure. — A  glass  bulk  of  about  300  to  400  c.c.  capacity,  of 
the  form  shown  in  Fig.  95,  is  procured  and  its  volume  accurately 

*  Luther  and  Inglis,  Z.  phys.  Chem.,  48,  208  (1903). 

fBer.  34,  1184(1901). 

J  Tread  well  and  Anneler,  Z.  anorg.  Chem.,  48,  86  (1905). 


6y8 


VOLUMETRIC  ANALYSIS. 


determined  by  weighing  it  empty  and  then  filled  with  water, 
applying  the  correction  for  temperature  as  described  on  p.  517  et  seq. 
The  bulb  is  then  connected  with  a  gas  delivery  tube,  making  use 
of  Babo  flanged  joints  (Fig.  95,  c  and  d)  which  are  pressed  together 
by  means  of  a  steel  clamp,  lined  with  cork.  The  delivery  tube 
is  connected  with  the  supply  of  ozone  and  oxygen,  with  which 
the  water  in  the  bulb  in  replaced.  During  the  filling  of  the  bulb, 
but  little  of  the  ozone  is  absorbed  by  the  water.  When  the 


FIG.  95. 


FIG.  96. 


FIG.  97. 


tube  is  filled,  the  lower  stop-cock  is  closed  first  and  the  upper  one 
a  few  seconds  later.  The  bulb  is  then  disconnected  with  the  gas 
delivery  tube,  inverted,  the  upper-stop  cock  opened  quickly  for 
an  instant  in  order  to  establish  atmospheric  pressure  in  the  bulb, 
Lnd  then  connected  by  means  of  rubber  tubing  with  the  gas 
reservoir  N  which  is  filled  with  double-normal  potassium  iodide 
solution  (Fig.  97).  The  air  imprisoned  in  the  rubber  tubing  is 
allowed  to  escape  through  the  three-way  stop-cock  b  and  after 
properly  setting  the  cock,  about  20  to  30  c.c.  of  the  iodide  solution 


DETERMINATION  OF  OZONb   IN  OZONIZED  OXYGEN.          679 

are  introduced  into  the  bulb.  Finally  the  stop-cock  6  is  closed 
and  the  rubber  tubing  disconnected  The  contents  of  the  bulb 
are  vigorously  shake*  and  allowed  to  stand  for  half  an  hour; 
at  the  end  of  this  time  the  absorption  of  the  ozone  will  be 
complete. 

An  Erlenmeyer  flask  is  then  placed  under  the  stop-cock  &'; 
tkis  k  opened  and  immediately  afterwards  the  upper  stop-cock  also. 
The  bulb  is  washed  out  first  by  introducing  some  potassium 
iodide  solution  through  a  and  finally  with  pure  water.  The 
contents  of  the  flask  are  then  acidified  with  dilute  sulphuric 
acid  and  the  liberated  iodine  titrated  with  tenth-normal  sodium 
thiosulphate. 

The  computation  takes  place  as  follows: 

Contents  of  the  bulb=  V  c.c. 
Ozone  found  by  titration  =  p  gms. 

Temperature  =  t,  barometer  reading  =  B,  aqueous  tension 
=  w. 

The  volume  of  the  bulb  at  0°  and  760  mm.  pressure  is 

F(Jg-u>)273 

760(273+0"' 

When  filled  with  oxygen  this  would  weigh: 

32-  Fo 


Therefore  the  weight  of  oxygen  and  ozone  in  the  bulb  is 

32-  FQ      p 
22,391     3' 


and  the  per  cent,  of  ozone  in  the  mixture  is 

100-  p          6,717,300-  p 
32.F0|p 
22,391  +  3 


680  VOLUMETRIC  ANALYSIS. 

(b)  Method  of  Soret-Thenard* 

Ozone  is  absorbed  quantitatively  by  means  of  sodium  arsenite 
solution  in  accordance  with  the  following  equation: 

Na3AsO3  +  O3  =  Na3AsO4  +  O2, 

although  A.  Ladenburg  f  finds  that  the  absorption  takes  place 
much  more  slowly  than  by  means  of  potassium  iodide.  When, 
therefore,  the  ozone  is  passed  through  the  arsenite  solution,  there 
is  danger  of  getting  too  low  results.  If  the  absorption  takes  place 
in  a  glass  bulb,  however,  the  results  are  good. 

Ozone  is  also  absorbed  by  alkali  bisulphite  J  solutions  and  may 
be  estimated  in  this  way,  by  titrating  the  excess  of  bisulphite 
with  iodine.  Ladenburg,  §  however,  has  shown  that  the  method 
is  not  as  accurate  as  the  potassium  iodide  one,  so  that  it  will  not 
be  considered  further  here. 

19.  Determination  of  Hydrogen  Peroxide.  Kingzstt's  Method. || 
1000  c.c.  ^  Na2S2O3  solution  =  ^^  =  ^|p  =1.7008  gms.  H2O2. 

The  hydrogen  peroxide  solution  is  diluted  until  its  H2O2 
content  corresponds  to  about  0.6  per  cent,  by  weight  and  of  this 
solution  10  c.c.  are  used  in  the  analysis. 

Procedure. — About  2  gms.  of  potassium  iodide  are  placed  in  an 
Erlenmeyer  flask  and  dissolved  in  200  c.c.  of  water,  30  c.c.  of 
sulphuric  acid  (1:2)  are  added,  and  then,  with  constant  stirring, 
10  c.c.  of  the  hydrogen  peroxide  solution  are  added  from  a  pipette. 
After  standing  five  minutes,  the  iodine  liberated  in  accordance 
with  the  equation 

H2O2  +  2KI  +  H2S04  =  K2SO4  +  2H2O  + 12 
is  titrated  by  means  of  tenth-normal  thiosulphate  solution. 

*Compt.  rend.,  38,  445  (1854),  75,  174  (1872). 
fBer.,  36,  115  (1903). 

J  Neutral  alkali  sulphite  is  not  suitable  here,  because  it  is  not  oxidized 
quickly  by  pure  oxygen  alone. 
§  Loc.  cit. 
II  J.  Chem.  Soc.,  1880,792. 


DETERMINATION  OF  IRON.  68 1 

Remark. — This  method  is  rather  better  than  that  described 
on  p.  627  because  the  titration  can  take  place  in  the  presence  of 
glycerol,  salicylic  acid,  etc.,  which  are  sometimes  used  as  pre- 
servatives in  commercial  hydrogen  peroxide  preparations.  These 
substances  will  render  the  results  obtained  by  the  permanganate 
titration  less  accurate. 

20.  Determination  of  Iron. 

This  method  was  first  proposed  by  Carl  Mohr  *  and  is  based 
upon  the  following  reaction: 

FeCl3 + HI  «=»  HC1 + FeCl2 + 1. 

As  the  reaction  is  reversible,  it  is  necessary  to  have  an  excess 
of  hydriodic  acid  present  in  order  that  it  may  take  place  quanti- 
tatively in  the  direction  from  left  to  right. 

Procedure. — The  hydrochloric  acid  solution  containing  a 
weighed  amount  of  the  ferric  salt  is  placed  in  a  300-c.c.  glass- 
stoppered  bottle,  the  greater  part  of  the  acid  is  neutralized  by  means 
of  sodium  hydroxide,  and  the  air  removed  by  means  of  a 
current  of  carbon  dioxide.  After  this  about  5  gms.  of  potas- 
sium iodide  are  added,  the  bottle  closed,  shaken,  and  allowed 
to  stand  in  the  cold  for  twenty  minutes.  The  liberated  iodine 

N 
is  then  titrated  with  —  sodium  thiosulphate  solution.      As  soon 

as  the  blue  color  has  disappeared  f  more  carbon  dioxide-  is  con- 
ducted through  the  solution,  the  bottle  is  stoppered  and  allowed 
to  stand  for  a  few  minutes  to  see  whether  the  blue  color  wiL  re- 
appear. Should  this  be  the  case,  more  thiosulphate  is  added, 
the  flask  again  stoppered  and  allowed  to  stand.  If  a  blue 
color  again  appears,  the  solution  contains  too  little  potassium 
iodide,  so  that  it  is  necessary  to  repeat  the  entire  analysis, 
using  1-2  gms.  more  of  it.  With  sufficient  potassium  iodide 
and  only  little  free  hydrochloric  acid,  the  reaction  is  always  com- 
plete at  the  end  of  twenty  minutes.  The  results  obtained  are 
satisfactory. 


*  Ann.  d.  Chem.  u.  Pharm.,  105,  p.  53. 
f  Starch  is  added  in  all  these  titrations. 


682  YOLU METRIC  ANALYSIS. 

21.  Determination  of  Copper.     Method  of  Haen*-Low.f 
1000  c.c.  ^  Na2S203  solution =^-l  =  6.357  gms.  Cu. 

Principle. — If  an  acid  solution  of  a  cupric  salt  is  treated  with 
an  excess  of  potassium  iodide,  all  the  copper  is  precipitated  as 
cuprous  iodide, 

2CuSO4  +  4KI  =  Cu2I2  +  2K2SO4  + 12, 

and  there  is  liberated  one  atom  of  iodine  for  each  atom  of  copper 
present.  The  iodine  is  titrated  with  sodium  thiosulphate  solution. 
This  method  has  found  extensive  application,  especially  at 
the  copper  mines  in  the  United  States  and,  if  the  proper  conditions 
are  fulfilled,  the  results  are  just  as  accurate  as  those  obtained  by 
electrolysis.  The  method  has  been  studied  by  Gooch  and  Heath,  { 
who  find  that  the  above  reaction  takes  place  preferably  in  a 
solution  containing  not  over  3  c.c.  concentrated  sulphuric  acid, 
hydrochloric  acid,  nitric  acid  free  from  lower  oxides,  or  25  c.c. 
of  50  per  cent,  acetic  acid.  Obviously  the  solution  must  not 
contain  ferric  iron  or  any  other  oxidizing  agents  which  will  react 
with  potassium  iodide  under  the  same  conditions. 

Standardization  of  the  Thiosulphate  Solution. 

In  technical  work  it  is  customary  to  standardize  the  thio- 
sulphate solution  against  pure  copper.  About  0.2  gm.  of  pure 
copper  is  weighed  into  a  200-c.c.  Erlenmeyer  flask  and  dissolved 
in  5  c.c.  of  a  mixture  of  equal  parts  nitric  acid  (sp.  gr.  1.42)  and 
water.  The  solution  is  diluted  with  25  c.c.  water  and  boiled  a 
few  minutes  to  get  rid  of  the  greater  part  of  the  reduced  oxides 
of  nitrogen.  To  remove  the  last  of  the  nitrous  oxides,  5  c.c. 
of  bromine  water  are  added  and  the  solution  boiled  until  the 
excess  bromine  is  expelled.  The  flask  is  then  removed  from 
the  flame  and  strong  ammonia  added  to  its  contents  until  a 
slight  excess  is  present.  After  boiling  off  the  excess  of  ammonia, 

*  Ann.  Chem.  Pharm.,  91,  237  (1854). 
t  Technical  Methods  of  Ore  Analysis, . 
t  Z.  Anorg.  Chem.,  55,  129  (1907). 


DETERMINATION  OF  COPPER  IN  ORES.  683 

7  c.c.  of  strong  acetic  acid  are  added,  which  dissolves  any  copper 
oxide  that  has  deposited.  After  cooling  to  room  temperature, 
3  gms.  of  potassium  iodide  are  added  and  the  brown  solution 
titrated  with  sodium  thiosulphate  until  nearly  colorless.  Then 
starch  solution  is  added  and  the  titration  finished.  In  making 
the  titration  for  the  first  time,  one  is  bothered  somewhat  by  the 
fact  that  the  cuprous  iodide  is  of  a  light-brown  color  but  after 
a  little  practice  there  is  no  difficulty  in  getting  the  correct  end- 
point.  If  t  c.c.  of  the  solution  were  used  in  titrating  a  gms.  of 

copper  then  1  c.c.  of  thiosulphate  =  —  gm.  Cu. 

t 

Determination  of  Copper  in  Ores.     Low's  Method. 

Principle. — The  ore  is  dissolved  in  acid,  the  copper  separated 
from  iron,  etc.,  by  precipitating  it  upon  metallic  aluminium,  the 
deposit  dissolved  in  nitric  acid  and  treated  as  in  the  standardiza- 
tion. 

Procedure. — To  0.25-0.50  gms.  of  finely  ground  ore  weighed 
into  a  250-c.c.  Erlenmeyer  flask,  add  6  c.c.  of  nitric  acid  (sp.  gr. 
1.42),  and  boil  gently  until  nearly  to  dry  ness.  Add  5  c.c.  of 
strong  hydrochloric  acid  and  heat  again.  As  soon  as  the  in- 
crusted  matter  has  dissolved  add  7  c.c.  of  concentrated  sulphuric 
acid  and  heat  until  the  sulphuric  acid  fumes  freely.  Cool  and 
add  25  c.c.  of  water.  Then  heat  until  any  anhydrous  ferric 
sulphate  is  dissolved,  and  filter  to  remove  insoluble  sulphates 
and  silica.  Wash  the  flask  and  filter-paper  until  the  volume  of 
the  filtrate  amounts  to  about  75  c.c.,  receiving  it  in  a  No.  2 
beaker.  Place  on  its  edge  in  the  beaker  a  piece  of  aluminium- 
foil  bent  into  triangular  shape.  Cover  the  beaker  and  boil 
gently  for  seven  to  ten  minutes,  which  will  be  sufficient  to  pre- 
cipitate all  the  copper,  provided  the  solution  does  not  much 
exceed  75  c.c.  Avoid  boiling  to  very  small  bulk.  The  alu- 
minium should  now  appear  clean,  the  copper  being  detached 
or  loosely  adhering.  Remove  from  the  heat  and  wash  down 
the  cover  and  sides  of  the  beaker  with  cold  water.  There  is 
danger  of  finely-divided  copper  being  oxidized  and  dissolved. 
To  prevent  this,  and  at  the  same  time  to  remove  any  traces  of 
copper  remaining  in  solution,  add  15  c.c.  of  strong  hydrogen- 


684  VOLUMETRIC  A 'N 'A LYSIS. 

sulphide  water.      The  next  step  depends  upon  the  amount  of 
copper  in  the  ore. 

(a)  When   there   is   apparently    less   than   20   per   cent.   Cu 
present,  decant  the  liquid  through  a  filter  and  then  without  delay 
transfer  by  means  of  a  jet  of  hydrogen-sulphide  water  from  a 
wash-bottle,  the  copper  to  the  filter  leaving  the  foil  as  clean  as 
possible  in  the  beaker.     Wash  the  copper  and  filter  thoroughly 
with  this  hydrogen-sulphide  water,  being  careful  not  to  allow 
the  filter  to  stand  empty  until  the  washing  is  finished.     (The 
filtrate  should  not  show  a  brown  tinge  of  copper  sulphide.)     Now 
place  the  original  flask  under  the  funnel.     Pour  over  the  alu- 
minium in  the  beaker  5  c.c.  of  a  mixture  of  equal  parts  concen- 
trated nitric  acid  and  water.     Heat  just  to  boiling  and  pour 
the  hot  acid  very  slowly  upon  the  filter,  lifting  the  fold  if  neces- 
sary.    Now,  before  washing,  pour  5  c.c.  of  bromine  water  into 
the  filter  and  wash  the  beaker  and  filter  with  hot  water.     Finally 
remove  the  filter  and  wash  any  residue  upon  it  into  the  flask. 
If  the  bromine  was  not  sufficient  to  give  a  slight  tinge  to  the 
filtrate  more  of  it  must  be  added.     Boil  the  filtrate,  which  does 
not  exceed  75  c.c.,  to  expel  the  excess  of  bromine,  but  do  not 
concentrate  to  small  volume.     Remove  from  the  heat  and  add 
.1  slight  excess  of  strong  ammonia  (usually  7  c.c.).     Boil  off  the 
excess  of  ammonia  and  add  3  or  4  c.c.  of  strong  acetic  acid.     Cool 
to  room  temperature,  add  3  gms.  of  potassium  iodide,  and  titrate 
with  thiosulphate,  adding  starch  toward  the  end  of  the  reaction. 

(b)  With  high  percentages  of  copper  it  is  better  to  wash  the 
copper  by  decantation  instead  of  on  the  filter.     Transfer  the 
liquid  and  copper  to  the  original  flask,  and  set  the  beaker  and 
aluminium  aside  temporarily.     Allow  the  liquid  in  the  flask  to 
settle,  decant  through  a  filter  and  wash  three  or  four  times  by 
decantation  with  hydrogen-sulphide  water,   using  about  20  c.c. 
each  time.     Now  place  the  flask  containing  the  copper  under  the 
funnel.     Heat  the  5  c.c.  of  nitric  acid  (1  vol.  cone.  HNOs  to  1  vol. 
water)  in  the  beaker  with  the  aluminium-foil  and  pour  it  through 
the   filter.      Remove   the   flask,  place   the  beaker   in   its    place 
under  the  funnel,  and  heat  the  acid  until  all  the  copper  has 
dissolved  and  the  red  fumes   are  mostly  expelled.     Now  return 
the  flask  under  the  funnel,  add  the    bromine  as  in  the  above 
method  of  analysis  (a),  and  continue  as  described  above. 


DETERMINATION  OF  ANTIMONY   T RIO X IDE  COMPOUNDS.     685 


22.  Analysis  of  Arsenious  Acid. 

The  titration  is  effected  in  the  same  way  as  in  the  standard- 

N 
ization  of  the  -rr  iodine  solution,  described  on  p.  650. 


23.  Determination  of  Antimony  Trioxide  Compounds. 


1000  c.c.  ^iodine  solution  =      2-3  =  7.21  g.  Sb2O3  =  6.01g.  Sb. 

The  titration  is  carried  out  exactly  as  in  the  case  of  arsenious 
acid  (cf.  p.  650)  except  that  tartaric  acid,  or  Rochelle  salt,  must 
be  added  to  the  solution  in  order  to  prevent  the  precipitation  of 
antimonous  acid,  or  antimony  oxychloride,  as  a  result  of 
hydrolysis. 

Examples: 

(a)  Determination  of  Antimony  in  Tartar  Emetic. 

If  an  aqueous  solution  of  tartar  emetic  be  treated  with  iodine  in 
the  presence  of  starch,  the  first  few  drops  of  reagent  will  impart  a 
permanent  blue  color  to  the  solution.  If,  however,  a  little  sodium 
bicarbonate  is  added  to  the  solution,  the  trivalent  antimony  is 
oxidized  quantitatively  to  the  pentavalent  condition. 

K(SbO)  C4H4O6  +  6NaHCO3  +  12  = 

=  Na3Sb04  +  2NaI  +  KNaC4H4O6  +  3H2O  +  6C02. 


N  .    ,.          .   ..         K(SbO)C4H4O6 
1000  c.c.        iodine  solution  =  —  * 


=  16.617  gms. 

8.309  gms.  of  tartar  emetic  are  dissolved  in  water,  the  solution 
diluted  to  exactly  500  c.c.  and  well  mixed.  Of  this  solution,  20 
c.c.  are  removed  by  a  pipette,  diluted  to  100  c.c.,  treated  with 
20  c.c,  of  2  per  cent,  sodium  bicarbonate  solution,  and  titrated  with 


686  VOLUMETRIC  ANALYSIS. 

tenth-normal  isodine  solution,  using  starch  as  an  indicator.     If 
t  c.c.  are  used  for  the  titration,  the  salt  contains: 

1.6617X25X* 

—  =  5  •  t  =  per  cent,  tartar  emetic, 


or 

=  1.809-£  =  per  cent,  antimony. 

(6)   Determination  of  Antimony  in  Stibnite. 

Not  over  0.5  gm.  stibnite  is  dissolved  in  a  small  covered  beaker 
by  means  of  10  c.c.  concentrated  hydrochloric  acid  (sp.  gr.  1.2). 
The  acid  is  allowed  to  act  in  the  cold  for  about  ten  minutes,  after 
which  the  contents  of  the  covered  beaker  are  heated  gently  on  the 
water  bath  for  ten  or  fifteen  minutes.  Three  gms.  of  powdered 
tartaric  acid  are  then  added  and  the  heating  is  continued  for  ten 
minutes  longer,  but  care  is  taken  not  to  allow  the  liquid  to  evapor- 
ate sufficiently  to  expose  any  part  of  the  bottom  of  the  beaker. 
When  this  precaution  is  taken,  there  is  no  volatilization  of  the 
antimony,  and  all  of  the  hydrogen  sulphide  is  expelled. 

Sb2S3  4-  6HC1  =  2SbCl3  +  3H2S. 

The  solution  is  now  removed  from  the  water  bath,  allowed  to  cool 
to  the  room  temperature,  and  very  cautiously  diluted  with  water, 
which  is  added  at  first  drop  by  drop,  until  a  volume  of  about  100 
c.c.  is  obtained.  If,  in  the  meantime,  a  red  coloration  due  to 
antimony  sulphide  appears  during  the  dilution,  the  solution  should 
be  at  once  heated  until  it  disappears,  and  the  diluting  then 
continued. 

The  diluted  solution  is  nearly  neutralized  with  ammonia,  but 
is  left  slightly  acid.  The  cold,  slightly  acid  solution  is  poured  into 
a  700  c.c.  beaker  containing  3  gms.  of  sodium  bicarbonate  dissolved 
in  200  c.c.  of  water,  starch  paste  is  added,  and  the  solution  titrated 
with  iodine  to  the  appearance  of  a  permanent  blue. 

Remarks.  —  Antimony  chloride  is  volatile  with  steam  from  its 
concentrated  solutions,  so  that  the  solution  should  not  be  boiled 
until  it  has  been  diluted.  The  heating  on  the  water-bath  can  be 


DETERMINATION  OF  ANTIMONY  PEN  TO  A  IDE  COMPOUNDS.     687 

carried  out,  however,  without  fear  of  losing  antimony  provided 
the  acid  is  not  allowed  to  evaporate  to  any  extent.  This  heating 
serves  to  remove  all  the  hydrogen  sulphide  which  would  otherwise 
precipitate  the  antimony  as  trisulphide  upon  diluting  the  solution. 
If  insufficient  tartaric  acid  is  present,  antimony  oxychloride, 
SbOCl,  precipitates  and  if  the  solution  is  titrated  in  this  condition 
it  is  impossible  to  obtain  a  permanent  end-point.  Such  a  pre- 
cipitate may  be  filtered  off,  dissolved  in  concentrated  hydrochloric 
acid  and  the  solution  treated  by  itself  as  above  described.  The 
value  of  the  iodine  solution  in  terms  of  Sb  is  given  in  the  previous 
process  (a). 


24.  Determination  of  Antimony  Pentoxide  Compounds 
(A.  Weller).* 

By  heating  a  pentavalent  antimony  compound  with  concen- 
trated hydrochloric  acid  and  potassium  iodide  in  the  Bunsen 
apparatus  (Fig.  94,  p.  662),  the  antimonic  acid  is  reduced  to 
antimonous  acid  with  separation  of  iodine  : 


The  iodine  is  distilled  over  into  potassium  iodide  solution  and 

N 
titrated  with  —  Na2S2Oa  solution.    The  results  are  a  little  low. 

25.  Determination  of  Hydrogen  Sulphide. 


1000  c.c.        Na&O.  solution  =        =  =1.704  gms. 

If   a  solution  of  hydrogen  sulphide  is  treated  with  iodine,  it 
is  oxidized  with  separation  of  sulphur: 


H2S+2I 

For  the  determination  of  the  amount  of  the  gas  present  in 
hydrogen  sulphide  water,  a  measured  amount  is  transferred  by 

*  Annal.,  213,  264. 


688  VOLUMETRIC  ANALYSIS. 

N 
means  of  a  pipette  to  a  known  amount  of  —  iodine  solution  and 

the  excess  of  the  latter  is  titrated  with  thiosulphate  solution.* 

If  the  amount  of  hydrogen  sulphide  present  is  not  very  large, 
correct  results  are  obtained  without  difficulty.  With  consider- 
able hydrogen  sulphide,  on  the  other  hand,  the  deposited  sulphur 
is  likely  to  enclose  some  of  the  iodine  solution,  as  shown  by  its 
brown  color;  this  iodine  escapes  the  titration  with  thiosulphate. 
In  such  a  case,  the  film  of  sulphur  floating  on  the  surface  of  the 
liquid  is  removed  with  a  glass  rod  after  the  completion  of  the 
thiosulphate  titration,  transferred  to  a  glass-stoppered  cylinder, 
and  shaken  with  1-2  c.c.  of  carbon  bisulphide.  The  latter  dis- 
solves the  iodine  with  a  violet  color  and  the  color  is  discharged 
by  the  addition  of  sodium  thiosulphate  solution. t  In  this  way 
the  total  amount  of  the  iodine  that  remains  can  be  titrated. 

Remark. — This  method  can  be  used  to  advantage  for  deter- 
mining the  sulphur  present  in  soluble  sulphides.  The  sulphides 
are  decomposed  as  described  on  p.  350  by  means  of  acid,  and 
the  hydrogen  sulphide  evolved  is  conducted  into  a  definite  amount 

N 
of  -TQ  iodine  solution.     The  excess  of  the  latter  is  titrated  as 

above  with  sodium  thiosulphate  solution. 

Determination  of  Hydrogen  Sulphide  in  Mineral  Waters. 

N 

A  measured  amount  of  —   iodine  solution  and  2  gms.  of  potas- 
1UU 

sium  iodide  are  placed  in  a  tall  liter  cylinder,  1000  c.c.  of  the  water 
to  be  analyzed  are  added,  and  after  thoroughly  shaking,  the  excess 

N 
of  the  iodine  is  titrated  with  — -^  thiosulphate.     The  standardiza- 

JL\J\J 

*  Correct  results  cannot  be  obtained  by  titrating  directly  with  iodine,  cf . 
O.  Brunck,  Z.  Anal.  Chem.,  45,  541  (1906). 

f  The  separation  of  the  sulphur  into  a  coherent  film  can  be  prevented 
by  sufficiently  diluting  the  solution  with  boiled  water.  O.  Brunck  (Z.  anal. 
Chem. ,'45,  541)  therefore,  recommends  using  hundredth-normal  iodine  instead 
of  tenth-normal  solution,  and  this  is  certainly  advisable  in  the  case  of  small 
quantities  of  hydrogen  sulphide  as,  for  example,  in  a  mineral  water.  On 
the  other  hand,  when  a  relatively  large  volume  of  hydrogen  sulphide  is 
liberated  from  a  sulphide  by  means  of  acid  (see  page  350)  it  is  advisable 
to  use  tenth-normal  iodine,  as'otherwise  the  volume  of  solution  will  be  too 
large  unless  a  very  small  weight  of  substance  is  used  in  the  analysis. 


ANALYSES   OF  ALKALI  SULPHIDES.  689 

tion  of  the  iodine  solution  used  is  accomplished  by  measuring 
off  10  c.c.  of  the  solution,  adding  2  gms.  of  potassium  iodide, 

diluting  to  1  liter  with  boiled  water,  and  titrating  with  -^  thio- 

1UU 

sulphate  solution. 


26.  Analysis  of  Alkali  Sulphides. 

TV  T?  ^         S       S2  07 

1000  c.c.  ^  iodine  solution=^-  or  —  =—^-  =  1.604  gms.  S. 

A  measured  volume  of  the  alkali  sulphide  solution  is  allowed 
to  run  slowly,  with  constant  stirring,  into  a  very  dilute  solution 
of  iodine  which  is  acid  with  hydrochloric  acid.  The  excess  of 
iodine  is  titrated  with  sodium  thiosulphate  solution. 

27.  Analysis  of  a  Mixture  of  Alkali  Sulphide  and  Alkali 
Sulphydrate. 

If  a  solution  of  alkali  sulphide  and  alkali  sulphydrate  is  treated 
with  an  acid  solution  of  iodine,  the  following  reactions  take  place: 


(a) 
(b) 
(c)  H2S  +  I2  =  2HI+S. 


It  is  evident  from  these  equations  that  in  the  case  of  the 
sulphide,  the  quantity  of  hydriodic  acid  formed  by  the  oxida- 
tion of  the  hydrogen  sulphide  is  equivalent  to  the  quantity  of 
hydrochloric  acid  required  to  decompose  the  sulphide;  in  this 
case,  therefore,  the  acidity  of  the  solution  remains  unchanged. 
In  the  case  of  the  sulphydrate,  however,  which  is  the  acid  salt 
of  hydrosulphuric  acid,  the  quantity  of  hydriodic  acid  formed  is 
equivalent  to  twice  the  quantity  of  hydrochloric  acid  required  to 
decompose  the  sulphydrate.  Thus  the  acidity  of  the  solution  is 
a  measure  of  the  quantity  of  sulphydrate  present.  Moreover, 
if  t  c.c.  of  tenth-normal  alkali  is  required  to  titrate  this  acid  then 


690  VOLUMETRIC  ANALYSIS. 

2t  c.c.  of  tenth-normal  iodine  must  have  been  required  to  oxidize 
the  hydrogen  sulphide  from  the  sulphydrate. 

Procedure. — A  known  volume  of  tenth-normal  iodine  together 
with  a  known  volume  of  tenth-normal  hydrochloric  acid  *  is 
diluted  in  a  beaker  to  a  volume  of  about  400  c.c.  and  the  solution 
containing  the  sulphide  and  sulphydrate  is  added  slowly  from  a 
burette,  with  constant  stirring,  until  the  solution  becomes  pale 
yellow.  Starch  indicator  is  added  and  the  excess  of  iodine 
titrated  with  tenth-normal  thiosulphate  solution.  Finally,  the 
acid  in  the  solution  is  titrated  with  tenth-normal  sodium  hydroxide 
solution. 

Computation.— In  the  analysis,  the  volumes  of  standard 
solutions  used  were:  T  c.c.  tenth-normal  iodine;  tt  c.c.  tenth- 
normal  thiosulphate;,  t2  c.c.  tenth-normal  hydrochloric  acid; 
t3  c.c.  of  tenth-normal  sodium  hydroxide:  and  V  c.c.  of  the 
sulphide  mixture. 

Then  (T—tJ  c.c.  =  total  volume  of  iodine  required,  and 
(£3  — £2)  c.c.  =  the  volume  of  sodium  hydroxide  required  to  neu- 
tralize the  acid  formed  by  the  reaction  with  the  sulphydrate. 
Then 


03- y  X0.005608  gm.  NaSH, 
and 


are  present  in  V  c.c.  of  solution. 


*  The  quantity  of  hydrochloric  acid  present  must  be  sufficient  to  decompose 
all  the  sulphide  and  sulphydrate;   an  excess  does  no  harm. 


DETERMINATION  OF  SULPHUROUS  ACID.  691 

28.  Analysis  of  a  Mixture  of  Hydrogen  Sulphide  and  Alkali 
Sulphydrate. 

The  analysis  is  carried  out  exactly  as  in  the  case  just  described 
and  the  computation  is  similar. 

N  N  N 

Let  T  =  c.c.  —  iodine,  t^c.c.  —  thiosulphate,  Z2  =  c.c.  —  acid, 

N 
and  £3=  c.c.  —  alkali.     Then 


-y  -(<,-*,)]  X0.005608  gm.  NaSH 
and 

gm.  H2S. 


Remark.  —  The  last  two  methods  of  analysis  are  applicable 
only  to  solutions  containing  no  other  compounds  decomposable 
by  hydrochloric  acid  than  sulphide  and  sulphydrate,  and  no 
other  substance  that  will  react  with  iodine.  The  analysis  may 
be  carried  out  without  the  addition  of  any  hydrochloric  acid. 
In  this  case  the  solution  of  sulphides  is  diluted  to  about  400  c.c., 
starch  added,  and  the  titration  with  iodine  carried  out  directly. 
The  hydriodic  acid  formed  is  titrated  with  caustic  soda,  using 
lacmoid  *  as  indicator.  The  reactions  are 


29.  Determination  of  Thiosulphate  in  the  Presence  of  Sulphide 
and  Sulphydrate. 

A  measured  volume  of  the  solution  is  treated  in  a  200-c.c. 
graduated  flask  with  an  excess  of  freshly-precipitated  cadmium 
carbonate.  After  shaking  thoroughly,  the  liquid  is  diluted  to 
the  mark,  filtered  through  a  dry  filter  and  100  c.c.  of  the  filtrate 


*  Methyl  orange  can  be  used,  but  it  is  not  so  easy  to  distinguish  the  end- 
point.  Phenolphthalein  works  well  but  no  better  than  the  lacmoid.  Care 
should  be  taken  to  titrate  the  acid  against  the  alkali  during  the  standardiza- 
tion at  the  same  dilution  as  to  be  used  in  the  analysis  and  an  appreciable 


692  VOLUMETRIC  ANALYSIS. 

titrated  with  iodine  solution.  By  shaking  with  cadmium  car- 
bonate, the  sulphide  and  sulphydrate  are  removed  and  the 
thiosulphate  remains  in  solution. 

30.  Determination  of  Sulphurous  Acid. 

1000  c.c.  ^  iodine  solution  =  |^2=^|~?=  3. 203  gms.  SO* 
The  determination  is  based  upon  the  following  reaction: 
SO2 + H2O  +  21  =  2HI  +  SO3, 

the  sulphurous  acid  being  oxidized  to  sulphuric  acid.  If  starch 
is  added  to  a  solution  of  sulphurous  acid,  and  a  titrated  iodine 
solution  is  run  into  it  from  a  burette,  the  blue  color  will  not  be 
obtained  until  all  of  the  sulphurous  acid  has  been  acted  upon. 
Bunsen,  however,  in  1854  showed  that  this  sensitive  reaction, 
which  was  first  used  by  Dupasquier,  will  only  take  place  quanti- 
tatively according  to  the  above  equation  when  the  solution  does 
not  contain  more  than  0.04  per  cent,  by  weight  of  SO2.  With 
greater  concentrations  uniform  results  are  not  obtained.  This 
irregularity  was  ascribed  to  the  reversibility  of  the  reaction,  so 
that  it  was  suggested  that  the  titration  be  performed  in  alkaline 
solution,*  thus  removing  the  hydriodic  acid  as  fast  as  it  is  formed. 
But  the  results  then  obtained  are  still  inaccurate,  f  Finkener,  t 
qpi  the  other  hand,  states  that  correct  values  will  be  obtained  if 
the  sulphurous  acid  is  allowed  to  run  into  the  iodine  solution. 

J.  Volhard  §  has  confirmed  the  results  of  Finkener  and  shown, 
that  the  anomalous  results  obtained  on  titrating  sulphurous  acid 
with  iodine  are  not  due  to  the  reversibility  of  the  reaction,  for 
the  direct  addition  of  20  per  cent,  sulphuric  acid  is  without 

*  Addition  of  Mg€O3  or  NaHCO3  (Fordos  and  Gelis). 

t  E.  Rupp,  Ber.,  35,  3694  (1902),  states  that  it  is  possible  to  obtain  good 
results  by  the  method  of  Fordos  and  Gelis  if  the  sulphurous  acid  is  allowed 
to  act  for  at  least  half  an  hour  upon  an  excess  of  iodine  in  the  presence  of 
sodium  bicarbonate.  The  solution  is  then  titrated  with  sodium  thiosulphate. 
According  to  E.  Miiller  and  O.  Diefenthaler,  however,  this  is  theoretically  in- 
correct, for  the  iodine  tends  to  form  a  little  hypoiodite:  I2+  H2O«^HI-f-HIO, 
and  the  latter  reacts  with  sodium  thiosulphate:  Na2S2O3+4HIO+H2O  = 
Na2S04+H2S04+4HI. 

%  Finkener- Rose,  Quantitative  Analyse  (1871),  p.  937. 

§  Ann.  d.  Chem.  u.  Pharm.,  242,  94. 


DETERMINATION  OF  SULPHUROUS   ACID.  693 

influence.  The  incomplete  oxidation  of  the  sulphurous  acid  is 
caused  by  the  fact  that  the  hydriodic  acid  reduces  a  part  of  the 
sulphurous  acid  to  free  sulphur  :  * 

(1)  S02+4HI  =  4I  +  2H20  +  S. 

If  sulphurous  acid,  whether  dilute  or  concentrated,  is  allowed. 
to  run  into  a  solution  of  iodine  with  constant  stirring,  there  is 
complete  oxidation  of  the  SO2: 


(2)  S02+2I  +  2H2O=2HI+H2SO4. 

If,  on  the  contrary,  iodine  solution  is  run  into  the  solution 
of  sulphurous  acid,  both  reactions  will  take  place: 


(3)  3SO2+4HI  +  2H2O  =  2H2SO4+4HIf  +  S. 

According  to  Raschrg,{  however,  Volhard's  explanation  is 
also  incorrect,  for  he  finds  that  no  separation  of  free  sulphur  takes 
place  if  the  iodine  is  allowed  to  act  upon  sulphur  dioxide  in  a 
dilute  solution.  Raschig  believes  that  the  error  that  results 
when  iodine  is  added  to  the  sulphurous  acid  solution  is  due  to  a 
loss  of  SO2  by  evaporation. 

Correct  results  are  always  obtained  if  the  sulphurous  acid  is 
added  slowly,  with  constant  stirring  ,  to  the  iodine  solution  until 
the  latter  is  •decolorized. 

In  the  analysis  of  sulphites,  the  sulphite  solution  is  added  from 
a  burette  to  the  solution  of  iodine  and  hydrochloric  acid. 

*  If  iodine  solution  is  added  slowly  to  a  not  too-dilute  sulphurous  acid 
solution,  a  distinct  separation  of  sulphur  is  soon  apparent. 
t  The  HI  acts  as  a  catalyser  according  to  Volhard. 
J  Z.  Angew.  Chem.,  1904,  580. 


694  VOLUMETRIC  ANALYSIS. 


31.  Determination    of    Formaldehyde    (Formalin).     Method    of 

G.  Romijn.* 

1000  c.c.  N.  iodine  solution  =  — - —  =— j- —  =15.01  gms.  formaldehyde. 

Principle. — Formaldehyde  is  quantitatively  oxidized  to  formic 
acid  by  remaining  in  contact  with  iodine  for  a  short  time  in 
alkaline  solution: 

HCHO  +  H2O  + 12  =  2HI  +  HCOOH. 

Procedure. — The  aqueous  solution  of  formaldehyde,  known  com- 
mercially as  "  formaline/'  contains  about  40  per  cent,  of  for- 
maldehyde. For  analysis,  10  c.c.  of  the  formaldehyde  solution  are 
diluted  to  400  c.c.,  and  of  this  1  per  cent,  solution,  5  c.c.  ( =0.125  c.c. 

N 
of  the  original  solution)  are  taken   for   analysis.      40  c.c.  of   — 

iodine  solution  are  added,  and  immediately  afterwards  strong 
sodium  hydroxide  solution,  drop  by  drop,  until  the  color  of  the 
solution  is  a  light  yellow;  it  is  then  placed  one  side  for 
ten  minutes.  The  solution  is  then  acidified  with  hydro- 

N 
chloric  acid,  and  the  unused  iodine  is  titrated  back  with  —  sodium 

thiosulphate  solution. 

N 
1  c.c.       iodine  solution= 0.001501  gm.  formaldehyde. 


32.  Determination  of  Hydroferricyanic  Acid.j 


1000  c.c.         iodine  solution^  «=  J*g|g  =32.92  gms.  K,Fe(CN)e. 

Principle.  —  If  a  neutral  solution  of  potassium  ferricyanide  is 
treated  with  an  excess  of  potassium  iodide,  the  ferricyanide  ion 
is  reduced  to  ferrocyanide  ion  with  separation  of  free  iodine  : 
2  A3Fe(CN)  6  +  2KI<=±2K4Fe(CN)  e  +  12. 

*  Zeitschr.  f.  anal.  Chem.,  36  (1897),  p.  19. 

t  Lenssen,  Ann.  Chem.,  91,  240.     Mohr.,  Ibid.,  105,  60. 


DETERMINATION   OF  PHENOL.  695 

Lenssen  titrates  the  liberated  iodine  with  sodium  thiosulphate, 
but  the  results  are  not  concordant,  because  the  reaction  is  a 
reversible  one.  The  reaction  is  quantitative,  however,  as  Mohr 
first  showed,  if  the  ferrocyanide  is  removed  from  the  solution  as 
fast  as  it  is  formed.  This  is  accomplished,  according  to  Mohr, 
by  adding  an  excess  of  zinc  sulphate,  free  from  iron,  to  the 
solution.  According  to  the  experiments  of  E.  Miiller  and  O. 
Diefenthaler,  *  the  titration  should  take  place  in  a  solution 
which  is  as  nearly  neutral  as  possible,  but  not  in  one  made  alkaline 
by  the  addition  of  sodium  bicarbonate  (see  p.  692). 

Miiller  and  Diefenthaler' s  Procedure. — About  0.7  gm.  of  the 
ferrocyanide  is  weighed  into  a  glass-stoppered  flask,  dissolved  in 
about  50  c.c.  water,  and  treated  with  3  gms.  potassium  iodide 
and  1.5  gms.  of  zinc  sulphate  free  from  iron.  If  an  acid  solution 
of  ferricyanide  is  to  be  analyzed,  it  is  carefully  neutralized  with 
caustic  soda  until  barely  alkaline  and  then  just  acidified  with 
a  drop  of  sulphuric  acid.  Alkaline  solutions  must  always  be 
neutralized  with  acid. 


33.  Determination  of  Phenol.     Method  of  W.  Koppeschaar.f 
1000  c.c.       NaAO.-^--l.W7  gms.  CflH5OH. 


Principle.  —  If  an  aqueous  solution  of  phenol  is  treated  with 
an  excess  of  bromine,  the  phenol  is  converted  quantitatively  into 
tribromophenol  : 

C6H5OH+3Br2=3HBr+C9H2Br3(OH). 

The  tribromophenol  is  a  pale  yellow,  crystalline  substance  which 
is  quite  insoluble  in  water  (43,700  parts  of  water  dissolve  1  part 
of  tribromophenol).  If,  after  the  reaction  has  taken  place, 
potassium  iodide  is  added  to  the  solution,  iodine  is  liberated 
corresponding  to  the  excess  of  bromine,  and  by  titrating  this 

*  Z.  anorg.  Chem.,  1910,  418. 
t  Z.  anal.  Chem.,  15,  233  (1876). 


696  VOLUMETRIC    ANALYSIS. 

iodine  with  sodium  thiosulphate  solution,  it  is  easy  to  find  how 
much  bromine  reacted  with  the  phenol. 

Requirements.  —  A  tenth-normal  solution  of  bromine  and  a 
tenth-normal  solution  of  sodium  thiosulphate. 

On  account  of  the  volatility  of  free  bromine,  Koppescharr 
uses  a  solution  of  potassium  bromate  and  bromide  which,  upon 
being  acidified,  gives  a  known  amount  of  bromine  in  accordance 
with  the  equation: 

KBr03  +  5KBr  +  6HC1  =  6KC1  +  3H2O  +  3Br2. 

Thus,  to  obtain  a  tenth-normal  solution  of  bromine  which  will 
keep  indefinitely,  exactly  2.784  gms.  of  pure  potassium  bromate 
(dried  at  100°)  and  about  10  gms.  of  potassium  bromide  are 
dissolved  in  water  and  the  solution  diluted  to  one  liter.  An 
excess  of  bromide  does  no  harm  : 

KBrO       167.02 
^ 


60 

5KBr     5X119.02 
"~60~:       ~~60~ 


_ 


Procedure.  —  About  0.5  gm.  of  phenol  is  weighed  out  in  a 
weighing  beaker,  dissolved  in  a  little  water,  the  solution  rinsed 
into  a  liter  flask  and  well  shaken.  Of  this  solution,  100  c.c.  are 
withdrawn  in  a  pipette,  transferred  to  a  second  liter  flask,  diluted 
with  water  up  to  the  mark,  mixed  and  170  c.c.  this  solution 
transferred  to  a  stoppered  bottle  of  about  250-c.c.  capacity, 
treated  with  50  c.c.  of  the  bromate  solution,  shaken,  acidified 
with  5  c.c.  concentrated  hydrochloric  acid,  shaken  again,  and 
allowed  to  stand  fifteen  minutes.  At  the  end  of  this  time,  2  gms. 
of  potassium  iodide  are  added  and  the  liberated  iodine,  corre- 
sponding to  the  excess  of  bromine,  is  titrated  with  tenth-normal 
thiosulphate  solution,  using  starch  as  indicator.  Then  if  t  c.c. 
of  the  last  solution  are  used  and  the  weight  of  phenol  was  a  gms.  : 


(50-OX0.1567     _ 

-  =  %  phenol. 


REDUCTION  METHODS.  697 

Remark. — Before  making  an  analysis,  a  blank  experiment 
should  always  be  made  with  the  bromate  solution  to  make  sure 
that  its  strength  corresponds  to  the  theoretical  value. 

This  method  is  suitable  for  the  analysis  of  pure  preparations 
of  phenol  (carbolic  acid)  but  not  for  crude  phenol,  creosote  oil, 
etc* 


B.  REDUCTION  METHODS. 

i.  Determination  of  Ferric  Iron  (Fresemus).f 

In  the  case  of  all  methods  previously  discussed,  it  was 
necessary  to  reduce  the  iron  to  the  ferrous  condition  before  it 
could  be  determined  volumetrically.  In  the  following  method, 
first  suggested  by  Penny  and  Wallace, J  but  improved  by  Fresenius, 
the  iron  in  the  ferric  condition  may  be  determined  with  accuracy 
and  rapidity. 

The  hydrochloric  acid  solution  containing  ferric  chloride  is 
titrated  hot  with  stannous  chloride  solution  until  the  former 
becomes  colorless.  By  this  means  the  ferric  salt  will  be  reduced 
to  ferrous  salt: 

2FeCl3 + SnCl2  =  SnCl4  +  2FeCl2. 

Inasmuch  as  it  is  not  very  easy  to  determine  the  end-point 
with  accuracy,  because  the  last  part  of  the  iron  is  reduced  very 
slowly,  it  is  customary  to  run  over  the  end-point  and  to  titrate 
the  excess  of  the  stannous  chloride  with  iodine  solution. 

Solutions  Required.  1.  A  Ferric  Chloride  Solution  Containing  a 
Known  Amount  of  Iron. — It  is  prepared  by  dissolving  exactly 
10.03  gms.  of  bright  iron  wire  in  hydrochloric  acid  within  a  long- 
necked  flask  held  in  an  inclined  position;  the  iron  is  oxidized 
with  potassium  chlorate  and  the  excess  of  chlorine  is  completely 

*  J.  Toth,  Z.  anal.  Chem.,  25,  160  (1886). 
f  Z.  anal.  Chem.,  1,  p.  26. 
j  Dingl.  polyt.  J.,  149,  440. 


698  VOLUMETRIC  ANALYSIS. 

expelled  by  boiling.  The  solution  of  ferric  chloride  is  washed 
into  a  liter  flask  and  diluted  up  to  the  mark  with  wnter;  50  c.c.  of 
this  solution  contain  0.5  gm.  of  pure  iron.* 

2.  A  Stannous  Chloride  Solution. — 25  gms.  of  tin-foil  are  heated 
for  two  hours  on  the  water-bath  with  50  c.c.  of  hydrochloric  acid 
of  specific  gravity  1.134  and  a  few  drops  of  hydrochlorplatinic  acid 
in  a  porcelain  dish  which  is  covered  with  a  watch-glass.     After  this, 
150  c.c.  of  hydrochloric  acid  and  an  equal  volume  of  water  are 
added,  the  solution  filtered  and  diluted  up  to   1  liter.     As  stan- 
nous  chloride  is  oxidized  by   contact  with  the   air,  it   is   placed 
in  a  flask  which  on  one  side  is  connected  with  the  burette  as  shown 
in  Fig.   87,  p.  556  and  on  the  other  side  with  a  Kipp  carbon  di- 
oxide generator. 

3.  An  Iodine  Solution  Approximately  Tenth-normal. 
Procedure. — (a)  Standardization  of  the  Solutions. 

First  of  all,  the  stannous  chloride  and  iodine  solutions  are 
titrated  against  one  another.  About  2  c.c.  of  the  former  are 
measured  from  the  burette,  diluted  to  about  60  c.c.,  a  little  starch 
solution  added,  and  the  mixture  titrated  with  iodine  until  a 
blue  color  is  obtained. 

Next,  50  c.c.  of  the  acid  ferric  chloride  solution  containing 
a  known  amount  of  iron  are  titrated  against  the  stannous  chloride 
solution. 

(6)  Determination  of  Iron  in  Hematite.  5  gms.  of  the 
finely-divided  ore  are  ignited  in  order  to  destroy  any  organic 
matter  which  may  be  present,  then  placed  in  a  long-necked  flask 
and  boiled  with  concentrated  hydrochloric  acid  and  a  little  potas- 
sium chlorate  until  the  iron  oxide  is  all  dissolved,  leaving  behind 
nothing  but  a  white  sandy  residue.  After  this  20  c.c.  more  of 
hydrochloric  acid  are  added  and  the  boiling  is  continued  while 
a  current  of  air  is  passed  through  the  solution,  until  all  the  excess 
of  chlorine  is  completely  removed  and  the  escaping  vapors  will 
no  longer  set  free  iodine  when  passed  into  a  potassium  iodide 
solution.  The  solution  thus  obtained  is  diluted  to  exactly  500  c.c. 
and  50  c.c.  of  it  are  taken  for  the  analysis. 

*  The  assumption  being  made  that  the  iron  wire  contained  99.7  per  cent 
pure  iron. 


DETERMINATION  OF  HYPOCHLOROUS  ACID.  99 

Example. 

1.  Standardization  of  the  reagents: 

2   c.c.  of  stannous  chloride  solution  require 
7.2  c.c.  of  iodine  solution.     1  c.c.  iodine  solution  =0.278  c.c.  SnCl2 

50  c.c.  ferric  chloride  solution  (  —  0.5  gm.  iron) 
require  for  decolorization  .....................     30.34  c.c.  SnCl2 

and  for  the  titration  of  the  excess  0.51  c.c.  or 
iodine  solution  =  0.51  X  0.28  ...................  .       0.14  c.c.  SnCl2 

Consequently  ,  50  c.c.   ferric  chloride  solution 
=  0.5  gm.  iron  ..........................  .....  =30.20  c.c.  SnCl2 

and  1  c.c.  SnCl2  =  ^|_  =0.01656  gm.  Fe. 

2.  Titration  of  the  solution  to  be  analyzed: 

50  c.c.  (  =0.5  gm.  of  iron  ore)  require  .........     18.96  c.c.  SnCl2 

and  for  the  titration  of  the  excess,  0.64  c.c.  of 
iodine  =  0.64X0.28  ...........................  =  0.18  c.c. 


so  that  0.5  gm.  of  ore  corresponds  to  ..........     18.78  c.c.  SnCl2 

and  contain,  therefore,  18.78  X  0.01656  =  0.31  10  gm.  Fe, 
and  in  per  cent.  : 

0.5:0.3110  =  100:z 
Z  =  62.20  per  cent.  Fe. 

2.  Determination  of  Ferric  Iron  by  Means  of  Titanous  Chloride 
(Knecht  and  Hibbert).* 

1000  c.c.  —  TiCl3  solution  =  —  ==0.8  g.  oxygen  =~  =  5.585  g.  Fe. 

Principle.  —  If  an  acid  solution  of  a  ferric  salt  is  treated  with 
titanous  chloride,  the  iron  is  immediately  reduced  in  the  cold  to 
the  ferrous  condition: 

FeCl3  +  TiCl3  =  TiCl4  +  FeCl2  . 

Preparation  of  Titanous  Chloride  Solution.—  A  concentrated 
solution  of  titanous  chloride,  prepared  by  the  electrolysis  of  TiCl4, 
can  now  be  obtained  on  the  market.  Such  a  solution  is  treated 

*Ber.  36,  1551  (1903). 


7.oo  VOLUMETRIC  ANALYSIS. 

with  an  equal  volume  of  concentrated  hydrochloric  acid,  boiled,* 
and  then  diluted  with  ten  times  as  much  boiled  water. 

The  solution  is  maintained  in  contact  with  an  atmosphere  of 
hydrogen,  or  carbon  dioxide,  and  kept  in  a  bottle  such  as  is  shown 
in  Fig.  87,  p.  556  which  is  connected  with  a  burette,  and  in  this 
case  with  a  Kipp  hydrogen,  or  carbon  dioxide,  generator  instead  of 
the  soda  lime  tube. 

Standardization  of  the  Titanous  Chloride  Solution. — A  ferric 
chloride  solution  known  of  strength  is  prepared  as  described  on 
p.  697,  and  of  this  solution  50  c.c.  are  measured  out  into  a  beaker, 
and  the  titanium  trichloride  is  slowly  added  with  constant  stirring, 
while  a  current  of  carbon  dioxide  is  constantly  being  passed  into 
the  beaker.  After  the  solution  is  nearly  decolorized,  a  drop  of 
potassium  sulphocyanate  solution  is  introduced,  and  the  addition 
of  titanous  chloride  is  continued  to  the  disappearance  of  the 
red  color. 

The  analysis  proper  is  carried  out  in  exactly  the  same  manner. 

3.  Determination  of  Ferrous  and  Ferric  Iron  by  the  Titanium 

Method. 

The  ferrous  iron  is  first  titrated  by  means  of  permanganate  in 
the  presence  of  manganous  sulphate  (cf.  p.  607)  and  then  the  total 
iron  is  determined  as  above  with  titanous  chloride. 

The  method  can  be  carried  out  very  rapidly  and  the  results  are 
accurate. 

4.  Determination  of  Hydrogen  Peroxide.f 

If  titanous  chloride  is  run  into  an  acid  solution  of  hydrogen 
peroxide,  the  latter  is  colored  first  yellow,  then  a  deep  orange,  and 
as  soon  as  the  maximum  depth  of  color  is  produced,  it  begins 
to  fade  upon  the  further  addition  of  titanous  chloride  until  finally 
the  solution  becomes  colorless,  which  is  taken  as  the  end-point. 

The  reaction  takes  place  in  two  stages: 

Ti2O3 + 3H2O2  =  2TiO3  +  3H20 
2Ti03  +  2Ti2O3  =  6Ti02 
or  combining  the  two  equations : 

2TiCl3  +  H2O2  +  2HC1  =  2TiCl4  +  2H2O. 

*  The  boiling  serves  to  expel  any  hydrogen  sulphide  that  is  present. 
fKnecht  and  Hibbert,  Ber.,  38,  3324  (1905). 


DETERMINATION  OF  FERRIC  IRON  BY  TiCl,.  7°i 

On  account  of  the  fact  that  the  value  of  the  titanous  chloride 
solution  is  not  very  permanent,  it  is  standardized  against  ferric 
chloride  before  each  series  of  experiments. 

If  t  c.c.  of  titanous  chloride  solution  of  which  1  c.c.  =  a  gms.  Fe 
were  required  for  the  reduction  of  1  c.c.  of  hydrogen  peroxide,  then 
the  amount  of  the  latter  is 


34.02  Xa* 

£  = 

and  in  per  cent. 


. 
x  =  —  gms. 


30.  46a£  =   er  cent.  H20. 


If  it  is  desired  to  express  the  per  cent,  in  per  cent,  by  volume 
of  active  oxygen  (cf.  p.  628)  the  following  proportion  holds: 

10023  -at  =  per  cent,  oxygen  by  volume. 

According  to  Knecht  and  Hibbert,*  persulphuric  acid  may 
likewise  be  estimated  by  titration  with  titanous  chloride.  The 
solution  of  the  persulphate  is  treated  with  titanous  chloride 
solution  and  the  excess  of  the  latter  is  titrated  with  ferric  chloride 
in  an  atmosphere  of  carbon  dioxide. 

5.  Determination  of  Hypochlorous  Acid  by  Means  of  Arsenious 

Acid. 

N 
1000  c.c.  yrr  As2O3=  3.546  gms.  chlorine. 

On  adding  arsenious  acid  to  a  solution  of  a  hypochlorite,  the 
former  is  oxidized  to  arsenic  acid;  while  the  latter  is  reduced  to 
chloride  : 

2XaOCl  +  As2O3  =  As2O5  +  2NaCl. 

The  end-point  is  reached  when  a  drop  of  the  solution  added 
to  a  piece  of  iodo-starch  paper  will  cause  no  blue  coloration. 

Alkali  hypochlorites   and   chloride  of  lime  may  be  analyzed 
by  this  method  and  the  results  obtained  are  more  reliable  than 
in  the  case  of  those  obtained  by  the  iodimetric  method  described 
on  p.  669,  for  the  presence  of  chlorate  has  no  effect  in  this  case. 
*  Knecht  and  Hibbert,  Ber.  38,  3324  (1905). 


702  VOLUMETRIC  ANALYSIS. 

III.  PRECIPITATION   ANALYSES. 
I.  Determination  of  Silver.     Method  of  Gay-Lussac. 

This  exceedingly  accurate  determination,  which  is  extensively 
used  for  testing  silver  alloys,  depends  upon  the  precipitation  of 
silver  chloride  from  nitric  acid  solution.  Common  salt  is  used 
as  the  precipitant. 

Solutions  Required.  1.  Sodium  Chloride  Solution  of  Known 
Concentration. — For  convenience,  it  is  customary  to  make  the 
solution  of  such  a  strength  that  1000  c.c.  correspond  to  exactly  5 
gms.  of  silver.  It  is  more  practical,  however,  to  use  a  some- 
what weaker  solution,  consequently  2.700  gms.  of  chemically  pure 
salt  are  dissolved  in  distilled  water  and  diluted  to  1  liter. 

2.  Decimal  Solution  of  Sodium  Chloride. — 100  c.c.  of  the  above 
solution  are  diluted  with  distilled  water  to  1  liter. 

In  laboratories  where  silver  determinations  are  frequently 
made,  the  above  solutions  are  made  up  in  much  larger  quanti- 
ties and  kept  in  bottles  similar  to  the  one  shown  in  Fig.  87,  p.  556. 
The  stronger  solution  is  connected  with  a  100-c.c.  pipette  and 
the  decimal  solution  with  a  burette. 

Standardization  of  the  Sodium  Chloride  Solution.  —  Exactly 
0.5  gm.  of  chemically  pure  silver  is  weighed  into  a 
200-c.c.  flask  provided  with  a  well-ground  glass  stopper,  and 
dissolved  in  10  c.c.  of  nitric  acid  of  specific  gravity  1.2,  free 
from  chlorine.  The  solution  is  hastened  by  heating  on  a  sand- 
bath.  When  the  silver  has  dissolved,  the  solution  is  heated  to 
boiling  in  order  to  expel  the  nitrous  acid  formed.  The  brown 
vapors  collecting  in  the  flask  are  removed  by  blowing  in  air. 
As  soon  as  no  more  of  these  are  formed,  the  flask  is 
removed  from  the  sand-bath,  and  allowed  to  cool.  To  the 
silver  sclution  exactly  100  c.c.  of  the  stronger  salt  solution  are 
added,  the  flask  stoppered,  and  vigorously  shaken  until  the  pre- 
cipitated silver  chloride  collects  together,  and  the  supernatant 
liquid  appears  clear. 


DETERMINATION  OF  SILVER.  7°3 

As  the  salt  solution  was  made  up  a  little  weak,  the  precipita- 
tion of  the  silver  is  not  quite  complete  and  consequently  more 
sodium  chloride  must  be  added.  For  this  purpose  half  a  cubic 
centimeter  of  the  decimal  salt  solution  is  added  from  the  burette, 
so  that  the  solution  runs  down  the  sides  of  the  flask  upon  the 
surface  of  the  liquid,  causing  a  distinct  cloud  of  silver  chloride 
to  be  formed.  The  liquid  is  shaken,  allowed  to  settle,  again 
treated  with  half  a  cubic  centimeter  of  the  decimal  salt  solution> 
and  the  process  repeated  until  finally  the  addition  of  the  ^alt 
solution  fails  to  produce  any  further  turbidity;  the  last  half  cubic 
centimeter  is  not  used  in  the  calculation. 

Example. — 0.5  gm.  of  chemically  pure  silver  (fJHHJ-  fine)  re- 
quired 100  c.c.  of  the  standard  salt  solution +1  c.c.  of  the  decimal 
solution,  i.e.,  100.1  c.c.  of  the  salt  solution  correspond  to  1000 
silver;  *  this  is  the  value  of  the  salt  solution. 

Silver  Determination. — In  order  to  obtain  absolutely  accurate 
results  it  is  necessary  to  employ  the  same  amount  of  silver  for 
the  analysis  as  was  used  in  the  standardization  of  the  solution, 
consequently  the  approximate  amount  of  silver  present  in  the  alloy 
must  be  determined.  This  can  be  accomplished  by  cupellation, 
or  volurnetrically  by  the  method  of  Volhard,  described  further  on. 

Example. — It  was  found  by  cupellation  that  an  alloy  contained 
about  /oVo  fi116  silver;  for  the  titration  an  amount  must  be  taken 
which  will  contain  0.5  gm.  of  silver;  we  have  then 

l:0.8  =  :c:0.5 
z=0.625  gm. 

We  weigh  out,  therefore,  0.625  gm.  (  =  1250f)  of  the  alloy  and 
proceed  exactly  as  in  the  standardization. 

1250  of  alloy  require  for  the  precipitation  of  the  silver  100  c.c. 
of  the  standard  salt  solution +  3  c.c.  of  the  decimal  solution,  i.e., 
1250  parts  of  the  alloy  require  100.3  c.c.  of  the  standard  salt 


*  For  convenience  in  calculation,  0.5  gm.  of  pure  silver    is    designated 
by  1000,  0.25  gm.  by  500,  and  0.1  gm.  by  250,  etc. 

f  If  0.5  gm.=  1000,  then  0.5: 1000=  0.625 :x\   x=1250. 


704  VOLUMETRIC  ANALYSIS. 

solution.     Since    100.1    c.c.    of   this   salt  solution   correspond   to 
1000  parts  of  pure  silver,  we  have 


100.1::  1000  =  100.3:a;; 

1000  X  100  3 

x=  —  —  =1002  parts  silver  in  1250  parts  of  alloy; 

lUU.l 

so  that  in  1000  parts  of  the  alloy  there  will  be 

1250:  1002  =  1000:  x 
#=801.6  parts  fine  of  silver. 

This  procedure  is  designated  as  the  French  method  in  contrast 
to  the  German  or  Dutch  method.  In  the  latter  case,  0.5  gm. 
of  the  alloy  (  =  1000)  is  weighed  out  and  the  same  amount  of 
silver  is  added  which  the  alloy  lacks  in  fineness.  In  this  way 
one  more  weighing  is  necessary,  but  the  calculation  is  somewhat 
simpler. 

Example.  —  By  cupellation  an  alloy  is  found  to  contain  fipfo 
silver.  In  order  to  make  the  silver  equal  1000,  200  parts  of 
fine  silver  must  be  added.  For  the  analysis,  therefore,  0.5  gm. 
of  the  alloy  and  0.1  gm.  of  pure  silver  (=200)  are  taken,  dis- 
solved in  nitric  acid,  and  titrated  with  sodium  chloride. 

For  the  titration  of  the  alloy,  100.25  c.c.  of  the  stronger 
salt  solution  were  required,  or  of  the  decimal  solution,  1002.5  c.c. 
and  for  the  titration  of  1000  fine  silver  (0.5  gm.)  .......  1001.0  c.c. 

Difference  ............................    =   1.5  c.c. 


As  1  c.c.  of  the  decimal  solution  corresponds  to  y-oVir*  silver, 
it  is  evident  that  1.5  c.c.  are  equivalent  to  T^T  silver.  If  this 
amount  is  added  to  the  assumed  silver  contents  (in  this  case  800), 
the  true  fineness  of  the  silver  alloy  will  be  obtained;  i.e.  801.5 
parts  fine  silver. 


*  100.1  c.c.  of  the  stronger  salt  solution=  5  gms.  {%%%  silver,  then  1001  c.c. 
of  the  decimal  solution  correspond  to  the  same  amount,  and  1  c.c.  =  7^5 
silver. 


DETERMINATION  OF  SILVER.  7°5 


2.  Determination  of  Silver  (Volhard). 
1000  c.c.  ^  KCNS=^=  1-^p  =  10.788  gms.  Ag. 

If  to  a  silver  solution  containing  iron  ammonium  rlum,  free 
from  chloride  but  containing  enough  nitric  acid  to  discharge  the 
brown  color  of  the  iron  salt,  a  solution  of  alkali  sulphocyanate  is 
added,  white  insoluble  silver  sulphocyanate  is  precipitated: 

AgN03+KCNS  =  KN03+AgCNS. 

When  all  the  silver  is  precipitated,  the  next  drop  of  the  sulpho- 
cyanate solution  will  cause  a  permanent  red  coloration  due  to  the 
formation  of  ferric  sulphocyanate. 

Requirements.  1.  Tenth-normal  Silver  Solution.  —  10.788  gms. 
of  chemically  pure  silver  are  dissolved  in  nitric  acid  free  from  chlo- 
ride, boiled  until  the  nitrous  acid  is  all  removed,  and  diluted 
with  distilled  water  to  a  volume  of  1  liter. 

2.  Tenth-normal  Potassium  (or  Ammonium)  Sulphocyanate  Solu- 
tion. —  As  both  of  these  salts  are  hygroscopic  and  cannot  be  dried 
without  decomposition,  an  exactly  tenth-normal  solution  cannot 
be  prepared  by  weighing  out  the  solid  salt.     Approximately,  the 
right   amount  (about  10  gms.  KCNS   or  9  gms.  NH4CNS)  is  dis- 
solved in  a  liter  of  water  and  the  solution  standardized  against 
the  silver  solution. 

3.  Iron-ammonium  Alum  Solution.  —  A  cold,  saturated  solution 
of   ferric    alum   to    which   enough    nitric  acid  is  added  to  cause 
the   disappearance   of   the   brown   color.     Of   this   indicator   the 
same  amount  is  used  for  all  titrations,  about  1  or  2  c.c.  for  100  c.c. 
of  the  silver  solution. 

For  the  standardization  of  the  sulphocyanate  solution,  20  c.c. 
of  the  silver  solution  are  placed  in  a  beaker,  diluted  with  about 
50  c.c.  of  water,  and  1  c.c.  of  the  indicator  added.  The  sulpho- 
cyanate solution  is  then  added  from  a  burette,  with  constant 
stirring,  until  a  permanent  red  color  is  obtained. 


706  VOLUMETRIC  A NA LYSIS. 


Determination  of  Silver  in  Silver  Alloys. 

About  0.5  gm.  of  the  brightly  polished  metal  is  dissolved  in 
nitric  acid  of  specific  gravity  1.2,  the  solution  boiled  to  expel 
the  nitrous  acid,  diluted  with  cold  water  to  about  50  c.c.,  and 
after  the  addition  of  1  c.c.  of  the  ferric  alum  solution  it  is  titrated 
with  the  sulphocyanate  solution  as  in  the  standardization  of  the 
latter.  The  presence  of  metals  whose  salts  are  colorless  doe: 
not  influence  the  accuracy  of  this  determination,  except  tha 
mercury  must  be  absent  because  its  sulphocyanates  are  insol- 
uble. Nickel  and  cobalt  must  not  be  present  to  any  extent, 
because  their  salts  are  colored,  and  not  more  than  60  per  cent, 
of  copper  in  an  alloy  is  permissible.  In  case  more  copper  is  present 
the  following  procedure  must  be  used:  The  silver  is  precipitated 
by  means  of  an  excess  of  alkali  sulphocyanate,  washed  completely 
with  water,  the  funnel  placed  over  an  Erlenmeyer  flask,  the  apex 
of  the  filter  broken,  its  contents  washed  into  a  flask  by  means  of 
concentrated  nitric  acid  (sp.  gr.  1.4),  and  the  liquid  heated  to  gentle 
boiling  for  three-quarters  of  an  hour.  As  the  sulphuric  acid 
formed  will  have  some  influence  upon  the  subsequent  titration, 
the  solution  is  diluted  with  water  to  about  100  c.c.,  and  a  con- 
centrated barium  nitrate  solution  is  added  drop  by  drop  until 
the  sulphuric  acid  is  all  precipitated,  after  which  the  silver  is 
titrated  with  sulphocyanate  solution  without  filtering  off  the 
barium  sulphate. 

Remark. — From  experiments  in  his  laboratory  carried  out  by 
Osann,  the  author  concludes  that  the  Volhard  method  is  less 
reliable  than  that  of  Gay-Lussac.  Apparently  the  experiments 
of  Hoitsema  *  indicate  that  the  precipitate  adsorbs  potassium 
thiocyanate.  If,  however,  the  solution  is  standardized  against 
very  nearly  the  same  quantity  of  silver  (or  the  equivalent  amount 
of  silver  nitrate)  as  is  taken  for  analysis,  this  error  is  compensated 
and  the  results  are  very  exact. — (TRANSLATOR.) 

*  Z.  angew.  Chem.,  1904,  647. 


DETERMINATION  OF  CHLORINE.  7<>7 

3.  Determination  of  Chlorine, 
(a)   Volhard's  Method. 

N  Cl 

1000  c.c.  YQ  AgNO3  solution  =  —  =  3.546  gms.  chlorine. 

According  to  Volhard's  original  directions,  the  chloride  solution 
treated  with  tenth-normal  silver  nitrate  solution  and  then, 
without  filtering  off  the  precipitate,  5  c.c.  of  the  ferric-ammonium 
alum  solution  were  added  and  the  excess  of  silver  titrated 
with  tenth-normal  potassium  or  ammonium  thiocyanate  (see 
p.  705). 

The  results  are  satisfactory  with  large  quantities  of  chlorider 
but  in  the  titration  of  small  quantities  of  chloride  too.  high 
results  are  obtained,  as  was  first  shown  by  G.  Drechsel  *  and 
later  confirmed  by  JL  A.  Rosanoff  and  A.  E.  Hill.f  Drechsel 
showed  that  it  was  impossible  to  get  the  true  end-point  of  the 
reaction,  as  the  red  coloration  gradually  disappeared  on  stirring, 
remaining  permanent  only  after  a  considerable  excess  of  thio- 
cyanate had  been  added.  The  reason  for  this  is  that  silver 
chloride  is  more  soluble  than  silver  thiocyanate.  Thus  the 
precipitate  gradually  reacts  with  the  red  ferric  thiocyanate,  as 
follows : 

3  AgCl  +  Fe  (CNS)  3 = 3  AgCNS  +  FeCl3. 

To  avoid  this  error  Drechsel  proceeds  as  follows: 
The  chloride  solution  is  placed  in  a  200-c.c.  graduated  flask, 
an  excess  of  0.1X  AgXO3  solution  added,  the  solution  acidified 
with  nitric  acid,  and  the  stoppered  flask  shaken  until  the  pre- 
cipitate coagulates  enough  to  give  a  clear  supernatant  liquid. 
The  solution  is  then  diluted  up  to  the  mark,  thoroughly  mixed 
and  filtered  through  a  dry  filter,  rejecting  the  first  10  c.c.  of 
filtrate.  Of  the  filtrate,  50  or  100  c.c.  are  taken,  the  ferric  alum 
indicator  added,  and  the  excess  of  silver  titrated  with  0.1N 
thiocyanate  solution.  The  results  thus  obtained  are  excellent. 

*  Z.  anal.  Chem.,  16,  351  (1877). 
t  J.  Am.  Chem.,  Soc.  29,  269. 


708  VOLUMETRIC  ^ ANALYSIS. 

Remark. — V.  Rothmund  and  A.  Burgstaller  *  find  that  it  is 
possible  to  obtain  correct  results  without  filtering  off  the  silver 
chloride  precipitate.  They  heat  the  solution  after  the  addition 
of  the  excess  of  silver  nitrate,  until  the  precipitate  coagulates 
thoroughly,  in  which  form  it  reacts  less  readily  with  a  soluble 
thiocyanate.  After  cooling,  the  ferric  alum  indicator  is  added 
and  the  titration  finished.  Rothmund  and  Burgstaller  also  find 
that  the  coagulation  of  the  silver  chloride  precipitate  by  ether  f 
suffices  to  make  the  filtration  unnecessary.  The  chloride  solution 
is  placed  in  a  flask  with  tightly  fitting  glass  stopper,  5  c.c.  of 
ether  added,  and  an  excess  of  silver  nitrate  solution.  After 
shaking  a  -few  minutes,  the  supernatant  solution  becomes  clear 
and  the  titration  can  be  finished  with  accuracy. 

(6)  Fr.  Mohr's  Method. 

If  the  neutral  solution  of  an  alkaline  or  alkaline-earth  chloride 
containing  a  few  drops  of  potassium  chromate  solution  *  is  treated 
with  silver  nitrate  solution,  added  from  a  burette,  a  red  precipi- 
tate of  silver  chromate  is  formed  which,  on  stirring,  disappears  on 
account  of  its  being  decomposed  by  the  alkali  chloride  to  silver 
chloride  and  alkali  chromate: 

Ag2CrO4  +  2NaCl  =  2  AgCl + Na2Cr04. 

When  all  of  the  chlorine  is  changed  to  insoluble  silver  chloride, 
the  next  drop  of  the  silver  solution  will  impart  a  permanent 
reddish  color  to  the  liquid.  For  small  amounts  of  chloride  in 
concentrated  solutions  this  method  gives  very  sharp  results.  If, 
however,  the  volume  of  the  solution  is  too  large,  the  results  are 
not  very  accurate.  In  all  cases,  a  blank  experiment  must  be 
made  to  see  how  much  of  the  silver  solution  is  necessary  to 
produce  the  red  shade  used  in  the  titration  when  no  chloride  is 
present,  and  this  amount  must  be  deducted  from  that  used  in 
the  analysis. 

*  Z.  anorg.  Chem.,  63,  330  (1909). 
t  Cf.  E.  Alefeld,  Z.  anal.  Chem.,  48,  79  (1909). 

J  Lunge  uses  sodium  arseniate  as  indicator,  and  this  is  to  be  recommended 
on  account  of  the  change  from  colorless  to  brown  being  very  easy  to  detect. 


DETERMINATION  OF  BROMINE  AND  IODINE.  709 

Remark. — If  it  is  desired  to  titrate  free  hydrochloric  acid, 
the  solution  is  first  neutralized  with  ammonia.  In  the  case  of 
colorless  chlorides  having  an  acid  reaction  (A1C13)  the  solution  is 
treated  with  an  excess  of  neutral  sodium  acetate  solution  and 
then  titrated.  With  colored  metal  chlorides,  the  metal  is  pre- 
cipitated with  caustic  potash  or  sodium  carbonate,  filtered, 
washed,  the  filtrate  acidified  faintly  with  acetic  acid,  and  the 
tit  rat  ion  then  made. 

4.  Determination  of  Bromine, 
(a)   Volhard's  Method. 

1000  c.c.  ^  AgXO3  solution  =  ^  =  7. 992.  gms.  bromine. 

The  solution  of  the  bromide  is  treated  with  an  excess  of 
0.1X  silver  solution  and  the  solution  titrated  with  ammonium 
thiocyanate,  using  ferric  alum  as  indicator.  From  the  required 
volume  of  silver  nitrate,  the  quantity  of  bromine  is  com- 
puted. 

Remark. — It  is  not  necessary  to  filter  off  the  silver  bromide, 
because,  unlike  the  chloride,  silver  bromide  is  more  insoluble 
than  is  silver  thiocyanate. 

(6)  Fr.  Mvhr's  Method. 

The  procedure  is  the  same  as  in  the  case  of  the  chloride  deter- 
mination. 

5.  Determination  of  Iodine. 
Volhard's  Method. 

1000  c.c.  ^  AgNO3  solution-^  =12.692  gms.  iodine. 

If  silver  iodide  is  produced  in  a  solution  of  an  iodide  by  the 
addition  of  silver  nitrate,  the  precipitate  will  usually  enclose  a 


7io  VOLUMETRIC  ANALYSIS. 

measurable  amount  of  either  the  soluble  iodide  or  the  silver  nitrate, 
so  that  the  analysis  cannot  be  accomplished  in  the  same  way  as 
in  the  analysis  of  chlorides  and  bromides. 

The  solution  is  placed  in  a  glass-stoppered  flask,  diluted  to 
200-300  c.c.,  and  the  silver  solution  is  added  with  vigorous  shak- 
ing until  the  yellow  precipitate  col'ects  together  and  the  superna- 
tant liquid  appears  colorless.  As  long  as  the  solution  appears  milky 
the  precipitation  is  not  complete.  A  little  more  silver  nitrate  is 
finally  added  and  the  solution  again  shaken  in  order  to  precipitate 
any  iodide  in  the  pores  of  the  silver  iodide.  Then  ferric  alum 
solution*  is  added,  the  excess  of  silver  titrated  with  potassium 
sulphocyanate,  and  the  iodine  calculated  from  the  amount  of 
silver  used.  In  this  way  Volhard  obtained  exact  results. 

6.  Determination  of  Cyanogen. 

(a)  Volhard 's  Method. 

1000  c.c.  ^  AgNO,  solution  =—     -  =  6.511  gms.  KCN. 

If  an  excess  of  silver  nitrate  is  added  to  a  solution  containing 
potassium  cyanide  and  we  attempt  to  titrate  the  excess  of  the 
former  by  means  of  potassium  sulphocyanate,  using  a  ferric  salt 
as  an  indicator,  there  will  be  no  distinct  end-point,  because  the 
silver  cyanide  reacts  with  the  ferric  sulphocyanate: 

SAgCN + Fe(CNS)3 + 3HNO3  =  3  AgCNS  +  3HCN + Fe(  NO3)3. 

The  red  color  obtained  in  the  titration  will  disappear  on  stirring. 
If,  however,  the  neutral  cyanide  solution  is  treated  with  an  excess 
of  the  silver  solution,  then  slightly  acidified  with  nitric  acid,  di- 
luted up  to  a  definite  volume  in  a  measuring-flask  and  filtered 
through  a  dry  filter,  the  excess  of  silver  can  then  be  titrated  in 
an  aliquot  part  of  the  filtrate. 

*  The  ferric  solution  must  not  be  added  before  the  iodine  is  completely 
precipitated,  because  in  acid  solution  it  oxidizes  the  hydriodic  acid  with 
separation  of  iodine.  Silver  iodide,  however,  is  without  action  on  ferric 
salts. 


DETERMINATION   OF  CHLORINE  AND   CYANOGEN.  711 

(6)  Liebig's  Method* 

1000  c.c.  —  AgNO3  solution  =  - =  13.022  gms.  KCN. 

10  5 

On  adding  silver  nitrate  solution  drop  by  drop  to  a  neutral 
or  alkaline  alkali  cyanide,  a  white  precipitate  is  formed  when 
the  two  liquids  first  come  in  contact  with  one  another,  but  on 
stirring  it  redissolves  owing  to  the  formation  of  potassium  silver 
cyanide : 

AgCN + KCN  =  Ag(CN)2K. 

As  soon  as  all  of  the  cyanogen  is  transformed  into  potassium 
silver  cyanide,  the  next  drop  of  the  silver  solution  will  produce 
a  permanent  turbidity: 

Ag(CN)2K+AgN03  =  KN03+2AgCN. 
The  total  reaction  is,  therefore, 

2KCN+AgN03=KN03+Ag(CN)2K. 

1  Ag  corresponds  to  2  CN  and  the  end-point  of  the  reaction 
is  shown  by  the  formation  of  a  permanent  precipitate. 

The  alkali  cyanide  solution  is  placed  in  a  beaker,  a  little  potas- 
sium hydroxide  is  added,  and  the  solution  diluted  to  a  volume 
of  about  100  c.c.  The  beaker  is  placed  upon  a  piece  of  black 
glazed  paper  and  titrated  with  constant  stirring  until  the  tur- 
bidity is  obtained. 

For  the  analysis  of  free  hydrocyanic  acid,  the  solution  is  satu- 
rated with  potassium  hydroxide  and  treated  as  above. 

Determination  of  Chlorine  and  Cyanogen  in  the  Presence 
of  One  Another. 

First,  the  cyanogen  is  determined  by  the  method  of  Liebig,  and 
then  enough  silver  solution  is  added  to  convert  all  of  the  cyanogen 
and  chlorine  into  their  silver  salts.  The  solution  is  acidified 
with  nitric  acid,  diluted  with  water  to  a  definite  volume,  filtered 

*  Ann.  d.  Chem.  und  Pharm.,  77.  p.  102. 


712  VOLUMETRIC  ANALYSIS. 

through  a  dry  filter,  and  an  aliquot  part  of  the  filtrate  used  for 
the  titration  of  the  excess  of  silver  by  means  of  potassium  sulpho- 
cyanate,  according  to  Volhard.  The  calculation  of  the  cyanogen 
and  chlorine  is  illustrated  by  the  following  example : 

10  c.c.  of  the  solution  required  for  the  production  of  a  per- 

N  N 

manent  turbidity  t  c.c.  —  silver  solution.     Then  an  excess  of  -— 

silver  solution  is  added  (T  c.c.  being  the  total  amount  used),  the 
solution  acidified  with  nitric  acid,  diluted  to  exactly  200  c.c.,* 
filtered  through  a  dry  filter,  and  the  excess  of  the  silver  titrated 

N 
in  100  c.c.  of  the  filtrate;  this  required  ^  c.c.  —  potassium  sulpho- 

cyanate  solution.  Consequently  the  amount  of  cyanogen  pres- 
ent is  *X 0.005202  gm.,  and  the  chlorine  present  amounts  to 


7.  Determination  of  Sulphocyanic  Acid.     Volhard's  Method. 

N  HPN^ 

1000  c.c.        AgNO3  solution  =  £  =  5.909  gms.  HCNS. 


This  is  the  reverse  of  the  silver  determination  (p.  705).     An 

N 
excess  of  —  silver  solution  is  added  to  the  solution  containing 

the  sulphocyanate,  and  the  excess  of  silver  is  titrated  with  potas- 
sium sulphocyanate  solution,  using  ferric  alum  as  an  indicator. 

Determination  of  Sulphocyanic  and  Hydrocyanic  Acids  in  the 
Presence  of  One  Another. 

A  little  potassium  hydroxide  is  added  to  the  solution,  and 
after  diluting  to  about  100  c.c.,  the  cyanogen  is  titrated  by  the 
method  of  Liebig  (p.  711).  Then,  after  adding  an  excess  of  silver 
solution,  nitric  acid  is  added  to  acid  reaction,  and  the  excess  of 
the  silver  is  titrated  with  potassium  sulphocyanate  in  an  aliquot 
part  of  the  filtrate. 

*  The  operation  is  performed  in  a  measuring-flask.  After  the  addition 
of  the  acid,  the  flask  is  filled  up  to  the  mark  with  water,  thoroughly  mixed, 
and  then  filtered. 


DETERMINATION  OF  HYDROCHLORIC  ACID,  ETC.  7*3 


Determination  of  Hydrochloric,  Hydrocyanic,  and  Sulphocyanic 
Acids  in  the  Presence  of  One  Another. 

In  one  portion  the  cyanogen  is  determined  according  to  Liebig. 

N 
A  second  portion  is  treated  with  an  excess  of  —  silver  solution, 

acidified  with  nitric  acid,  filtered,  the  precipitate  washed  with 
water,  and  the  excess  of  silver  in  the  filtrate  determined  according 
to  Volhard.  The  filter  containing  the  precipitate  is  washed  by 
means  of  concentrated  nitric  acid  into  a  flask  and  boiled  for 
three-quarters  of  an  hour.  By  this  means  the  cyanide  and  sulpho- 
cyanate  of  silver  go  into  solution,  while  the  silver  chloride 
remains  undissolved.  The  solution  is  diluted  to  about  100  c.c., 
a  sufficient  amount  of  barium  nitrate  is  added  to  precipitate  the 
sulphuric  acid  formed,  and  the  silver  corresponding  to  the  cyanide 
and  sulphocyanate  is  titrated  with  potassium  sulphocyanate  with- 
out filtering  off  the  silver  chloride  or  barium  sulphate. 
The  calculation  is  accomplished  as  follows: 

N 

1.  For  the  titration  of  the  cyanide  in  alkaline  solution,  t  c.c.  -^ 

silver  solution  were  necessary,  and  for  the  precipitation  of   the 

N 

same  amount  of  cyanogen  in  acid  solution  2  t  c.c.  —  silver  solu- 
tion were  required. 

2.  For  the  precipitation  of  the  chlori ne-h  cyanogen  +  suipho- 

N 
cyanogen  in  acid  solution,  T  c.c.  of  —  silver  solution  were  used. 

N 

3.  Finally,  ^  c.c.  yp:  KCNS  solution  were  used  for  the  precipi- 
tation of  the  silver  cyanide  +  sulphocyanide. 

Then 

1.  Cyanogen  =tX 0.005202  gm.  CN. 

2.  Sulphocyanogen  =  (*i-20  X  0.005808  gm.  CNS. 

3.  Chlorine  =  (77- «  X0.003546  gm.  Cl. 


VOLUMETRIC  ANALYSIS. 


8.  Determination  of  Sulphuric  Acid  by  Benzidine  Hydrochloride.* 
1000  c.c.        NaOH--5-4.904  gms.  H2SO4. 


Benzidine,  Ci2H8(NH2)2,  is  a  weak  organic  base.  It  forms 
stable  salts  with  strong  mineral  acids,  of  which  the  ^sulphate  is 
characterized  by  its  slight  solubility,  particularly  in  water  con- 
taining hydrochloric  acid.  The  base  itself  is  neutral  toward 
phenolphthaleiin.  On  account  of  being  such  a  weak  base,  there- 
fore, the  aqueous  solutions  of  its  salts  undergo  hydrolysis.  Thus 
benzidine  hydrochloride  is  decomposed  according  to  the  equation  : 


into  hydrochloric  acid  and  benzidine  hydroxide,  and  the  latter 
breaks  down  further  into  benzidine  and  water: 

C12H8(NH2)2(HOH)2^C12H8(NH2)2  +  2H20. 

In  other  words,  an  aqueous  solution  of  benzidine  hydrochloride 
behaves  like  a  mixture  of  hydrochloric  acid  and  benzidine,  and 
the  amount  of  acid  present  may  be  titrated  with  alkali,  using 
phenolphthalei'n  as  indicator. 

There  are  two  methods  which  have  been  used  for  the  volumetric 
estimation  of  sulphuric  acid  by  means  of  benzidine.  Miiller  treated 
the  neutral  solution  of  the  sulphate  with  a  solution  of  benzidine 
hydrochloride  of  known  acidity. 

Ci2H8(NH2)2  -  2HC1  +  Na2SO4  =  2NaCl  +  C12H8(NH2)2  .  H2SO4. 

Insoluble 

The  precipitate  of  benzidine  sulphate  was  filtered  off  and  the 
nitrate  titrated  with  a  standard  solution  of  alkali.  The  loss  in 

t  W.  Muller,  Ber.,  35,  1587  (1902);  Muller  and  Diirkes,  Z.  anal.  Chem., 
42,  477  (1903);  F.  Raschig,  Z.  angew.  Chem.,  1903,  617  and  818;  von  Knorre, 
Chem.  Ind.,  28,  2;  and  Friedheim  and  Nydegger,  Z.  angew.  Chem.,  1907,  9. 


DETERMINATION  OF  SULPHURIC  ACID.  715 

acidity  corresponded  to  the  amount  of  sulphuric  acid  present. 
Haschig,  on  the  other  hand,  recommends  treating  the  neutral 
or  acid  solution  of  the  sulphate  with  benzidine  hydrochloride 
solution,  filtering  off  the  precipitated  benzidine  sulphate,  washing 
it,  and  then  suspending-  it  in  water  and  titrating  the  sulphuric  acid 
with  tenth-normal  sodium  hydroxide. 

The  latter  method  constitutes  a  direct  determination  and 
has  the  advantage  that  it  does  not  require  a  neutralization 
of  the  solution  before  the  treatment  with  benzidine  hydro- 
chloride. 

In  the  precipitation  of  sulphuric  acid  by  means  of  benzidine, 
there  are  two  sources  of  error.  In  the  first  place  the  precipitate 
is  not  perfectly  insoluble,  so  that  an  appreciable  amount  of 
sulphuric  acid  is  not  precipitated.  In  the  second  place,  the 
precipitate  tends  to  adsorb  some  benzidine  hydrochloride.  These 
two  sources  of  error  influence  the  results  of  an  analysis  in  opposite 
directions  and  either  wholly  or  partly  compensate  one  another. 
Friedheim  and  Xydegger  have  studied  the  method  from  this 
point  of  view  and  have  apparently  succeeded  in  working  out  the 
conditions  whereby  the  sum  of  the  errors  becomes  practically 
zero. 

Procedure. — To  prepare  the  solution  of  benzidine  hydrochloride, 
6.7  gms.  of  the  free  base,  or  the  corresponding  amount  of  the 
hydrochloride,*  is  rubbed  up  in  a  mortar  with  20  c.c.  of  water. 
The  paste  is  rinsed  into  a  liter  flask,  20  c.c.  of  hydrochloric  acid 
(sp.  gr.  1.12)  are  added,  and  the  solution  diluted  up  to  the  mark. 
(1  c.c.  of  this  solution  corresponds  theoretically  to  0.00357  gms. 
H2SO4,)  The  solution  has  a  brown  color  and  may  be  filtered  if 
necessary.  After  some  time  brown  flakes  are  likely  to  separate, 
but  these  do  no  harm. 

The  solution  of  the  sulphate  is  diluted  with  water  until  its 
volume  corresponds  to  not  less  than  50  c.c.  for  each  0.1  gm.  of 
sulphuric  acid  present.  An  equal  volume  of  the  reagent  is  added 
while  stirring  vigorously.  A  filter  is  prepared  by  placing  a  Witt 
perforated  porcelain  filter  plate  in  a  funnel  and  covering  it  with 
two  moistened  filter  papers,  one  of  exactly  the  same  size  as  the 


*  The  commercial   salt  contains  varying  amounts  of  hydrochloric  acid. 
This  can  be  determined  by  titration  with  alkali. 


716  VOLUMETRIC  ANALYSIS. 

plate  and  the  upper  one  a  little  larger.  After  ten  minutes,  tho 
precipitate  is  filtered  off  upon  this  filter,  using  gentle  suction.  The 
last  portions  of  the  precipitate  are  transferred  to  the  filter  with 
the  aid  of  small  portions  of  the  clear  filtrate,  and  then  the  beaker 
and  precipitate  are  washed  with  20  c.c.  of  cold  water,  added  in 
several  portions.  The  precipitate  and  filter,  but  not  the  plate, 
are  then  transferred  to  an  Erlenmeyer  flask,  50  c.c.  of  water  are 
added  and  the  contents  of  the  stoppered  beaker  shaken  until  a 
homogeneous  paste  is  obtained.  The  rubber  stopper  is  then 
removed  from  the  flask,  rinsed  off  with  water,  a  drop  of  phenol- 
phthalei'n  added,  the  water  heated  to  about  50°  and  titrated  with 
tenth-normal  sodium  hydroxide.  When  the  end  point  is  nearly 
reached,  the  liquid  is  boiled  for  five  minutes,  and  the  titration 
then  finished. 

Remark. — According  to  Friedheim  and  Nydegger,  this  method 
gives  excellent  results  in  the  analysis  of  all  sulphates  provided 
no  substances  are  present  which  attack  benzidine,  and  provided 
the  amount  of  other  salts  and  acids  present  is  not  too  great.  There 
should  not  be  more  than  10  mol.  of  HC1,  15  mol.  HNO3,  20  mol. 
HC2H3O2,  5  mol.  alkali  salt,  or  2  mol.  ferric  iron  present  to  1  mol. 
H2SO4.  A  satisfactory  determination  of  the  sulphur  in  pyrite  may 
be  made  by  dissolving  0.5  of  the  sample  according  to  the  Lunge 
method  (see  p.  362),  evaporating  off  the  nitric  acid,  taking  up  the 
residue  in  a  little  hydrochloric  acid,  diluting  to  500  c.c.  and  using 
100  c.c.  for  the  treatment  with  benzidine  hydrochloride. 


9.  Determination  of  Sulphuric  Acid:  (Hinman).* 


1000  c.c.       Na2S203=p-   :=3.269  gms.  H2SO4. 

The  solution  of  the  sulphate  is  treated  with  an  excess  of  a 
hydrochloric  acid  solution  of  barium  chromate,  by  which  means 
barium  sulphate  is  precipitated,  while  an  equivalent  amount 
of  chromic  acid  is  set  free.  If  the  solution  is  then  neutralized 

*  Am.  J.  Sci.  and  Arts,  114,  478  (1877).  Cf.  Andrews,  Am.  Chem.  J.,  2, 
567;  Pennock  and  Morton,  J.  Am.  Chem.  Soc.,  1903,  2265;  Bruhns,  Z.  anal. 
Chem.,  45,  573  (1906);  Holliger,  Ibid.,  19,  84  (1910),  and  M.  Reuter,  Chem 
Ztg.,  1898,  357. 


DETERMINATION  OF  SULPHURIC  ACID.  717 

with  ammonia  or  calcium  carbonate,  the  excess  of  barium  chro- 
mate  is  precipitated  and  can  be  filtered  off  with  the  barium  sul- 
phate. In  the  filtrate,  the  amount  of  free  chromic  acid  is  deter- 
mined volumetrically  by  acidifying  with  hydrochloric  acid,  adding 
potassium  iodide  and  titrating  the  deposited  iodine  with  sodium 
thiosulphate  solution. 

The  barium  chromate  used  for  this  determination  must  be  com- 
pletely free  from  soluble  chromate  and  can  contain  no  soluble 
barium  salt  nor  barium  carbonate;  the  presence  of  barium  sul- 
phate exerts  no  influence. 

It  is  best  to  prepare  the  barium  chromate  by  precipitating 
barium  chloride  with  potassium  chromate  at  the  boiling  tem- 
perature. The  precipitate  is  washed  first  with  boiling  water  con- 
taining a  little  acetic  acid,  then  with  pure  water  and  dried.  The 
solution  used  for  the  analysis  is  obtained  by  dissolving  2  to  4  gms, 
of  the  dry  salt  in  1  liter  of  normal  hydrochloric  acid. 

Procedure. — The  solution  of  the  sulphate,  which  at  the  most 
should  contain  not  over  2  per  cent,  of  S03,  is  almost  neutralized 
(in  case  it  reacts  acid)  with  potassium  hydroxide  solution,  pre- 
cipitated at  the  boiling  temperature  with  an  excess  of  the  hydro- 
chloric acid  solution  of  barium  chromate  and  boiled  for  one  minute. 
If  the  solution  originally  contained  carbonate,  the  boiling  is  con- 
tinued for  five  minutes.  The  precipitated  barium  sulphate  always 
carries  a  little  barium  chromate  with  it  and  consequently  appears 
yellow  in  color. 

To  the  boiling  solution,  calcium  carbonate  free  from  alkali  is 
added  in  small  amounts  until  there  is  no  further  evolution  of  carbon 
dioxide,  after  which  the  hot  liquid  is  filtered  and  the  precipitate 
washed  with  hot  water. 

After  cooling,  an  excess  of  potassium  iodide  is  added  and  5  c.c. 
of  concentrated  hydrochloric  acid  for  each  100  c.c.  of  the  solution; 
the  liberated  iodine  is  titrated  as  described  on  p.  647. 

Remark. — When  iron,  nickel  or  zinc  salts  are  contained  in  the 
solution,  the  acid  present  cannot  be  neutralized  with  calcium  carbon- 
ate because  these  salts  when  boiled  with  calcium  carbonate  and  a 
soluble  chromate  form  insoluble  basic  chromates,  so  that  too  little 
chromic  acid  will  be  found  in  the  filtrate  corresponding  to  too  little 
sulphuric  acid.  In  such  a  case  the  neutralization  is  effected  with 


7i«  VOLUMETRIC  ANALYSIS. 

ammonia,  an  excess  being  added,  the  solution  boiled  until  the 
excess  is  almost  entirely  expelled  and  then  filtered. 

10.  Determination  of  Phosphoric  Acid.     Method  of  Pincus. 

Principle.  —  If  a  neutral  solution,  or  one  slightly  acid  with 
acetic  acid,  is  treated  with  uranyl  acetate,  a  greenish-  white  pre- 
cipitate of  uranyl  phosphate  is  formed: 

KH2PO4  +  UO2(C2H3O2)2  =  KC2H3O2  +  HC2H3O2  +  UO2HPO4. 

If  at  the  same  time  ammonium  salts  are  present,  ammonium 
is  contained  in  the  precipitate: 


KH2P04+U02(C2H302)2 

=  KC2H3O2  +  2HC2H3O2  +  UO2NH4PO4. 

The  end  of  the  precipitation  is  determined  by  testing  a  drop 
of  the  solution  on  a  porcelain  tile  with  potassium  ferrocyanide. 
As  soon  as  all  of  the  phosphoric  acid  is  precipitated  and  the  solu- 
tion contains  a  trace  of  uranyl  acetate  in  excess,  the  ferrocyanide 
solution  produces  a  brown  coloration. 

In  order  to  completely  precipitate  the  phosphoric  acid,  it  is 
necessary  to  titrate  in  a  boiling  hot  solution.  As,  however,  a  solu- 
tion of  calcium  phosphate  will  become  turbid  on  boiling,  owing 
to  the  formation  of  secondary  calcium  phosphate  (CaHPO4),  it  is 
best  to  precipitate  the  greater  part  of  the  phosphoric  acid  in  the 
cold,  then  heat  to  boiling  and  complete  the  titration. 

Requirements.  1.  Potassium  Phosphate  Solution.  —  This  is  ob- 
tained by  dissolving  19.17  gms.  (corresponding  to  10  gms.  P2O5) 
of  chemically  pure  monopotassium  phosphate  (which  can  be  ob- 
tained commercially)  in  1  liter  of  water. 

The  concentration  of  the  solution  is  confirmed  by  evaporating 
50  c.c.  to  dryness  in  a  large  platinum  crucible,  igniting  the  residue 
over  the  full  flame  of  a  Bunsen  burner  and  weighing  as  KPO3;  also 
by  precipitating  another  portion  as  magnesium  ammonium  phos- 
phate and  weighing  as  magnesium  pyrophosphate. 

50  c.c.  of  the  solution  correspond  to  0.5  gm.  P2O5 

and  should  yield  ..................  0.8315  gm.  KPO3 

and  ...........................  0.7839  gm.  Mg2P2O7. 


DETERMINATION  OF  PHOSPHORIC  ACID.  719 

2.  Calcium  Phosphate  Solution.— 5  A§1  gms.   of   Ca3P2O8,  cor- 
responding to  2.5  gms.  P2O5,  are  dissolved  in  a  little  nitric  acid, 
diluted  with  water  to  a  volume  of  1  liter,  and  the  concentration  oi 
the  solution  tested  by  means  of  the  molybdate  method  of  Woy 
(p.  437). 

3.  Uranyl  Aceta'e  Solution. — This  is  made  by  dissolving  about 
35  gms.  of  uranyl  acetate  in  a  liter  of  water. 

4.  Ammonium  Acetate  Solution. — 100  gms.  of  pure  ammonium 
acetate  and  100  c.c.  of  acetic  acid,  sp.  gr.  1.04,  are  diluted  with 
water  to  a  volume  of  1  liter. 

5.  Potassium  Ferrocyanide.— The  salt  is  used  in  the  powdered 
form. 

Procedure. 

(a)  Standardization  of  the  Uranium  Solution, 
50  c.c.  of  the  potassium  phosphate,  or  calcium  phosphate, 
solution  are  treated  with  10  c.c.  of  the  ammonium  acetate  solution, 
and  to  it  the  uranyl  acetate  solution  is  run  in  from  a  burette  until 
a  :'rop  of  the  solution  will  show  a  brown  coloration  when  treated 
with  solid  potassium  ferrocyanide  upon  a  white  porcelain  tile.  The 
solution  is  then  heated  to  boiling,  when  a  drop  of  it  will  no  longer 
react  with  the  ferrocyanide.  To  the  hot  solution  more  of  the 
uranium  solution  is  added,  until  the  brown  color  is  obtained  once 
more. 

If  for  the  precipitation  of  the  phosphoric  acid  contained  in 
50  c.c.  of  the  potassium  phosphate  solution  (0.5  gm..  P2O5)T  T 
c.c.  of  the  uranium  solution  were  required,  its  concentration  is 

~  gm.  P2O5  per  c.c. 

For  the  analysis  of  alkali  phosphates,  the  solution  is  standard- 
ized against  the  potassium  phosphate  solution,  while  for  the 
analysis  of  an  alkaline-earth  phosphate  the  solution  of  calcium 
phosphate  is  used. 

(b)  Determination  of  Phosphoric  Acid  in  Alkali  Phosphates. 

The  solution  to  be  analyzed  should  be  of  about  the  same  con- 
centration as  that  of  the  potassium  phosphate  used  for  the  stand- 
ardization, and  titrated  as  above.  Phosphate  solutions  of  different 
concentrations  give  different  results  by  the  titration. 


720  VOLUMETRIC  AHA LYSIS. 

(c)  Determination  of  Phosphoric  Acid  in  Calcium  Phosphate. 

A  weighed  amount  of  calcium  phosphate  is  dissolved  in  dilute 
nitric  acid,  ammonia  is  added  to  the  solution  until  a  permanent 
precipitate  is  produced,  which  is  redissolved  in  a  little  acetic  acid, 
10  c.c.  of  the  ammonium  acetate  solution  are  added,  and  the 
solution  is  titrated  with  the  standardized  solution  of  uranyl 
acetate. 

Remark. — In  the  presence  of  iron  and  aluminium  this  method 
will  not  give  accurate  results  because  the  phosphates  of  these 
metals  are  insoluble  in  acetic  acid.  In  such  cases,  the  turbid 
acetic  acid  solution  is  filtered  and  the  phosphoric  acid  determined 
in  the  filtrate  by  the  above  titration.  The  precipitate  consisting 
of  iron  and  aluminium  phosphates  is  ignited,  weighed,  and,  if 
it  amounts  to  less  than  0.01  gm.;  half  its  weight  is  taken  as  P2O5; 
otherwise  the  phosphoric  acid  in  the  precipitate  must  be  deter- 
mined by  the  molybdate  method. 

ii.  Determination  of  Nickel  by  Potassium  Cyanide.* 

This  method,  which  permits  the  volumetric  estimation  of 
nickel  with  speed  and  accuracy  even  in  the  presence  of  iron, 
manganese,  chromium,  zinc,  vanadium,  molybdenum,  and 
tungsten,  depends  upon  the  fact  that  nickel  ions  react  with 
potassium  cyanide  in  slightly  ammoniacal  solution,  to  form  a 
complex  anion,  Ni(CN)4" 

Ni(NH3)6Cl2  +4KCN  =  K2[Ni(CN)4]  +  6NH3  +  2KC1. 
If  the  solution  of  the  nickel  salt  contains  a  precipitate  of  silver 
iodide,  produced  by  adding  a  known  amount  of  silver  nitrate  and 
a  few  drops  of  potassium  iodide  solution,  the  turbidity  will  not 
disappear  until  all  of  the  nickel  has  entered  into  reaction  with  the 
potassium  cyanide. 

Agl  +  2KCN  =  K[Ag(CN)2]  +  KI. 

*Cf.  Campbell  and  Andrews,  J.  Am.  Chem.  Soc.,  17,  126  (1895);  Moore, 
Chem.  News,  72,  92  (1895) ;  Goutal,  Z.  angew.  Chem.,  1898,  177;  Brearley  and 
Jarvis,  Chem.  News,  78,  177  and  190  (1898);  Johnson,  J.  Am.  Chem.  Soc.,  29, 
1201  (1907);  Campbell  end  Arthur  ibid.,  30,  1116  (1908);  and  Grossmann,' 
Chem.  Ztg.,  32,  1223  (1908). 


DETERMINATION  OF  NICKEL  BY  POTASSIUM   CYANIDE.     72: 

The  titration  is  finished  by  adding  just  enough  more  silver  nitrate 
to  cause  the  precipitate  of  silver  iodide  to  reappear. 

K[Ag(CN)  2]  +  AgN03  =  2AgCN  +  KNO3. 

Requirements. — The  potassium  cyanide  solution  should  be  about 
equivalent  to  a  tenth-normal  silver  solution,  and  is  prepared  by 
dissolving  13.5  gms.  of  pure  potassium  cyanide  and  5  gms.  of 
caustic  potash  in  water  and  diluting  to  a  volume  of  one  liter. 
The  addition  of  the  alkali  serves  to  make  the  solution  more  stable. 

The  silver  nitrate  solution  is  made  exactly  tenth-normal  and 
is  prepared  by  dissolving  8.495  gms.  AgNOs  in  water  and  diluting 
to  exactly  500  c.c.  1  c.c.  of  this  solution  is  equivalent  to  0.01302 
gms.  KCX,  or  to  0.002934  gms.  Ni.  It  is  used  for  standardizing 
the  potassium  cyanide  solution,  and  in  the  analysis  itself. 

The  potassium  iodide  solution  contains  2  gms.  KI  in  100  c.c. 

Standardization  of  the  Potassium  Cyanide  Solution. — About 
30  c.c.  of  the  potassium  cyanide  solution  are  diluted  to  100  c.c., 
5  c.c.  of  the  potassium  iodide  solution  added,  and  the  solution 
titrated  with  silver  nitrate  solution  until  a  faint  permanent 
opalescence  is  obtained  which  is  cleared  up  by  a  small  drop  of 
potassium  cyanide  solution. 

Analysis. — The  solution,  containing  not  more  than  0.1  gm.  of 
nickel  and  having  a  volume  of  about  100  c.c.,  is  treated  with 
ammonia  until  slightly  alkaline*  and  then  with  5  c.c.  of  the 
potassium  iodide  solution  and  0.5  c.c.  of  the  0.1N  silver  nitrate 
solution,  the  latter  being  accurately  measured  from  a  burette. 
While  stirring  constantly  with  a  glass  rod,  the  standard  potassium 
cyanide  solution  is  added  until  the  precipitated  silver  iodide 
dissolves  completely.  Then  the  silver  solution  is  added  until  a 
faint,  permanent  opalescence  is  obtained  which  is  cleared  up  by 
a  small  drop  of  the  potassium  cyanide. 

Assuming  that  the  silver  solution  was  exactly  tenth-normal, 
that  one  c.c.  of  potassium  cyanide  =  ti  c.c.  of  the  silver  nitrate 
solution,  and  that  T  c.c.  of  potassium  cyanide  and  t  c.c.  of  silver 


*  If  the  addition  of  ammonia  does  not  give  a  clear  solution,  in  few  cubic 
centimeters  of  ammonium  chloride  solution  should  be  added. 


722  VOLUMETRIC  ANALYSIS. 

nitrate  were  used  in  titrating  a  gms.  of  a  substance,  then  the 
percentage  of  nickel  is  found  by  the  following  equation: 

-  0X0.2934 

—  =  per  cent.  Ni. 


Instead  of  working  with  two  solutions,  F.  Button  *  states  that 
equally  reliable  results  can  be  obtained  by  using  a  potassium 
cyanide  solution  to  which  a  little  silver  nitrate  has  been  added. 
Thus,  to  the  above  solution  of  potassium  cyanide  there  may 
be  added  about  0.50  gm.  of  silver  nitrate  which  is  first  dissolved 
in  water  by  itself.  If  this  solution  is  used  for  titrating  a  nickel 
solution  to  which  potassium  iodide  solution.  has  been  added,  a 
precipitate  of  silver  iodide  is  formed  at  once  which  increases 
with  the  further  addition  of  the  potassium  cyanide-silver  nitrate 
solution  until  all  the  nickel  is  converted  into  potassium  nickelo- 
cyanide,  but  the  precipitate  eventually  disappears  upon  the 
further  addition  of  the  solution.  When  this  modification  of  the 
potassium  cyanide  method  is  used,  however,  it  is  necessary  to 
standardize  the  potassium  cyanide  solution  against  a  solution 
containing  a  known  quantity  of  nickel. 

Remarks.  —  The  method  can  be  carried  out  in  the  presence  of 
most  of  the  other  elements  of  the  ammonium  sulphide  group.  f 
If  copper  is  present  in  amounts  not  exceeding  0.4  per  cent.,  the 
copper  will  replace  almost  exactly  three-quarters  of  its  weight 
of  nickel.  In  case  chromium  is  present,  the  dark  color  due  to 
presence  of  chromic  salts  may  be  obviated  by  adding  to  the 
original  sulphuric  acid  solution  a  2  per  cent,  solution  of  potassium 
permanganate  until  a  slight  permanent  precipitate  of  manganese 
dioxide  is  obtained,  whereby  the  chromium  is  oxidized  to  chromic 
acid.  The  solution  is  filtered,  concentrated  in  a  400  c.c.  beaker 
to  about  60  c.c.,  then  treated  with  sodium  pyrophosphate,  as 
described  above.  The  method  is  not  applicable  in  the  presence 
of  cobalt,  the  presence  of  which  is  betrayed  by  the  solution  as- 
suming a  dark  color  upon  the  addition  of  potassium  cyanide, 

*  Volumetric  Analysis,  8th  edition,  p.  252. 

t  The  addition  of  alkali  phosphate  is  usually  necessary,  as  in  the  following 
method  for  determining  nickel  in  steel. 


DETERMINATION  OF  NICKEL    IN  NICKEL  STEEL.  723 

but  when  the  amount  of  the  latter  does  not  exceed  one-tenth  the 
amount  of  nickel  present,the  titration  can  e  carried  out  successfully 
and  the  results  represent  the  amount  of  nickel  and  cobalt  present. 
The  temperature  of  the  solution  should  not  be  much  above 
20°,  for  in  hot  solutions  the  results  are  not  concordant.  The 
quantity  of  ammonia  present  should  not  be  too  great,  because 
the  tendency  is  for  ammonia  to  impede  the  reaction  if  more  than 
a  slight  excess  is  present.  Potassium  cyanide  containing  sulphide 
cannot  be  used;  the  reagent  should  be  the  purest  obtainable. 
The  results  are  accurate.  The  method  has  been  modified  so  that 
it  can  be  used  to  advantage  for  the 

Determination  of  Nickel  in  Nickel  Steel. 

One  gram  of  steel  is  dissolved  in  a  casserole  with  10  to  15  c.c. 
of  nitric  acid  (sp.  gr.  1.2),  adding  a  little  hydrochloric  acid  if 
necessaiy.  After  the  steel  has  dissolved,  6  or  8  c.c.  of  sulphuric 
acid  (1:1)  are  added,  and  the  solution  is  evaporated  until  fumes 
of  sulphuric  anhydride  begin  to  come  off.  The  residue  is  cooled, 
30  to  40  c.c.  of  water  are  added,  and  the  contents  of  the  casserole 
boiled  until  the  ferric  sulphate  has  all  dissolved.  The  solution  is 
then  transferred  to  a  400-c.c.  beaker,  filtering  if  necessary,  and 
13  gms.  of  sodium  pyrophosphate  *  dissolved  in  60  c.c.  of  water  at 
about  60°  are  added.  The  pyrophosphate  solution  must  not  be 
boiled,  as  this  causes  the  formation  of  normal  phosphate.  The 
addition  of  the  sodium  pyrophosphate  causes  the  formation  of  a 
heavy  white  precipitate  of  ferric  pyrophosphate.  The  liquid  is 
cooled  to  room  temperature,  whereupon  dilute  ammonia  (1:1)  is 
added  drop  by  drop,  while  stirring  constantly,  until  the  greater 
part  of  the  precipitate  has  dissolved  and  the  solution  has  assumed 
a  greenish  tinge.  At  this  point,  it  should  react  alkaline  toward 
litmus  but  should  not  smell  of  free  ammonia.  Now  on  gently 
heating  the  solution,  while  stirring,  the  remainder  of  the  pyro- 
phosphate will  dissolve,  giving  a  perfectly  clear  light  green 
solution.  If  the  ammonia  is  added  too  fast,  or  the  solution  is 
not  carefully  stirred,  a  brownish  color  is  likely  to  result,  but  this 

*  Instead  of  using  sodium  pyrophosphate  to  prevent  the  interference  of 
iron  and  other  metals,  many  chemists  use  citric  or  tartaric  asid. 


724  VOLUMETRIC  ANALYSIS. 

can  usually  be  overcome  by  carefully  adding  a  few  drops  of  dilute 
sulphuric  acid.  The  clear  solution  is  cooled  to  room  temperature 
and  0.5  c.c.  of  the  standard  silver  nitrate  solution  is  added  together 
with  5  c.c.  of  the  potassium  iodide.  The  solution  is  then  titrated 
with  potassium  cyanide,  which  is  added  until  the  precipitate  of 
silver  iodide  has  disappeared.  The  titration  is  finished  by  adding 
just  enough  more  of  the  silver  nitrate  to  cause  the  formation  of  a 
slight  turbidity  again. 

12.  Determination  of  Copper  by  the  Potassium  Cyanide  Method.* 

Principle. — If  an  ammoniacal  solution  of  a  cupric  salt  is 
treated  with  potassium  cyanide,  the  intense  blue  color  gradually 
disappears.  The  reaction  is  essentially  as  follows: 

2Cu(NH3)4S04.H20+7KCN<=K3NH4Cu2(CN)6+NH4CNO 

+  2K2SO4  +  6NH3  +  H20. 

The  temperature  of  the  solution,  the  ammonia  concentration, 
and  the  quantity  of  ammonium  salts  present  effect  the  reaction 
so  that  a  given  quantity  of  copper  does  not  always  react  with 
the  same  quantity  of  potassium  cyanide.  The  potassium  cyanide 
solution,  therefore,  must  be  standardized  under  exactly  the  same 
conditions  as  under  which  the  analysis  is  carried  out. 

Standardization  of  the  Potassium  Cyanide  Solution. — Twenty 
grams  of  pure  potassium  cyanide  are  dissolved  in  a  liter  of  water 
and  the  solution  titrated  against  pure  copper.  About  0.2  gm. 
of  pure  copper  wire  or  foil  is  weighed  out  into  a  200-c.c.  Erlen- 
meyer  flask  and  dissolved  in  5  c.c.  of  concentrated  nitric  acid. 
After  solution  is  complete,  25  c.c.  of  water  and  5  c.c.  of  bromine 
water  are  added  and  the  solution  boiled  to  expel  the  excess  of 
bromine.  Then  50  c.c.  of  water  and  10  c.c.  of  strong  ammonia 
(sp.  gr.  0.90)  are  added,  the  solution  cooled  to  room  temperature 
by  placing  the  flask  in  cold  water,  and  the  potassium  cyanide 
added  slowly  from  a  burette,  while  constantly  rotating  the 

*  Cf.  Steinbeck,  Z.  anal.  Chem.  8,  8  (1869).  Dulin,  J.  Am.  Chem.  Soc.,  17, 
346.  A.  H.  Low,  Technical  Methods  of  Ore  Analysis. 


DETERMINATION  OF  COPPER.  725 

contents  of  the  flask.  When  the  solution  has  become  a  pale 
blue,  water  is  added  to  make  the  total  volume  150  c.c.  and  the 
addition  of  the  potassium  cyanide  continued  until  the  solution 
is  just  decolorized.  The  weighed  amount  of  copper  divided  by 
the  number  of  cubic  centimeters  of  potassium  cyanide  required 
gives  the  titer  of  the  solution. 

Low's  Method  for  Analyzing  Copper  Ores.  — About  0.5  gm.  of  a 
rich  ore,  or  from  two  to  four  times  as  much  of  a  low-grade  ore, 
is  weighed  into  a  200-c.c.  Erlenmeyer  flask  and  treated  with 
6-10  c.c.  of  concentrated  nitric  acid  and  boiled  until  nearly  all 
the  red  fumes  are  expelled.  If  necessary,  5  c.c.  of  concentrated 
hydrochloric  acid  are  also  added  to  decompose  the  ore  and  the 
boiling  continued  for  a  short  time.  After  cooling  somewhat, 
7  c.c.  of  concentrated  sulphuric  acid  are  added  and  the  solution 
evaporated  until  dense  fumes  of  sulphuric  acid  are  evolved. 
Then,  after  allowing  to  cool  somewhat,  25  c.c.  of  cold  water  are 
added  and  a  drop  of  concentrated  hydrochloric  acid  to  pre- 
cipitate any  silver  as  chloride.  The  liquid  is  boiled  to  dissolve 
the  copper  and  ferric  sulphates  and  then  the  precipitated  lead 
sulphate  and  silicious  residue  is  filtered  off  and  washed  with  hot 
water.  The  filtrate  is  received  in  a  beaker  of  about  6  cm.  diam- 
eter and  care  is  taken  not  to  let  the  filtrate  exceed  75  c.c. 

The  copper  is  precipitated  from  the  filtrate  by  introducing  a 
piece  of  aluminium  foil,  about  14  cm.  long  and  2.5  cm.  wide, 
which  is  bent  into  a  triangle.  The  beaker  is  covered  with  a 
watch-glass  and  its  contents  boiled  about  ten  minutes,  whereby 
nearly  all  the  copper  is  deposited  as  spongy  metal.  The  beaker 
is  now  removed  from  the  flame  and  the  sides  washed  down  with 
cold  water.  To  precipitate  the  last  traces  of  copper  and  to 
prevent  the  oxidation  of  the  fine  deposit,  15  c.c.  of  strong  hydrogen 
sulphide  water  are  added,  after  which  the  liquid  is  decanted  through 
a  9  cm.  filter.  The  copper  is  washed  off  the  aluminium  by 
means  of  weak  hydrogen  sulphide  water  into  the  flask  in  which 
the  ore  was  dissolved  and  the  liquid  decanted  through  the  filter; 
the  beaker  containing  the  aluminium  foil  and  some  copper  is 
set  aside  temporarily.  The  operation  of  filtering  should  take 
place  without  interruption  and  the  filter  kept  well  filled  with 
liquid  to  prevent  the  oxidation  of  any  precipitate  upon  it,  which 


726  VOLUMETRIC  ANALYSIS. 

would  cause  it  to  dissolve  and  give  a  turbid  nitrate.  After 
washing  the  deposit  four  times,  using  in  each  case  20  c.c.  of  weak 
hydrogen  sulphide  water,  the  liquid  is  allowed  to  drain  from 
the  funnel,  and  then  the  beaker  containing  the  nitrate  is  replaced 
by  the  flask  containing  the  deposited  copper.  The  aluminium 
foil,  to  which  some  copper  usually  adheres,  is  now  covered  with 
10  c.c.  of  nitric  acid  (1:1),  heated  nearly  to  boiling,  and  the  hot 
acid  poured  through  the  filter.  The  flask  is  replaced  by  the 
beaker  containing  the  foil,  and  the  contents  of  the  flask  heated 
until  all  the  copper  is  dissolved  and  the  greater  part  of  the  red 
fumes  expelled.  The  flask  is  again  placed  under  the  funnel,  the 
aluminium  foil  in  the  beaker  covered  with  5  c.c.  of  strong  bromine 
water,  which  is  poured  through  the  filter.  The  aluminium  foil 
and  the  filter  are  then  washed  with  hot  water.  The  solution  is 
boiled  to  expel  the  excess  of  bromine,  cooled  to  room  tem- 
perature, treated  with  10  c.c.  of  strong  ammonia,  and  titrated 
with  potassium  cyanide  exactly  as  in  the  standardization. 

13.  Determination  of  Lead  by  the  Molybdate  Method.* 

Principle. — The  lead  is  precipitated  as  molybdate  from  an 
acid  solution  and  the  termination  of  the  reaction  is  recognized 
by  testing  a  drop  of  the  solution  with  a  drop  of  tannin  solution, 
which  gives  a  yellow  coloration  when  an  excess  of  ammonium 
molybdate  is  present. 

Requirements. — 1.  A  solution  of  ammonium  molybdate  pre- 
pared by  dissolving  about  4.25  gms.  of  ammonium  molybdate  in 
water,  and  diluting  to  one  liter. 

2.  A  freshly  prepared  tannin  solution  containing  0.1  gm.  of 
tannin  in  20  c.c.  of  water. 

Standardization  of  the  Ammonium  Molybdate  Solution. — About 
0.2  gm.  of  pure  lead  foil  is  weighed  into  a  200-c.c.  Erlenmeyer 
flask,  dissolved  in  a  mixture  of  2  c.c.  concentrated  nitric  acid  and 
4  c.c.  of  water  and  the  solution  evaporated  nearly,  if  not  quite, 
to  dryness.  The  residue  is  taken  up  in  30  c.c.  water,  5  c.c.  of 
concentrated  sulphuric  acid  added,  the  liquid  shaken,  the  lead 

*  Low,  Technical  Methods  of  Ore  Analysis. 


DETERMINATION   OF  LEAD.  727 

sulphate  allowed  to  settle  completely,  filtered  and  washed  with 
dilute  sulphuric  acid  (1:10).  The  filter,  together  with  precipi- 
tate, is  thrown  into  an  Erlenmeyer  flask,  10  c.c.  of  concentrated 
acid  are  added,  and  the  liquid  boiled  until  the  filter  is  completely 
disintegrated.  Then,  after  adding  15  c.c.  more  of  the  concen- 
trated hydrochloric  acid,  and  25  c.c.  of  cold  water,  25  c.c.  of 
concentrated  ammonia  (sp.  gr.  0.90)  are  carefully  poured  into 
the  flask,  whereby  the  greater  part  of  the  acid  is  neutralized. 
A  piece  of  blue  litmus  paper  is  thrown  into  the  solution,  ammonia 
is  added  to  slightly  alkaline  reaction,  and  then  glacial  acetic 
acid  until  the  litmus  paper  turns  red.  The  solution  is  diluted 
to  about  200  c.c.  with  hot  water  and  about  two-thirds  of  the 
solution  transferred  to  a  beaker.  The  ammonium  molybdate 
solution  is  added  to  the  latter  from  a  burette  until  a  drop  of  the 
solution,  brought  in  contact  with  a  drop  of  the  tannin  indicator 
upon  a  white  porcelain  tile,  gives  a  brown  or  yellow  color.  Some 
more  of  the  lead  solution  is  added  to  the  beaker  and  the  opera- 
tion is  repeated  until  finally  but  a  few  cubic  centimeters  of  the 
lead  solution  remain  in  the  flask.  The  contents  of  the  beaker 
are  now  poured  into  the  flask,  then  back  again  to  the  beaker, 
and  the  titration  finished  by  adding  the  molybdate  solution  two 
drops  at  a  time.  If  t  c.c.  of  molybdate  are  used  in  titrating  a  gms. 
of  lead  the  titer  of  the  solution  is 

1  c.c.  ammonium  molybdate  =  —  gm.  lead. 

Procedure. — 0.5  gm.  of  the  ore  is  weighed  into  a  200-c.c. 
Erlenmeyer  flask,  10  c.c.  of  concentrated  hydrochloric  acid  and 
20  c.c.  of  water  added  and  the  liquid  boiled  until  all  the  hydrogen 
sulphide  is  expelled.  If  the  ore  should  not  dissolve  completely 
by  this  treatment,  a  little  concentrated  nitric  acid  is  added  and 
the  heating  continued  until  the  ore  is  completely  decomposed. 
As  soon  as  this  has  taken  place,  the  solution  is  allowed  to  cool, 
7  c.c.  of  concentrated  sulphuric  acid  added,  and  the  liquid 
evaporated  over  a  free  flame  until  dense  vapors  of  sulphuric 
acid  are  evolved.  After  allowing  to  cool,  20  c.c.  of  water  are 
added  and  the  liquid  boiled  for  fifteen  minutes  in  order  to  dissolve 
all  the  anhydrous  ferric  sulphate. 


728  VOLUMETRIC  ANALYSIS. 

After  cooling,  the  precipitated  lead  sulphate  and  silicious  residue 
is  filtered  off  and  washed  with  cold  dilute  sulphuric  acid  (1:10). 
The  lead  sulphate  is  often  contaminated  with  calcium  sulphate  or 
barium  sulphate,  and  before  the  titration  it  must  be  purified.  To 
this  end,  the  precipitate  is  rinsed  by  a  stream  of  cold  water  into  the 
original  flask,  5  gms.  of  pure  ammonium  chloride  are  added  and 
about  1  c.c.  of  concentrated  hydrochloric  acid.  By  boiling,  all  of 
the  lead  and  calcium  sulphates  are  dissolved  but  the  gangue,  which 
is  easily  distinguished  from  either  of  the  above  salts,  remains 
behind.  The  solution  is  neutralized  with  ammonia  and  treated 
with  an  excess  of  ammonium  sulphide.  The  precipitate  is 
allowed  to  settle  and  is  then  filtered  and  washed  with  hot  water 
until  the  filtrate  no  longer  gives  a  test  for  calcium  when  tested 
with  ammonium  oxalate.  As  the  lead  sulphide  may  be  con- 
taminated with  some  iron  sulphide,  it  is  again  rinsed  into  the 
original  flask,  by  means  of  as  little  hot  water  as  possible,  treated 
with  5  c.c.  of  dilute  sulphuric  acid,  shaken  until  the  precipitate 
is  well  broken  up,  treated  with  25  c.c.  of  strong  hydrogen  sulphide 
water,  filtered  through  the  same  filter  as  was  last  used,  and 
washed  with  cold  water.  By  this  time  it  is  safe  to  assume  that 
the  lead  sulphide  is  free  from  all  calcium  and  iron.  The  filter 
and  precipitate  are  once  more  returned  to  the  original  flask,  dis- 
solved by  boiling  with  5  c.c.  of  concentrated  hydrochloric  acid, 
boiled,  and  then,  when  the  hydrogen  sulphide  is  practically  all 
expelled,  treated  with  2  or  3  drops  of  concentrated  nitric  acid 
to  remove  the  last  traces  of  hydrogen  sulphide.  Now  25  c.c.  of 
cold  water  are  added,  and  the  solution  treated  exactly  as  in  the 
standardization  of  the  ammonium  molybdate. 

Remark. — The  smelter  chemists  in  the  western  part  of  the 
United  States  use  a  much  more  rapid  method,  which  gives  good 
results  in  the  hands  of  an  experienced  operator,  provided  the  lead 
content  of  the  ore  is  greater  than  fifteen  milligrams. 

Procedure.* — The  ore  is  dissolved  in  hydrochloric  acid  or 
hydrochloric  and  nitric  acids,  and  the  solution  is  filtered 
while  hot  without  diluting  any  more  than  to  prevent  the 
acid  attacking  the  paper.  The  residue  is  washed  rapidly 
with  a  hot  solution  of  ammonium  chloride  until  the  washings 

*  This  method  was  obtained  through  the  courtesy  of  Mr.  Franklin  G.  Hills 
of  the  American  Smelting  and  Refining  Co. 


DETERMINATION  OF  LEAD.  7280 

show  no  blackening  when  tested  with  ammonia  and  a  drop  of 
ammonium  sulphide.  The  filtrate  is  made  just  alkaline  with 
ammonia  and  a  slight  excess  of  ammonium  sulphide  added.  The 
liquid  is  heated  to  boiling  and  the.  precipitated  sulphides  are 
filtered  off  and  washed  with  hot  water.  (The  alkaline  earths  may 
be  determined  in  the  filtrate  if  desired.)  The  sulphides  are  dis- 
solved in  hot,  dilute  nitric  acid  and  the  resulting  solution  is  caught 
in  the  same  beaker  in  which  the  sulphides  were  precipitated. 
About  7  c.c.  of  concentrated  sulphuric  acid  are  added  and  the 
Ikiuil  evaporated  until  dense  vapors  of  sulphuric  acid  are  evolved. 
After  allowing  to  cool,  20  c.c.  of  water  are  added  and  the  liquid 
boiled  to  dissolve  the  anhydrous  ferric  sulphate.  The  precipitated 
lead  sulphate  is  filtered  off,  washed  free  from  acid,  dissolved  in  a 
slight  excess  of  ammonium  acetate  solution*  and  diluted  with 
water.  After  heating  to  boiling,  the  hot  solution  is  titrated  with 
ammonium  molybdate. 

The  above  procedure  serves  when  alkaline  earths  are  present ; 
but  when  these  are  known  to  be  absent,  the  original  solution  of 
the  ore  is  at  once  evaporated  with  sulphuric  acid  and  the  resulting 
lead  sulphate  can  be  dissolved  in  ammonium  acetate  solution  and 
titrated  without  any  purification. 

*  If  too  much  ammonium  acetate  solution  is  used,  a  transitory  end  point 
is  obtained  in  the  subsequent  titration.  It  is  necessary  to  use  a  hot  solution, 
which  does  not  contain  too  much  of  the  salt.  See  page  176. 


PART  III. 

GAS  ANALYSIS. 


THE  chemical  analysis  of  gas  mixtures  is  accomplished  usually 
by  measuring  and  rarely  by  weighing  the  individual  constituents, 
so  that  it  is  customary  to  express  the  results  in  per  cent,  by  volume. 
But  inasmuch  as  the  volume  of  a  gas  is  influenced  to  an  extraor- 
dinary extent  by  the  temperature  and  pressure,  it  is  necessary 
to  reduce  each  measurement  to  standard  conditions  of  tempera- 
ture and  pressure,  and  further  to  take  care  that  these  remain 
constant  during  the  whole  of  the  analysis.  A  volume  of  gas  V 
measured  over  water  at  t°  C.  and  B  mm.  barometric  pressure,* 
is  reduced  to  the  volume  which  it  would  assume  at  0°  C.  and 
760  mm.  pressure  in  a  dry  condition  by  means  of  the  formula 

_  V(B-w) 
K°    760(1  +at)' 

In  this  formula,  V0  represents  the  reduced  volume,  f  V  the 
volume  of  the  gas  at  t°  C.  and  B  mm.  pressure,  w  the  tension 
of  aqueous  vapor,  and  a  the  expansion  coefficient  of  the  gas 
(  =  0.003665). 

As,  however,  a =;™,  the  above  formula  may  be  written  as 


follows: 


=  V(B-w)273 
760(273  +  0' 


*  Here    is    understood    the    barometer    reading    reduced    to    0°  C.     The 
reduction  is  accomplished  by  means  of  the  formula: 


,, 

0     l  +  at 

in  which  B0  represents  the  reduced  reading,  B  the  actual  reading  at  t°,  a 
the  expansion  coefficient  of  mercury  (  =  0.000181),  /?  the  linear  coefficient 
of  expansion  of  glass  (  =  0.0000085).  For  most  purposes,  however,  the 
reduction  to  0°  C.  can  be  made  with  sufficient  accuracy  by  making  the 
following  deductions  from  the  actual  readings: 
5°-12°  ...................  1  mm. 


2 


21°-28° 3  mm. 

29°-35°..  .  4    " 


f  Or  volume  under  standard  conditions.  729 


730  GAS  ANALYSIS. 

Instead  of  reducing  the  observed  volume  to  the  standard 
conditions  by  computation,  it  can  be  effected  mechanically  by 
compression  (see  p.  388). 

The  Collection  and  Confinement  of  Gas  Samples. 

Since  all  gases  diffuse  rapidly  into  one  another  even  when 
separated  by  porous  solid  bodies  or  liquids,  it  is  evident  that  the 
collection  of  the  sample  and  its  preservation  offers  certain  diffi- 
culties. If  a  gas  is  confined  in  a  bell  jar  over  water  and  thus  kept 
out  of  contact  with  the  air,  it  will  be  found  that  different  results 
will  be  obtained  in  the  analysis  of  the  gas  from  day  to  day.  The 
air  gradually  penetrates  through  the  water  into  the  bell  jar  and 
in  the  same  way  the  gas  within  the  jar  gradually  diffuses  into 
the  atmosphere.  This  process  will  continue  until'  finally  the 
composition  of  the  gas  both  within  and  without  the  jar  is  the 
same.  The  rapidity  of  the  diffusion  depends  upon  the  extent 
to  which  the  gases  are  absorbed  by  the  liquid  which  separates 
them.  Those  liquids  which  absorb  the  gases  readily,  allow  them 
to  pass  through  it  rapidly,  and  consequently  cannot  be  used  for 
keeping  the  gases  apart.  Of  all  liquids,  mercury  is  best  suited 
for  the  purpose,  because  it  absorbs  only  minimum  amounts  of 
the  different  gases. 

Gases  which  combine  chemically  with  mercury,  such  as  chlo- 
rine, bromine  vapors,  hydrogen  sulphide,  etc.,  cannot,  of  course, 
be  collected  over  mercury;  it  is  best  to  collect  them  in  dry  glass 
tubes  and  to  seal  the  latter  by  fusing  together  the  open  ends  in 
case  the  gas  cannot  be  analyzed  immediately.  Through  glass 
there  is  no  diffusion,  so  that  gases  may  be  kept  unchanged  in  sealed 
tubes  for  years. 

If  the  gas  is  to  be  analyzed  within  a  few  days  after  the  tims 
of  collection,  it  can  be  kept  in  pipette-shaped  tubes.  The  ends 
are  closed  by  thick  pieces  of  rubber  tubing  into  each  of  which  is 
inserted  a  piece  of  glass  stirring-rod  with  rounded  ends ;  where  the 
rubber  tubing  comes  in  contact  with  the  glass  it  should  be  fastened 
tightly  with  wires.  It  is  not  permissible  to  keep  gases  in  such 
tubes  for  a  considerable  length  of  time,  for  rubber,  particularly 
when  it  has  become  hard,  permits  the  diffusion  of  gases  to  some 
extent. 


COLLECTION  AND  CONI1NEMEN7    OF  GAS  SAMPLES. 


731 


For  less  accurate  analyses,  the  gases  may  be  collected  over 
water  which  has  been  previously  saturated  with  the  gas  to  be 
analyzed,  and  the  analysis  must  be  made  immediately  afterwards. 

From  what  has  been  said,  it  is  evident  that  care  must  be  taken 
in  collecting  and  keeping  the  gas  to  be  analyzed.  We  will  now 
consider  briefly  a  few  practical  examples. 

(a)  Collection  of  Gases  in  Accessible  Places. 

1.  The  neck  of  a  200-c.c.  flask  is  drawn  out  somewhat  and 
a  glass  tube  is  inserted  and  about  800  c.c.  of  the  gas  to  be 
analyzed  are  drawn  through  the  flask  by  means  of  suction 
(Fig.  98).  The  neck  of  the  flask  is  closed  by  means  of  a  rubber 
cap  and  the  glass  is  fused  together. 

(b)  Collection  of  Gases  from  Inaccessible  Places. 

The  rubber  tubing  G,  Fig.  99,  is  connected  on  one  side  with 
the  aspirator  A  of  about  30  liters  capacity  and  on  the  other  with 


FIG.  98. 


FIG.  99. 


the  source  of  the  gas,  and  water  is  allowed  to  flow  quickly  from 
ihe  former.     After  5  or  6  liters  have  run  out,  the  air  is  usually 


732 


GAS  ANALYSIS. 


completely  expelled  from  the  rubber  tubing  and  replaced  by  the 
gas  to  be  analyzed,  so  that  it  is  now  ready  for  collecting  the  sample. 
For  this  purpose  the  stop-cock  H  is  turned  90°  to  the  right,  so  that 
til ,  vessel  R,  which  is  to  receive  the  gas,  is  in  communication 
with  the  outer  air,  and  the  air  is  expelled  from  it  by  raising  the 
mercury  reservoir  N.  The  stop-cock  is  then  turned  back  to 
the  position  shown  in  the  figure  and  R  is  filled  with  the  gas  by 
lowering  N.  As  the  tubing  between  the  T  tube  and  the  stop- 
cock contained  impure  gas,  R  is  again  filled  with  mercury  and 
the  gas  expelled  into  the  air.  After  the  process  has  been  repeated 
three  times,  the  receiver  is  filled  for  the  last  time  with  the  gas, 
H  is  closed,  N  is  lowered  so  that  the  pressure  in  the  tube  is  less 
than  that  of  the  atmosphere,  and  the  ends  of  R  are  fused  together 
first  at  a  then  at  6.  During  this  sealing  of  the  tube,  it  should 
be  removed  from  the  ring-stand  so  that  the  tube  can  be  revolved 
a  little  while  being  heated  in  the  flame. 

In  sealing  the  tube,  the  ends  are  drawn  out  into  a  capillary  as 
shown  in  R',  Fig.  99. 

If  it  is  necessary  to  obtain  the  gas  from  places  at  a  very  high 
temperature,  e.g.,  f  re  r_  blast-furnaces,  producers,  . . 

etc.,  glass  tubes   would  melt,  and   if  ordinary  cpp 


iron  tubes  were  not  melted  they  would  de-  "*>__ 
compose  the  gas.  In  this  case  it  is  best  to  a  — 
use  the  water-jacketed  iron  tube  devised  by 
St.  Claire  Deville  and  shown  in  Fig.  100.  Cold 
water  is  run  into  the  outer  condenser  at  a 
and  allowed  to  run  out  at  b,  and  the  gas  is  col- 
lected as  described  above  through  the  tube  c. 
It  is  important  that  the  water  should  run 
through  the  tube  fast  enough  to  ke<3p  the  inner 
tube  cold,  otherwise  the  gas  will  be  decom- 
posed. By  this  means  there  is  no  difficulty  in  collecting  gas 
samples  from  different  heights  of  the  glowing  layers  of  coal  in 
blast-furnaces  or  producers. 

Collection  of  Gases  Arising  from  Mineral  Springs. 

The  receiver  R  is  connected  with  the  funnel  T  by  means  of  the 
rubber  tubing  q  (Fig.  101).   All  these  parts  of  the  apparatus  are 


FIG.  100. 


COLLECTION  AND  CONFINEMENT  OF  GAS  SAMPLES. 


733 


FIG.  101. 


filled  with  spring-water  and  the  gas  is  allowed  to  ascend  up  through 

the  funnel  as  shown  in  the  illustration. 
In  order  that  the  gas  may  pass  from 
the  funnel  into  the  receiver,  R  is  raised 
so  that  only  the  tubing  p  remains  in 
the  water  while  the  funnel  is  lowered 
as  deep  as  possible,  causing  pressure 
enough  to  drive  the  gas  over.  The 
tubing  is  then  closed  just  above  a  by 
means  of  a  screw-cock,  a  beaker  filled 
with  spring-water  is  placed  under  p, 
the  apparatus  removed  from  the  spring, 
and  both  ends  of  R  are  fused  together 
with  the  blowpipe.  If  the  gas  is  to 
be  analyzed  within  two  or  three  days, 

the  receiver  may  be  closed  by  pieces  of  short  rubber  tubing  each 
containing  a  short  piece  of  glass  rod  with  rounded  ends.  All  of 
such  connections  must  be  fastened  by  means  of  wires  where  the 
glass  comes  in  contact  with  the  rubber.  According  to  the  above 
method  the  gas  arising  from  the  thermal  springs  of  Baden,  Switzer- 
land, was  collected  and  analyzed.  *  The  results  obtained  showed 
that  it  makes  but  little  difference  which  method  is  used  for  closing 
the  receiver,  provided  the  analysis  is  made  within  a  short  time.f 
100  c.c.  of  the  gas  contained: 

*  "  Chemische  Untersuchung  der  Schwefeltherme  von  Baden  (Kanton 
Aargau),"  by  F.  P.  Treadwell,  1896. 

t  The  following  analyses  illustrate  the  importance  of  analyzing  the  gas 
soon  after  it  has  been  collected.  Both  samples  were  taken  at  the  same  time 
and  one  was  analyzed  promptly  but  the  other  only  after  two  years  and  a 
half  had  elapsed.  The  gas  receiver  was  closed  by  fresh  rubber  tubing  con- 
taining a  piece  of  stirring  rod  with  rounded  edges,  and  precaution  was  taken 
to  wire  the  connections  tightly.  The  rubber  remained  soft  and  flexible. 

I.  II. 

C02=     8.52%  0.43% 

O2=   10.77  14.82 

CO=     0.17 
CH4=     0.15 
N2=  80.39  84.75 


100.00% 


100.00% 


734 


GAS  ANALYSIS. 


Nitrogen 69.13 

Carbon  dioxide 30.81 

Hydrogen  sulphide 0 . 05 

Oxygen 0 . 00 


99.99  100.10 

Sample  I  was  collected  and  the  ends  of  the  receiver  fused  to- 
gether, while  with  sample  II  the  ends  were  closed  by  means  of  rub- 
ber tubing  and  glass  rods,  and  analyzed  five  days  later. 

Collection  of  Gases  Absorbed  in  Spring-water. 
Of  the  many  different  methods  which  have  been  proposed  for 
the  analyses  of  the  absorbed  gases  in  spring-water,  the  author  has 

found  the  following 
to  give  the  best  re- 
sults. 

The  flask^l,  Fig. 
102,  is  filled  with 
spring- water  up  to  its 
upper  edge  and  the 
rubber  stopper  con- 
taining the  tube  L, 
which  is  fused  to- 
gether at  the  bottom 
but  has  an  opening 
on  the  side  at  I,  is  im- 
mediately placed  in 
the  neck  and  pressed 
down  to-  the  mark. 
The  tube  L  is  raised 
so  that  the  opening  I 
is  within  the  stop- 
per, thus  making  an 
air-tight  connection. 
The  bulb  K  is  now 
connected  with  L, 
filled  half  full  with  distilled  water  and  connected  with  the 
capillary  tubing  C,  although  the  latter  is  not  yet  connected  with 


FIG.  102. 


COLLECTION  4ND  CONFINEMENT   OF  GAS  SAMPLES.          735 

the  measuring-tube  B,  as  shown  in  the  illustration.  The  levelling- 
tube  N  is  next  raised  until  mercury  begins  to  flow  out  of  the 
right-angled  capillary  tube,  when  the  stop-cock  H  is  closed.  After 
this  the  water  in  the  bulb  K  (which  is  held  in  an  inclined  position) 
is  boiled  for  three  minutes,  meanwhile  warming  the  capillary  tub- 
ing connected  with  the  measuring-tube  Unless  this  last  precau- 
tion is  taken,  the  capillary  tubing  is  likely  to  break,  particularly 
in  winter.  After  the  water  in  K  has  boiled  vigorously  for  three 
minutes,  the  flame  is  removed,  C  is  quickly  connected  with  the 
measuring-tube  B  and  the  rubber  connection  is  securely  -?asta.:_ed 
with  wire.-  By  boiling  the  water  in  K,  a  complete  v«  .uum  is 
produced  in  the  bulb,  so  that  the  gas  can  be  at  once  collected 
from  the  spring-wrater.  For  this  purpose  the  tube  L  is  pressed 
down  through  the  rubber  stopper  until  the  opening  I  comes 
just  below  its  lower  edge,  the  le veiling-tube  N  is  lowered,  and 
the  stop-cock  H  is  opened.  At  once  there  is  a  lively  evolution 
of  gas  from  the  water  in  A  and  this  is  subsequently  maintained  by 
warming  the  water.  As  soon  as  the  eudiometer  is  full  the  stop- 
cock is  closed  and  the  volume  of  the  gas  read  after  bringing  the  mer- 
cury to  the  same  level  in  N  that  it  is  in  B.  At  the  same  time  the 
temperature  of  the  water  in  the  condenser  M  is  taken  by  the  ther- 
mometer T  and  the  barometer  is  read.  The  gas  is  then  driven  over 
into  the  Orsat  tube  0  containing  potassium  hydroxide  solution  (1:2) 
and  allowed  to  remain  there  for  the  time  being.  Meanwhile  the  boil- 
ing of  the  water  in  A,  measurement  of  the  gas  in  B,  etc.,  are  con- 
tinued until  finally  no  more  gas  is  evolved  from  the  spring-water. 
All  of  the  gas  is  driven  over  into  the  Orsat  tube  after  its  volume 
has  been  noted  and  by  means  of  the  caustic  potash,  the  carbonic 
acid  is  quantitatively  absorbed.  The  unabsorbed  gas  is  again 
driven  over  into  B  and  its  volume  read.  By  correctly  regulat- 
ing the  velocity  of  the  current  of  water  flowing  through  the  con- 
denser, it  is  easily  possible  to  maintain  a  constant  temperature 
throughout  the  whole  of  the  experiment.  The  residual  gas  remain- 
ing after  the  absorption  of  the  carbon  dioxide  consists  usually 
of  nitrogen,  oxygen,  and  in  some  cases  methane.  It  is  trans- 
ferred to  the  apparatus  of  Hempel,  and  analyzed  according  to 
methods  which  will  be  described  further  on. 

According   to    this   method,    the    determination   of   nitrogen, 


736  GAS  ANALYSIS. 

oxygen,  and  methane  gives  exact  results,  but  the  apparent  amount 
of  carbon  dioxide  is  sometimes  too  much  and  sometimes  too  little. 
If  the  water  contains  large  amounts  of  bicarbonate  in  solution, 
the  carbonic  acid  found  will  represent  more  than  was  originally 
present  in  the  free  state,  for  such  substances  are  partly  decom- 
posed by  boiling  their  aqueous  solution.  On  the  other  hand  if 
only  a  little  bicarbonate  is  present,  the  result  will  be  too  low,  for 
it  is  not  possible  to  remove  all  of  che  free  carbonic  acid  from  a 
solution  by  boiling  it  in  a  vacuum. 

Consequently,  in  all  cases  the  free  carbonic  acid  must  be  deter- 
mined by  computation.  For  this  purpose,  in  a  fresh  sample  of 
the  water,  the  total  carbonic  acid  is  determined  according  to  p.  393, 
and  then  if  the  composition  of  the  solid  constituents  present  is 
known,  the  volume  of  the  free  carbonic  acid  can  be  calculated. 

Example.  — 1000  gms.  of  Tarasper  -  Lucius  water  contain 
7.8767  gms.  of  total  carbonic  acid.  Of  this  amount,  a  part  of  it 
is  present  in  the  water  as  carbonate  (''combined  "  carbonic  acid), 
an  equal  amount  as  "half-combined"  carbonic  acid,  and  the  re- 
mainder is  free  carbonic  acid.  If  from  the  total  amount  of 
carbonic  acid  the  " combined"  and  "ha'f-combined"  ac'd  is 
deducted  (or  what  s  the  same  thing,  double  the  amount  of  the 
"combined"  carbonic  acid),  the  difference  represents  the  amount 
of  free  carbonic  acid  present. 

Calculation  of  the  "Combined"  Carbonic  Add. 

This  is  obtained  by  multiplying  the  difference  between  the 
number  of  cations  and  anions  (expressed  in  univalent  ions)  by 
the  molecular  weight  of  carbonic-acid  (CO3)  ions  and  dividing  by 
two,*  because  the  sum  of  the  cations  in  every  salt  solution  is 
equal  to  that  of  the  anions  present  when  both  are  expressed  in 
univalent  ions. 

The  ":univalent  ions  "  are  obtained  by  dividing  the  amount 
in  grams  of  each  element  (or  radical)  present  by  its  atomic  (or 
molecular)  weight  and  multiplying  by  the  valence. 

*  For  the  CO3  ion  is  bivalent. 


COLLECTION  AND  CONFINEMENT   OF  GAS  SAMPLES. 


737 


(a)   Calculation  of  the  Cations. 


1000  gms.  Lucius  water  contain: 

Grams. 

Sodium 3.90610 

Potassium 0. 16603 

Lithium 0.00914 

Ammonium 0.01298 

Calcium 0.62691 

Strontium 0.00879 

Magnesium 0 . 19040 

Iron 0.00603 

Manganese 0. 00021 

Aluminium .  .  .   0 . 00064 


Combining 
Weight. 

23.00= 

39.10= 

7.00 

18.04= 

40.09  =  0.01564 
87.62  =  0.00010  X2 
24.32  =  0.00783  X2 
55.85  =  0.00011  X2  = 
54.93  =  0.000004X2 
27.1  =0.00002  X3 


0.16983  X  1 
0.00425  Xl 
0.00131  Xl 
0.00072  Xl 
X2 


Univalent 

Ions. 
0.16983 
0.00425 
0.00131 
0.00072 
0.03128 
0.00020 
0.01566 
0.00022 
0.00001 
0.00006 


Sum  of  the  cations  =  0.22354 


(6)    Calculation  of  the  Anions. 


Chlorine  (Cl) 2.40000; 

Bromine  (Br) 0 . 02890 ; 

Iodine  (I) 0.00086: 

Sulphuric  acid  (SO4) 1  -  72098  ; 

Boric  acid  (BO2) 0 . 57600 ; 

Phosphoric  acid  (PO4) ....  0 . 00008  : 

Silicic  acid  (SiO,) 0.01421 ; 


:  35.46  =  0.06768 

;  79.92  =  0.00035 

:  126. 92  =  0.00001 

:  96.07  =  0.01701 

:  43.0  =0.01340 

:  95.0  =0.00000 

:  76.3  =0.00019 

Sum  of  the  anions 


Xl  =  0. 06768 
Xl  =  0. 00035 
XI  =  0.00001 
X2  =  0. 03582 
Xl  =  0. 01340 
X3  =  0. 00000 
X2  =  0. 00038 


=0.11764 


Sum  of  the  cations 
Sum  of  the  anions 


=  0.22354 
=0.11764 


CO3  anions  remaining  =  0.1 0590 


expressed  in  univalent  ions. 


As  CO4  is  a  bivalent  ion,  half  of  this  amount  represents  the  actual  amount 
of  CO3  ions  present: 


0.10590 


=0.05295. 


This  corresponds  to 0.05295X60  =  3. 177  gms.  CO3 

Or  the  "  combined  "  carbonic  acid =  2 . 330    ' '     CO3 


GAS    ANALYSIS. 


Calculation  of  the  Free  Carbonic  Acid. 

Total  amount  of  carbonic  acid  (CO2)  present 7.877  gins,  per  liter 

Amount  of  "combined"  carbonic  acid..  .2.330     "       "     " 


Amount  of  free  +  "  half-combined  "  carbonic  acid   .  5 . 547 
Amount  of  "half-combined"  carbonic  acid. .  .   2.330 


Amount  of  free  carbonic  acid 3 . 217     ' '       "     " 

This  weight  of  carbon  dioxide  occupies  1637  c.c.  under  standard  con- 
ditions. 

By  boiling  828.3  gms.  of  the  water,  1868.9  c.c.  of  C02  were 
obtained  at  8.4°  C.  and  651  mm.  pressure,  containing  only  traces 
of  nitrogen.  This  corresponds  to  1851.4  c.c.  at  0°  C.  and  760  mm. 
pressure,  per  liter,  which  is  more  than  the  calculated  amount, 
because  the  carbonic  acid  gas  consisted  partly  of  free  and  partly 
of  "  half-combined "  carbonic  acid. 

In  cases  where  the  amount  of  bicarbonate  present  is  very 
small,  the  total  amount  of  carbonic  acid  obtained  by  boiling  the 
water  is  always  too  small.  Thus  in  the  case  of  the  thermal  water 
of  Baden,  by  boiling  there  was  obtained : 

Nitrogen 14.43  c.c.  per  liter 

Carbon  dioxide.  .  .   112.12   "      "      " 


126.55   "      " 

while  from  the  analysis,  the  free  carbonic  acid  was  computed  to 
be  180.52  c.c.  The  absorbed  gas  in  the  thermal  water  of  Baden 
is,  therefore, 

Nitrogen 14. 43  c.c.  per  liter 

Carbon  dioxide .  .   180.52   "      "      " 


194.95    "      "      " 

Remark. — With  the  above  method  of  collecting;  the  gas,  it  is 
difficult  to  prevent  some  water  getting  into  the  measuring-tube  B, 
by  means  of  which  a  small  amount  of  the  gas  will  be  reabsorbed. 
This  difficulty  is  avoided,  however,  if  the  flask  shown  in  Fig.  103  is 
used  to  contain  the  water 


COLLECTION  AND  CONFINEMENT  OF  GAS  SAMPLES. 


739 


FIG.  103. 


This  flask  is  provided  with  a  short  tube  blown  into  its  neck 
near  the  top   and   connected  by  means  of   thick-walled   rubber 

tubing  w.th  the  mercury  reservoir 
R.  In  order  to  determine  the  con- 
tents of  the  flask,  a  scratch  is  made 
on  the  small  tube  about  4  cm. 
from  the  neck  of  the  flask,  the 
mercury  is  driven  over  just  to 
this  mark,  and  the  rubber  tubing 
tightly  closed  by  means  of  a  screw- 
cock.  The  reservoir  is  then  emptied 
of  mercury,  and  the  flask  is  weighed 
together  with  the  stopper,  glass 
tube  L,  rubber  tubing,  and  what 
mercury  remains  above  Q.  The 
flask  is  then  filled  with  water,  the 
stopper  pressed  down  to  the  mark 
in  the  neck  of  the  flask,  and  the  tube 
L  is  raised  until  the  lower  opening 

I  comes  within  the  stopper.  After  drying  the  tube  L  with  blotting- 
paper,  the  flask  and  its  contents  are  weighed.  Its  capacity  is 
then  etched  upon  it 

For  the  determination  of  the  gases  absorbed  in  a  liquid,  the 
flask  A  is  filled  with  it  in  the  same  way  as  in  the  determination 
of  its  capacity,  the  bulb-tube  K,  half  filled  with  distilled  water 
is  connected  with  L,  and  the  latter  is  connected  with  a  capillary 
tube  as  shown  in  Fig.  102.  The  air  is  removed  from  K  and  the 
capillary  tubing  by  boiling  the  water  in  the  former,  as  described 
on  p.  735,  and  the  capillary  is  then  connected  with  the  measuring- 
tube  B,  Fig.  102.  The  heavy  rubber  tubing  is  now  connected  with 
the  reservoir  as  shown  in  Fig.  103,  and  the  latter  is  placed  in  a 
beaker  of  hot  water.  The  tube  L  is  introduced  into  the  neck 
of  the  flask  until  the  opening  I  can  just  be  seen,  and  the  gas  is 
expelled  in  the  same  way  as  described  on  p.  735,  except  that  in 
this  case  the  liquid  is  not  allowed  to  rise  so  high  in  K.  After 
three-quarters  of  an  hour  the  gas  will  be  completely  expelled 
from  the  liquid.  The  last  portions  of  the  gas  are  driven  over  into 
B  by  lowering  the  levelling  tube  N  (Fig.  102),  raising  the  mercury 


740  G4S  ANA LYSIS. 

reservoir  R  (Fig.  103),  and  carefully  opening  the  screw-cock  Q. 
A  warm  stream  of  mercury  will  then  flow  into  the  flask;  expelling 
the  gas  into  the  measuring- tube.  As  soon  as  the  liquid  in  A  has 
been  driven  over  as  far  as  the  stop-cock  H,  this  is  immediately 
closed.  Otherwise  the  procedure  is  the  same  as  was  described 
on  p.  735. 

In  order  to  test  the  accuracy  of  this  method,  the  author  made 
a  few  determinations  of  the  oxygen  absorbed  in  the  lake-water  at 
Zurich,  and  the  results  were  compared  with  those  obtained  by 
E.  Martz  in  this  laboratory  by  means  of  the  method  of  L.  Winkler 
(seep.  760). 

OXYGEN    IN    1    LITER    OF   ZURICH     LAKE-WATER. 


Modified  Pettersson 

Method. 

Method  of  L. 

Winkler. 

I 

7.66c.c.           7 

II 
74  c.c. 

I 
7.  67  c.c. 

II 

7.  75  c.c. 

Collection  of  Gases  Absorbed  by  Defibrinated  Blood. 

The  author  has  used  for  this  purpose  the  apparatus  shown 
in  Fig.  104.  The  experiment  is  carried  out  as  follows : 

First,  the  rubber  tubing  which  connects  N'  and  A  is  filled 
with  mercury  by  raising  the  levelling  tube  N',  and  the  pinch- 
cock  h"  is  closed.  T}hen  the  gas  burette  C  is  likewise  filled  with 
mercury  by  raising  D  and  the  stop-cock  h  is  turned  to  the  position 
shown  in  the  drawing.  By  raising  the  leveling  tube  N,  the 
bulb  K  and  the  vessel  A  are  filled  with  mercury,  which  is  allowed 
to  flow  into  the  funnel  M  up  to  the  line  a  and  then  the  stop- 
cock h  is  closed.  The  blood  to  be  examined  is  poured  into  M, 
N'  is  lowered,  and,  by  carefully  opening  h,  the  mercury  is  allowed 
to  fall  from  M  until  it  just  reached  the  stop-cock  h,  which  is 
then  closed.  M  is  now  filled  with  blood  to  the  mark  6,  h  is 
opened,  and  the  blood  is  sucked  down  until  its  upper  level  is 
exactly  at  the  line  a,  when  h  is  closed  once  more.  The  levelling 
tube  N  is  lowered  until  a  good  vacuum  is  produced  in  A  and 
the  mercury  level  falls  to  near  the  bottom  of  K.  When  this  is 


COLLECTION  OF  GASES  ABSORBED  BY  DEFIURINATED  BLOOD.    741 


FIG.  104. 


742  GAS  ANALYSIS. 

accomplished,  the  gas  escapes  from  the  blood  so  rapidly  that 
all  of  A  is  filled  with  foam;  after  a  few  minutes,  however,  the 
effervescence  subsides  and  all  the  blood  collects  in  K.  Then, 
the  leveling  tube  N  is  raised  until  the  blood  reaches  the  cock  h' 
and  the  latter  is  then  closed.  In  this  way  the  greater  part 
of  the  gas  absorbed  is  separated  from  the  blood.  The  leveling 
tube  N'  is  raised,  h"  opened  so  that  the  gas  in  A  is  under  pressure, 
and  by  properly  opening  h  and  the  burette  stop-cock,  the  gas 
is  driven  over  into  the  measuring  burette  C;  when  this  is  accom- 
plished, h  is  closed,  N'  lowered  until  the  mercury  reaches  the 
cock  h"  which  is  then  closed  and  h'  opened.  The  blood  again 
effervesces,  but  not  so  vigorously  as  before.  The  bulb  K  is  now 
surrounded  by  water  at  55°,  which  causes  further  effervescence 
from  the  blood.  As  soon  as  the  foam  subsides,  the  gas  is  again 
driven  over  into  C  and  the  process  of  evacuating  is  continued 
until  the  blood  ceases  to  effervesce.  The  volume  of  the  gas  in  C 
is  finally  read,  the  temperature  and  pressure  noted,  and  the 
analysis  carried  out  as  described  on  p.  775  or  786. 

The  Transference  of  Gases  in  Sealed  Tubes  to  the  Apparatus 
Used  for  the  Analysis. 

We  will  assume  the  gas  to  be  contained  in  R,  Fig.  105.  Over 
one  of  the  short  tubes  connected  with  the  three-way  stop-cock  H  is 
placed  a  piece  of  thick-walled  rubber  tubing  which  contains  a  short 
piece  of  heavy  glass  tubing  r.  The  stop-cock  is  then  revolved  so 
that  the  rubber  tubing  is  above  it  and  the  latter  is  filled  with  mer- 
cury. H  is  then  turned  180°  toward  the  left  so  that  the  left  r.nd 
upper  tubes  communicate  with  one  another.  As  soon  as  tve 
mercury  begins  to  run  out,  the  stop-cock  is  closed.  One  end  of  R 
is  then  introduced  into  the  rubber  tubing  containing  the  mercury 
so  far  that  its  drawn-out  point  reaches  within  r,  and  the  rubber 
tubing  is  securely  fastened  by  wiring.*  In  a  similar  way,  the  other 
end  of  R  is  connected  with  the  rubber  tubing  filled  with  mercury 
of  the  levelling-tube  N,  and  after  this  the  stop-cock  H  is  connected 
with  the  measuring  apparatus  W  by  means  of  the  capillary  tubing 

*  Annealed  iron  wire  is  used.  Copper  or  brass  wire  would  be  likely  to 
become  amalgamated  with  mercury. 


CALIBRATING   GAS  MEASURING   VESSELS. 


743 


E.      By  raising  the  levelling  bulb  K,  the  air  is  expelled  from  W 
and  the  capillary  E,  and  the  mercury  is  allowed  to  rise  in  the  fun- 


FiG.105. 


nel  T.  The  stop-cock  H  is  turned  so  that  communication  is  estab- 
lished between  R  and  W,  and  the  ends  of  the  former  are  opened 
by  pressing  the  capillaries  against  r  and  r7.  Then,  by  raising  N 
and  lowering  K,  the  gas  is  readily  driven  over  into  W. 


Calibrating  Gas  Measuring  Vessels. 

When  vessels  are  purchased  to  be  used  in  measuring  gases, 
the  correctness  of  the  calibrations  should  always  be  tested;  the 
testing  can  be  done  with  water  or  with  mercury.  The  calibration 
with  water  is  carried  out  in  exactly  the  same  way  as  was  described 
for  vessels  to  be  used  in  measuring  liquids  (cf.  pp.  522-530). 


744 


GAS  ANALYSIS. 


The  calibration  by  means  of  mercury  will  be  illustrated  by  an 
example.  We  shall  assume  that  it  is  desired  to  calibrate  the 
apparatus  shown  in  Fig.  106.  The  vessel  must  be  thoroughly 

cleaned  and  then  placed  in  a  vertical 
position  as  shown  in  Fig.  106  II.  The 
lower  capillary  a  is  connected  by  means 
of  thick-walled  rubber  tubing  with  a 
leveling  vessel  containing  mercury,  and 
the  mercury  is  made  to  rise  slowly  in 
the  vessel  to  a  little  above  the  upper 
mark.  The  stop-cock  is  then  closed,  the 
leveling  tube  together  with  the  rubber 
tubing  is  removed,  and  the  mercury  al- 
lowed to  flow  out  slowly  until  the  highest 
point  in  the  meniscus  is  exactly  tangent 
to  the  horizontal  plane  through  a! a! '.  To 
avoid  a  parallax  error,  the  reading  is 
taken  by  means  of  a  telescope  placed 
2  or  3  mm.  away  from  the  glass.  The 
whole  contents  of  the  vessel,  including 
the  space  in  the  stop-cock,  is  next  al- 
lowed to  run  into  a  tared  flask,  which 
is  then  weighed  to  the  nearest  centigram. 
After  determining  the  temperature  of  the 
mercury,  its  volume  can  be  found  by 
means  of  the  table  (top  of  page  745)  pre- 
pared by  Schlosser.* 

If  the  weight  of  the  mercury  at  20.3° 
amounted    to    2025.26    gms.,   then    its    volume   corresponds  to 

'^=  149.41.    Since,  however,  mercury  forms  a  convex  menis- 
Io.o4oo 

cus  and  the  volume  is  desired  up  to  the  plane  aa',  it  is  evident 
that  the  volume  of  mercury  weighed  did  not  include  the  space 
a'a-aa'  and,  moreover,  since  the  instrument  is  to  be  used  in 
the  reversed  position,  the  error  is  really  twice  as  much,  as  is 
evident  from  the  inspection  of  Fig.  106  I.  This  is  called  the 


FIG.  106. 


*  Schlosser  and  Grimm,  Z.  Chem.  App.-Kunde,  2,  201  (1907). 


CALIBRATING   GAS  MEASURING    VESSELS. 


745 


WEIGHT  OF  1  C.C.  OF  MERCURY  IX  AIR  AT  TEMPERATURES  BETWEEN  15°  AND  20°. 

Normal  temperature  15°. 


Temperature          Weight 
of  .Mercury. 

Temperature         Weight 
of  Mercury. 

Temperature              Weight 
of  Mercury. 

Deg.  C. 

Gms. 

Deg.  C. 

Gms. 

Deg.  C. 

Gms. 

i 

15 

13  .  5593 

20 

13  .  5489 

25 

13.5385 

15.5 

13  .  5583 

20.5 

13.5479 

25.5 

13.5374 

16 

13.5573 

21 

13.5468 

26 

13.5364 

16.5 

13.5562 

21.5 

13.5458 

26.5 

13.5353 

17 

13.5552 

22 

13.5447 

27 

13.5343 

17.5 

13.5541 

22.5 

13.5437 

27.5 

13.5332 

18 

13.5531 

23 

13.5426 

28 

13.5322 

18.5 

13  .  5520 

23.5 

13.5416 

28.5 

13.5312 

19 

13.5510 

24 

13.5405 

29 

13.5301 

•     19.5 

13  .  5499 

24.5 

13.5395 

29.5 

13  .  5291 

30 

13.5280 

double  meniscus  correction.     Its  value  is  dependent  upon  the  bore 
of  the  tube,  as  is  shown  by  the   following  table: 

TABLE    OF   MENISCUS    CORRECTIONS.* 


Diameter  of 
Tube  in  mm. 

Double  Meniscus, 
Correction  for  Hg  in 
mgs. 

Double  Meniscus, 
Correction  for  thO  in 
mgs.  =  cubic  millimeters 

Simple  Meniscus  Cor- 
rection (H2O—  Hg)  in 
cubic  millimeters. 

3 

76 

12 

3 

4 

108 

20 

6 

5 

174 

31 

9 

6 

314 

44 

10 

7 

550 

61 

10 

8 

790 

81 

11 

9 

1038 

106 

15 

10 

1288 

134 

20 

11 

1540 

167 

27 

12 

1796 

204 

36 

13 

2058 

245 

46 

14 

2326 

289 

59 

15 

2596 

336 

72 

,    16 

2872 

387 

88 

17 

3152 

441 

104 

18 

3436 

499 

123 

19 

3726 

560 

143 

20 

4016* 

624 

164 

21 

4314 

691 

187 

22 

4614 

757 

208 

23 

4920 

821 

229 

24 

5230 

881 

247 

25 

5544 

938 

264 

26 

5864 

991 

279 

27 

6185 

1042 

293 

28 

6515 

1090 

308 

29 

6845 

1135 

315 

30 

7182 

1179 

324 

*  W.  Schlo.sser.  Private  Communication. 


746  GAS  ANALYSIS. 

If  the  diameter  of  the  vessel  in  question  is  20  mm.,  then  the 
correction,  according  to  the  table,  would  be  4.016  gms.  and  the 
true  volume  will  be 

2025.26 +4.016  _2029.276  _1/|r)7p 
13.5473         =  13.5483  " 

The  volume  of  this  instrument,  therefore,  is  0.22  cm.  less  than 
the  intended  150  c.c.  The  volume  of  the  narrower  parts  of  the 
tube  can  be  found  in  a  similar  manner. 

The  diameter  of  the  tube  is  best  determined  by  filling  with 
mercury  up  to  a  mark,  then  allowing  it  to  run  out  until  a  lower 
mark  is  reached,  weighing  the  escaped  mercury,  and  measuring 
the  distance  between  the  two  marks  with  a  millimeter  rule. 
If  the  weight  of  the  mercury  is  p,  the  distance  between  the  marks 
h,  the  temperature  of  the  mercury  20.3°,  then  the 


diameter  =: 


h  XnX  13.5483* 


In  many  cases  it  is  sufficiently  accurate  to  compute  the  diameter 
from  the  circumference  of  the  tube  and  then  subtract  twice  the 
thickness  of  the  glass. 

If  it  is  desired  to  determine  the  total  volume  of  a  tube  provided 
with  stop-corks  at  both  ends,  the  apparatus  is  weighed  empty 
and  then  filled  with  mercury.  In  this  case,  it  is  obvious  that  no 
meniscus  correction  is  necessary. 

For  a  measuring  vessel  calibrated  with  water,  when  in  a 
reversed  position,  the  meniscus  correction  is  obtained  from  the 
table  on  page  745.  If  an  instrument  calibrated  with  water  is 
to  be  used  subsequently  with  mercury,  the  water  meniscus  in 
calibrating  the  reversed  tube  occupies  a  similar  position  to  that 
of  the  mercury  meniscus  when  the  instrument  is  in  use  (see  Fig. 
107)  but  the  mercury  meniscus  is  not  so  deep  as  that  of  the  water. 
The  volume  of  the  gas  is  therefore  found  as  much  too  large  as  the 
difference  between  the  simple  meniscus  corrections  (H2O— Hg). 
Thus  if  the  volume  of  a  gas  measuring  instrument  of  70  mm. 
diameter  is  found  by  weighing  with  water  to  be  10.167,  according 


PURIFICATION  OF   MERCURY. 


747 


to  the  table  on  page  745,  then  if  the  instrument  is  to  be  used  with 
mercury,  the  gas  volume  will  be  10.167—0.020=10.147  c.c. 

Purification  of  Mercury.     Lothar  Mayer's  Method.  * 

The  mercury  used  for  gas-analytical  operations  must  be 
purified.  The  principal  impurities  are  copper,  cadmium,  zinc 
and  sometimes  silver  and  gold.  The  base  metals  are  removed 


FIG.  107. 


FIG.  108. 


most  readily  by  allowing  the  mercury  to  run  in  a  fine  stream 
through  about  a  meter  of  8  per  cent,  nitric  acid.  This  is  done 
in  the  apparatus  shown  in  Fig.  108.  The  bottom  of  the  tube  B 
is  first  filled  with  impure  mercury  and  the  nitric  acid  is  added. 
The  mercury  is  then  poured  through  the  funnel  A,  the  stem  of 
which  is  drawn  out  to  a  capillary  and  bent  to  an  angle  of  60°. 
This  causes  the  mercury  to  take  a  zig-zag  course. as  it  flows  slowly 
through  the  nitric  acid.  The  dry  mercury  that  first  passes  over 
into  the  flask  C  is  impure  and  must  be  poured  into  the  funnel  and 
allowed  to  flow  through  the  acid.  In  this  way  a  fairly  pure  mercury 

*  Z.  anal.  Chem.,  2,  241  (1863).  C.  J.  Moore  (Chem.  Ztg.,  1910,  735) 
has  used  a  similar  apparatus  for  purifying  large  quantities  of  mercury,  but 
niters  through  buckskin  before  allowing  it  to  fall  through  the  acid. 


748 


GAS  ANALYSIS. 


is  obtained  which  can  be  used  as  it  is  for  most  purposes.  If  the 
mercury  is  to  be  used  for  calibrating  apparatus,  it  must  be  distilled. 
For  this  purpose,  Hulett's  apparatus,  shown  in  Fig.  109,  may 
be  used.  The  mercury  is  placed  in  the  long-necked  flask  K  which 
is  connected  with  the  receiver  V.  The  flask  is  covered  with  a 
hood  of  asbestos  paper  and  heated  on  the  sand  bath.  Through 
the  arm  a,  the  receiver  is  connected  with  a  suction  pump  and  at 


FIG.  109. 

b  a  slow  current  of  nitrogen  (or  carbon  dioxide)  which  has  been 
dried  by  passing  over  calcium  chloride,  is  introduced  into  the 
flask  through  the  long  glass  tube  that  ends  in  a  capillary.  The 
distillation  is  regulated  so  that  the  mercury  condenses  in  the  glass 
arm  c,  where  it  leaves  the  hood  of  asbestos  paper.  About  150-200 
c.c.  of  mercury  can  be  distilled  in  an  hour  with  this  apparatus. 
Frequently,  especially  when  the  nitrogen  used  contains  a  little 
oxygen,  the  distilled  mercury  is  covered  with  a  thin  coating  of 
oxide.  This  may  be  removed  by  filtration.  To  filter  the  mercury, 
the  point  of  a  paper  filter  is  perforated  several  times  with  a 
needle,  the  filter  placed  in  a  funnel  and  the  mercury  poured 


SUBDIVISIONS  OF  GAS  ANALYSIS.  749 

through.     The  pure  metal  runs  through  the  holes  in  the  paper 
while  the  impurity  remains  behind. 


Subdivisions  of  Gas  Analysis. 

According  to  the  manner  of  determining  the  amount  of  gas, 
we  distinguish  between : 

1.  Absorption  Methods. 

2.  Combustion  Methods. 

3.  Volumetric  Methods. 


In  the  case  of  an  absorption  method  the  mixture  of  gases  is 
treated  with  a  series  of  absorbents.  The  difference  in  the  volumes 
of  the  gas  before  and  after  it  has  been  acted  upon  by  each  absorbent 
represents  the  amount  of  gas  absorbed.  The  absorption  of  the 
gas  may  take  place  in  the  measuring-tube  itself,  or,  what  is  better, 
in  separate  absorption  vessels. 

In  this  way,  the  amount  of  carbon  dioxide,  heavy  hydrocar- 
bons (ethylene,  benzene,  acetylene, etc.), oxygen,  and  carbon  mon- 
oxide may  be  determined  in  illuminating-gas,  producer  gas,  water- 
gas,  or  Dowson  gas. 

After  the  constituents  capable  of  absorption  have  been  re- 
moved, a  gas  residue  is  left  consisting  of  hydrogen,  methane, 
and  nitrogen;  the  two  former  constituents  are  determined  by 
combustion,  while  the  latter  is  always  determined  by  subtracting 
the  total  amount  of  other  gases  found  from  100  per  cent. 

For  a  combustion  analysis  the  unabsorbed  constituents  of  the 
gas  mixture  are  mixed  with  air,  or  oxygen,  in  more  than  sufficient 
amount  to  ensure  complete  combustion,  and  burnt  in  a  suitable 
apparatus;  the  amount  of  combustible  gas  is  determined  by 
measuring  the  contraction,  the  amount  of  carbon  dioxide  formed, 
and  the  excess  of  oxygen. 

Finally,  if  the  gas  evolved  by  means  of  a  chemical  reaction  is 
measured  and  from  the  volume  of  the  latter  the  weight  of  the  body 
producing  it  is  calculated,  we  have  made  use  of  what  is  called 
a  gas-volumetric  method.  (Cf.  Determination  of  Carbonic  and 
Nitric  Acids,  pp.  384  and  456). 


750  GAS  ANALYSIS. 

DETERMINATION  OF  THE  GASES. 
Carbon  Dioxide,  CO2.     Mol.  Wt.  44. 

Density  =  1.5290  *  (Air  =  1) .     Weight  of  1  liter  =  1.9767  gms. 
Molar  volume  =22.26  1. 
Critical  temperature  =  +31.5°  C. 

Carbon  dioxide  is  absorbed  to  a  considerable  extent  by  water; 
1  vol.  water  absorbs: 

At    0°  C 1.7967  c.c.  CO2 

"   15°  C , 1.0003    "      " 

"  25°  C 0.8843    "      " 

or  in  general 

/?t  =  1.7967  -0.07761  X* + 0. 001 6424  XZ2, 

Absorbent. — Potassium  Hydroxide  Solution  1 :2. 

1  c.c.  of  caustic  potash  of  the  above  strength  will  absorb  at 
least  40  c.c.  of  C02.  Sodium  hydroxide  solution  is  not  used  on 
account  of  the  difficult  solubility  of  sodium  bicarbonate. 

Small  amounts  of  C02  may  be  absorbed  by  means  of  a  definite 
amount  of  standardized  Ba(OH)2  solution,  and  the  excess  of  the 

N 
latter  titrated  with  —  HC1,  using  phenolphthalein  as  indicator. 

(See  p.  593). 

*  This  number  is  the  mean  from  the  observations  of  Lord  Rayleigh 
(1897)  =  1.52909,  Leduc  (1898)  =  1.52874,  and  Christie  (1905)  =  1.52930. 

t  @  is  called  the  absorption  coefficient  of  the  gas.  This  signifies  the 
volume  of  gas,  measured  at  0°  and  760  mm.  pressure,  which  1  c.c.  of  a  liquid 
at  t°  will  absorb  when  the  pressure  upon  the  surface  of  the  liquid  is  760  mm. 
If  h  c.c.  of  liquid,  at  t°  and  B  mm.  pressure,  absorb  Vt  c.c.  of  the  gas,  then 
the  absorption  coefficient  can  be  computed  by  the  equation: 


THE  HEAVY  HYDROCARBONS.  751 

The  Heavy  Hydrocarbons. 
Ethylene  (Ethene),  C2H4;  Benzene,  C6H6  ;  Acetylene  (Ethine), 


Ethylene,  C2H4.     Mol.  Wt.  28.03. 

Density  =0.9739*  (Air  =  1)  .     Weight  of  1  liter  =  1.2590  gms. 
Molar  volume  =22.27.     Critical  Temperature  =  +9°  C. 

Preparation  of  Ethylene.  —  One  of  the  most  satisfactory  methods 
consists  in  treating  an  alcoholic  solution  of  ethylene  bromide 
with  zinc  dust  f  '• 

C2H4Br2  +  Zn  =ZnBr2  +  C2H4. 

A  round-bottomed  flask  of  about  200  c.c.  capacity,  and  having 
a  short  wide  neck,  is  chosen  for  the  experiment.  In  the  neck  is 
inserted  a  rubber  stopper  with  three  holes,  carrying  respectively 
a  safety  tube  provided  with  mercury  seal,  a  gas  delivery  tube, 
and  a  dropping  funnel. 

A  sufficient  amount  of  zinc  dust,  moistened  with  alcohol,  is 
placed  in  the  flask  and  gently  heated  at  the  start  by  placing  the 
flask  in  a  bath  of  warm  water  at  about  50°  C.  A  mixture  of  1 
part  ethylene  bromide  and  20  parts  absolute  alcohol  is  placed  in 
the  dropping  funnel,  and  allowed  to  flow  slowly  upon  the  zinc 
dust.  The  escaping  gas  is  passed  first  through  olive  oil,  to  remove 
a  little  ethylene  bromide  which  is  carried  over  mechanically,  then 
through  caustic  potash  solution,  and  finally  through  water;  it  is 
collected  over  mercury,  perhaps  in  the  Drehschmidt  pipette 
(Fig.  105,  p.  743).  The  gas  thus  obtained  is  almost  pure,  par- 
ticularly when  the  mixture  of  ethylene  bromide  and  alcohol  has 
stood  for  some  time  over  anhydrous  sodium  carbonate  to  remove 
traces  of  hydrobromic  acid. 

A  sample  of  the  gas  prepared  by  W.  Misteli  was  found  to  con- 

*  M.  Bretschger  (Inaug-Dissert.  Zurich,  1911)  found  the  density  of  ethylene 
to  be  0.9724,  but  M.  Stahrfoss  and  P.  A.  Guye  (Arch.  soc.  phys.  et  nat,  28 
1909)  found  the  value  0.9758.  In  the  text  the  mean  of  these  two  values 
is  used. 

t  Gladstone  and  Tribe,  Berichte,  7,  364  (1874). 


752  VOL UME  TRIG  ANAL  YSIS. 

tain  98.84  per  cent,  of  ethylene,  1.00  per  cent,  of  hydrogen  and 
0.16  per  cent,  of  nitrogen. 

Absorption  Coefficient  for  Water. 
1  volume  of  water  absorbs  at 

0°  C 0.256  c.c.  C2H4 

15°  C 0.161    "      " 

20   C 0.149    "      " 

or  in  general, 

ft  =0.25629  =0.0091363U+0.000188108*2. 
Alcohol  absorbs  more  ethylene;  the  general  formula  is 
#=3.594984  -0.077162  •  £  4-0.0006812  •  t\ 

Absorbents. — 1.  Fuming  sulphuric  acid*  (with  20  to  25  per 
cent.  SO2),  1  c.c.  absorbs  8  c.c.  of  C2H4.  2.  Bromine  water,  f 

Ammoniacal  cuprous  chloride  solution  will  also  absorb  ethy- 
lene. 

By  means  of  bromine,  the  ethylene  is  absorbed  with  the  for- 
mation of  ethylene  bromide,  C2H4Br2.  If  the  absorption  is 
effected  with  a  titrated  bromine  water,  the  amount  absorbed 
can  be  determined  by  titrating  the  excess  of  bromine.  This 
excellent  method,  proposed  by  Haber,J  is  at  present  the  best 
known  for  the  determination  of  ethylene  in  the  presence  of 
benzene.  (See  p.  818.) 


Benzene,  C6H6.     Mol.  Wt.  78.04. 

78.04  gms.  of  benzene  vapor  occupy  a  volume  of  22.391  liters 
under  standard  conditions. 

Benzene  is  readily  soluble  in  alcohol,  ether,  carbon  bisulphide, 
caoutchouc,  ethylene  bromide,  bromine,  and  fuming  sulphuric 
acid. 

*  Ethionic  acid,  C2H6S2O7,  is  formed. 

f  Treadwell  and  Stokes,  Berichte,  21  (1888),  p.  3131. 

J  Haber  and  Oechelhauser,  Berichte,  29,  p.  2700. 


BENZENE.  753 

Absorbents. — Fuming  sulphuric  acid*  and  bromine  water  con- 
taining an  excess  of  bromine. 

Inasmuch  as  benzene  is  neither  brominated  nor  oxidized  by 
bromine  at  ordinary  temperatures,  it  was  difficult  to  under- 
stand why  bromine  water  should  absorb  it  quantitatively.  In 
fact,  Berthelotf  and  Cl.  Winklerf  disputed  it,  but  the  results 
of  Tread  well  and  Stokes  §  have  recently  been  confirmed  by 
Haber.  He  suggested  that  the  absorption  of  benzene  by  bromine 
was  of  a  purely  physical  nature,  and  M.  Korbuly||  has  shown 
that  such  is  the  case.  Just  as  bromine  can  be  removed  from 
aqueous  solution  by  shaking  with  benzene,  so  benzene  can  be 
removed  by  shaking  with  bromine,  or  even  ethylene  bromide  and 
other  like  solvents. 

By  means  of  highly  concentrated  nitric  acid  (specific  gravity 
1.52)  benzene  is  also  absorbed;  this  solvent  cannot  be  used  in  tbe 
analysis  of  gases  containing  carbon  monoxide,  for  the  latter  is 
quantitatively  oxidized  to  carbon  dioxide  by  nitric  acid  of  this 
strength,  and  is  therefore  removed  with  the  benzene  1T  when  the 
acid  vapors  are  neutralized  by  caustic  potash  solution. 

Behavior  of  Benzene  to  Water. 

Benzene  vapors  are  absorbed  to  a  considerable  extent  by  water 
and  all  aqueous  salt  solutions,  a  circumstance  which  must  be 
considered  when  an  exact  gas  analysis  is  to  be  made. 

In  order  to  determine  how  much  benzene  is  absorbed  by  water, 
M.  Korbuly  performed  the  following  experiments: 

Different  amounts  of  air  containing  3.16  per  cent,  of  benzene 
vapor  were  shaken  in  a  Drehschmidt's  pipette  with  the  same 
amount  of  water  (5  c.c.)  until  no  more  benzene  was  absorbed.  He 
obtained  the  following  results : 


*  Benzene  sulphonic  acid  is  formed,  C8H,SO3. 

t  Compt.  rend.,  83,  p.  1255. 

t  Zeitschr.  f.  anal.  Chem.,  1889,  p.  281. 

§  Treadwell  and  Stokes,  loc.  cit. 

II  Inaug.  Dissertation,  Zurich,  1902. 

1f  Treadwell  and  Stokes,  loc.  cit. 


754 


GAS  ANALYSE. 


Experiment. 

Gas  Taken  in  c.c. 

Per  Cent.  Benzene 
Present  by 
Volume. 

Amount  of  Benzene  Absorbed 
at  the  End  of  Three 
Minutes. 

1 

5S.-92 

3.16 

1.28c.c.  =  2.17% 

2 

61.14 

3.16 

0.80     '  =1.31% 

3 

53.32 

3.16 

0.52     '  =0.89% 

4 

5 

59.86 

60.78 

3.16 
3.16 

0.44     «  =0.73% 
0.2S     '  =0.46% 

6 

59.88 

3.16 

O.OS     '  =0.01% 

7 

60.20 

3.16 

0.02     '  =0.00% 

Potassium  hydroxide  behaves  similarly. 

In  the  analysis  of  a  mixture  of  carbon  dioxide  and  benzene, 
it  is  customary  to  first  remove  the  carbon  dioxide  by  means  of 
potassium  hydroxide  solution  and  then  the  benzene  with  fuming 
sulphuric  acid  or  bromine.  It  is  evident,  then,  that  both  of  the 
results  obtained  will  be  inaccurate  if  a  fresh  solution  of  potassium 
hydroxide  is  used  for  the  absorption  of  the  carbon  dioxide,  for 
this  will  absorb  not  only  the  whole  of  the  carbon  dioxide,  but  in 
many  cases  nearly  all  of  the  benzene.  Accurate  results  may  be 
obtained  by  using  a  solution  of  potassium  hydroxide  which  has 
been  saturated  with  benzene  vapors. 

Acetylene,  G2H2.    Mol.  Wt.  26.02. 

Density  =0.9134  *  (Air  =  1) .     Weight  of  one  liter  =  1.1808  gms. 
Molar  volume  =22.03  1.     Critical  temperature  =  +37°  C. 
Boiling  Point  =  -80.6°  C. 

Acetylene  is  quite  soluble  in  water;  1  volume  of  water  at  the 
ordinary  temperature  absorbs  an  equal  volume  of  this  gas.  In 
amyl  alcohol,  chloroform,  benzene,  glacial  acetic  acid,  and  acetone 
it  is  much  more  soluble;  thus  1  volume  of  acetone  absorbs  31 
volumes  of  acetylene. t 

*M.  Bretschger  (Inaug.  Dissert.  Zurich,  1911)  found  the  density  of 
acetylene  =  0.9157,  whereas  M.  Stahrfoss  and  P.  A.  Guye  (Arch.  sci.  phys.  et 
nat.,  28,  1909)  found  it =0.9120.  The  mean  of  these  two  values  is  0.91335. 

t  Hempel,  Gasanalytische  Methoden  (1900),  p.  206. 


PREPARATION  OF  PURE   ACETYLENE.  755 

Preparation  of  Pure  Acetylene. 
(a)   Method  of  M.  Bretschger.* 

The  crude  acetylene,  prepared  from  calcium  carbide,  is  passed 
through  an  acid  solution  of  copper  sulphate,  then  through  aqueous 
chromic  acid,  caustic  potash,  and  finally  over  slaked  lime;  it  is 
then  subjected  to  a  fractional  distillation.  The  gas  is  passed 
through  a  small  bulb  cooled  by  liquid  air  which  causes  the  acetylene 
to  solidify.  By  gentle  warming,  the  acetylene  is  then  evaporated 
and  is  caused  to  pass  through  calcium  chloride  tubes. 

(b)  Method  of  M.  Stahrfoss  and  P.  A.  Guyej 

The  impure  acetylene,  prepared  from  calcium  carbide,  is  passed 
through  a  solution  of  potassium  permanganate,  then  through 
caustic  potash  solution  and  finally  over  phosphorus  pentoxide. 
It  is  frozen  by  means  of  liquid  air  and  then  fractionated. 

The  method  of  preparing  acetylene  by  decomposing  copper 
acetylide  cannot  be  recommended,  because  the  gas  is  then  strongly 
contaminated  with  ethylene  (C2H4)  and  vinyl  chloride  (X^HsCl). 
Thus  M.  Bretschger  J  found  from  5  to  10  per  cent,  of  ethylene 
in  such  gas. 

Absorbents:  Fuming  sulphuric  acid.§  By  bromine  water 
acetylene  is  absorbed  extremely -slowly  in  the  cold,  a  fact  which 
permits  the  titration  of  ethylene  in  the  presence  of  acetylene 
(see  page  821). 

By  means  of  ammoniacal  cuprous  chloride,  acetylene  is 
absorbed  and  forms  red  copper  acetylide  (Cu2C2H2)O.  This 
reaction  is  so  characteristic  that  it  is  used  for  the 

Qualitative  Detection  of  Acetylene 

in  gas  mixtures.     This  test  is  best  performed  by  the  method  of 
L.  Ilosvay  von  Nagy  Ilosva.|| 

*  Loc.  tit. 

f  Private  Communication  from  Professor  Guye. 

%  Loc.  tit. 

§  C2H4SO4  is  formed. 

||  Berichte,  32  (1899),  p.  2698. 


756  GAS  ANALYSIS. 

Preparation  of  the  Reagent. — One  gram  of  copper  nitrate 
(chloride  or  sulphate)  is  placed  in  a  50-c.c.  measuring-flask  and 
dissolved  in  a  little  water.  To  the  solution  4  c.c.  of  concentrated 
ammonia  (20-21  per  cent.  NH3)  and  3  gms.  of  hydroxylamine 
hydrochloride  are  added,  and  the  liquid  is  shaken  until  it  becomes 
colorless,  when  it  is  immediately  diluted  with  water  up  to  the 
mark. 

The  Qualitative  Test. — A  few  cubic  centimeters  of  the  reagent 
are  placed  in  a  500-c.c.  glass-stoppered  cylinder  and  the  gas  to  be 
tested  for  acetylene  (illuminating-gas)  is  passed  over  it  until  the 
color  of  the  reagent  becomes  pink.  The  cylinder  is  then  stoppered 
and  its  contents  thoroughly  shaken.  If  acetylene  is  present,  a 
beautiful  red  precipitate  is  immediately  formed.  Another  method 
of  making  the  test  is  to  pass  the  gas  through  a  small  bulb-tube 
containing  glass-wool  moistened  with  the  reagent. 

Remark. — If  the  reagent  is  placed  under  petroleum  it  can  be 
kept  for  about  one  week,  but  if  copper  wire  is  added  to  the  solu- 
tion, it  can  be  kept  for  a  much  longer  time,  as L.Pollak  has  shown. 
Such  a  solution  gave  a  distinct  reaction  after  it  had  been  kept 
for  one  year,  but  the  precipitate  obtained,  instead  of  being  a  bright 
red,  was  more  the  color  of  sealing-wax.  The  solution  is  much 
less  permanent  when  it  is  prepared  from  the  chloride  or  sulphate, 
even  when  copper  is  added  to  it.  Without  the  copper,  the  chloride 
would  give  no  reaction  after  being  a  week  old,  and  with  the  addition 
of  copper  it  was  spoiled  at  the  end  of  two  weeks.  The  sulphate 
behaved  about  the  same. 

Separation  of  the  Heavy  Hydrocarbons  from  One  Another. 

It  has  been  attempted  repeatedly  to  separate  ethyleiie  from 
benzene,  but  usually  in  vain.  The  separation  as  proposed  by 
Berthelot,  of  absorbing  the  ethylene  with  bromine  water  and 
afterwards  removing  the  benzene  by  means  of  concentrated  nitric 
acid,  is  erroneous  in  every  respect.*  The  method  of  Harbeck 
and  Lunge  f  is  correct  in  principle  but  very  tedious,  and  the 
original  modification  of  Pfeiffer  J  always  gives  too  high  results. 

*  Treadwell  and  Stokes,  loc.  cit. 
t  Zeit.  f.  anal.  Chem.,  XVI  (1898),  p.  26. 

I  J.  f.  Gasbeleuchtung  und  Wasserversorgung,  1899,  p.  697,  and  Berichte, 
29,  p.  2700. 


OXYGEN. 


757 


Recently  Pfeiffer  *  has  improved  his  method  so  that  it  gives 
the  same  results  as  that  of  Harbeck  and  Lunge. 

Haber  and  Oechelhauser,f  on  the  other  hand,  have  devised 
a  method  which  is  accurate  and  to  be  recommended. 

Principle. — In  one  portion  of  the  gas,  the  sum  of  the  ethylene 
and  ten zene  is  determined  by  absorption  with  bromine  water  or 
fuming  sulphuric  acid,  while  in  a  second  portion  the  gases  are 
absorbed  in  titrated  bromine  water,  and  the  excess  of  the  latter 
is  determined  iodimetrically.  From  the  amount  of  bromine  re- 
quired the  ethylene  is  calculated: 

N 
1  c.e.  — 1=1.114  c.c.  C2H4  at  0°  G.  and  760  mm.  pressure. 

As  this  analysis  is  performed  in  the  Bunte  burette,  it  will  not 
be  described  in  detail  until  we  have  become  acquainted  with  this 
important  form  of  apparatus.  (See  p.  798.) 

Oxygen,  O  =  16.     Mol.  Wt.  32. 

Density  =1.1053    (Air=l).     Weight  of   1   liter  =1.4289   gms. 
Molar  volume  =  22.39  1.       Critical  temperature  =  -  119°  C. 

Oxygen  is  only  slightly  soluble  in  water;  according  to  the 
experiments  of  L.  W.  Winkler;J  one  liter  of  water  at  60  mm. 
pressure  will  absorb  the  following  quantities  of  air : 

ABSORPTION  COEFFICIENTS  OF  ATMOSPHERIC  AIR. 


Temperature. 
0° 

(1000  c 

Oxygen 

...     10  24  c 

.c.  absorbed) 

Nitrogen. 

c.         18.57c 
16.45  ' 
14.67   ' 
13.29 
12.19 
11.31 
10.59 
9.92 
9.35 
8.93 
8.59 
8.31 

Air. 
c.         28.81c 
25.43  ' 
22.64  ' 
20.45 
18.69 
17.24 
16.06 
15.03 
14.18 
13.51 
12.97 
12.53 

c. 

< 

i 

5° 

...     8  98 

10°            

7  97 

15°              

7.16 

20°          

6.50 

25° 

5  93 

30° 

.     5  47 

35° 

...     5  11 

40° 

4  83 

45°          

4  58 

50°  

4  .  38 

55°  

4  .  22 

*  See  Chem.  Ztg.,  1904  (884). 

f  J.  f.  Gasbeleuchtung  und  Wasserversorgung,  1896,  p.  804,  and  Berichte, 
29,  p.  2700. 

JBerichte,  34,  1410  (1901). 


758  GAS  ANALYSIS. 

From  these  data,  the  absorption  coefficient  of  pure  oxygen  for 
water  at  0  to  55°  can  be  computed. 

ABSORPTION  COEFFICIENTS  OF  OXYGEN  FOR  WATER. 
Temperature.  0  Temperature.  0 

0° 0.04890  30° 0.02608 

5° 0.04286  35° 0.02440 

10° 0.03802  40° 0.02306 

15° 0.03415  45° 0.02187 

20° 0.03102  50° 0.02090 

25° 0.02831  55° 0.02012 

Oxygen  can  be  determined  by  combustion  or  by  -absorption. 

The  Determination  of  Oxygen  by  Combustion. 

The  determination  of  oxygen  by  combustion  may  be  effected 
by  exploding  it  with  hydrogen  (Bunsen)  or  by  conducting  a 
mixture  of  the  two  gases  through  a  glowing  platinum  capillary 
(Drehschmidt),  exactly  as  in  the  determination  of  carbon  mon- 
oxide (cf.  p.  765).  In  both  cases  the  combustion  takes  place  in 
accordance  with  the  equation : 

0   +   H2   =    H2O 

1  vol.      2  vols.  0  vol. 

Three  volumes  of  gas,  therefore,  disappear  for  each  volume  of 
oxygen  present.  If  the  contraction  resulting  from  the  com- 
bustion of  a  mixture  of  oxygen  and  an  excess  of  hydrogen  is 
designated  by  Vc,  then  the  amount  of  oxygen  present  =JVc. 


The  Determination  of  Oxygen  by  Absorption. 

The  absorbents  of  oxygen  are : 

1.  Alkaline  Pyrogallol  Solution  (Liebig). 

One  volume  of  a  22  per  cent,  aqueous  solution  of  pyrogallol  is 
mixed  with  five  or  six  times  as  much  potassium  hydroxide  solution 
(3:2).  1  c.c.  of  this  solution  absorbs  12  c.c.  of  oxygen. 


OXYGEN.  759 

At  a  temperature  of  15°  C.,  or  higher,  the  absorption  takes 
place  quickly;  the  oxygen  in  100  c.c.  of  air  will  be  absorbed  in 
three  minutes  or  less. 

At  lower  temperatures  the  absorption  takes  place  less  readily 
and  at  0°  C.  the  above  quantity  of  oxygen  cannot  be  absorbed 
completely  in  half  an  hour. 

A  pyrogallol  solution  of  the  above  concentration  will  not  evolve 
carbon  monoxide  during  the  absorption.  . 

2.  Phosphorus  (Lindemann). 

The  absorption  of  oxygen  by  means  of  phosphorus  takes 
place  by  simply  allowing  the  gas  containing  the  oxygen  to  remain 
over  moist  phosphorus.  The  formation  of  white  clouds  indicates 
the  presence  of  oxygen,  and  their  disappearance  shows  that 
the  absorption  is  complete.  A  temperature  of  15  to  20°  C.  is 
best  suited  for  the  absorption. 

The  oxygen  is  completely  absorbed  at  the  end  of  three  min- 
utes from  100  c.c.  of  air  at  this  temperature.  At  lower  temper- 
atures the  absorption  requires  more  time  and  at  0°  more  than  an 
hour  is  necessary. 

If  the  gas  contains  more  than  60  per  cent,  of  oxygen,  moist 
phosphorus  will  absorb  none  of  it  at  the  ordinary  atmospheric 
pressures.  In  this  case  the  gas  must  be  diluted  with  nitrogen  or 
hydrogen  until  a  mixture  is  obtained  containing  less  than  60 
per  cent,  oxygen,  or  the  gas  must  be  allowed  to  act  upon  the  moist 
phosphorus  under  diminished  pressure.  In  the  latter  case,  how- 
ever, the  phosphorus  easily  becomes  heated  enough  to  melt  it. 

Further,  oxygen  is  not  absorbed  by  moist  phosphorus  if  the 
gas  contains  traces  of  heavy  hydrocarbons,  ethereal  oils,  alcohol, 
or  ammonia.  According  to  Hempel*  0.04  per  cent,  of  ethylene, 
and  according  to  Haberf  0.17  per  cent.,  suffices  to  prevent  com- 
pletely the  absorption  of  oxygen. 

*  Gasanalytische  Methoden. 

f  Experimental-Untersuchung  liber  Zersetzungen  und  Verbrennungen 
von  Kohlenwasserstoffen,  Habilitationschrift,  Munich,  1896. 


76°  GAS  ANALYSIS. 

3.  Chromous  Chloride. 

Consult  the  paper  by  Otto  von  der  Phordten,  Annal.  Chem. 
Phys.  228,  112. 

4.  Copper. 

The  gas  is  either  conducted  over  glowing  copper,  or  it  is  intro- 
duced into  a  Hempel  pipette  containing  rolls  of  copper  gauze  and 
an  ammoniacal  solution  of  ammonium  carbonate. 

5.  Sodium  Hydrosulphite,*  NazSzO*  (Franzen  f), 

An  alkaline  solution  of  sodium  hydrosulphite,  which  can  now 
be  obtained  commercially  at  a  low  price,  is  an  excellent  absorbent 
for  oxygen.  The  reagent  may  be  prepared  for  use  in  the  Hempel 
pipette  by  dissolving  50  gms.  of  the  salt  in  250  c.c.  water  and  40 
c.c.  caustic  potash  solution  (500  KOH:700H2O).  For  absorption 
in  the  Bunte  burette,  the  above  solution  is  too  concentrated;  in 
this  case  10  gms.  hydrosulphite  in  50  c.c.  water  plus  50  c.c.  of  10 
per  cent,  caustic  soda,  may  be  used. 

The  absorption  takes  place  in  accordance  with  the  equation: 

2Na2S2O4  +  2H2O  +  O2  =  4NaHS03. 

Sodium  hydrosulphite  has  the  advantage  over  all  other 
absorbents  that  the  absorption  is  always  complete  at  the  end  of 
five  minutes. 

Determination    of    Absorbed    Oxygen    in    Water.     Method    of 
L.  W.  Winkler.l 

1000  c.c.  JQ  Na^Og  solution  =—  =  2^=0.8  gm.  or  559.8  c.c.  oxygen  at  0° 

and  760  m.m  pressure. 

Principle. — If  water  containing  dissolved  oxygen  be  heated  in 
a  closed  vessel  with  manganese  hydroxide,  the  latter  is  oxidized 
to  manganous  acid  according  to  the  following  equation : 
Mn(OH)2  +  Q=H2MnQ3. 

*  This  is  really  sodium  hyposulphite,  although  sodium  thiosulphate,  Na2S2O3, 
is  commonly  called  "hyposulphite." 
t  Berichte,  39,  2069  (1896). 
I  Ibid,  21  (1888),  p.  2843. 


DETERMINATION  OF  ABSORBED  OXYGEN  IN  WATER         761 

The  amount  of  oxygen  taken  up  is  determined  iodimetrically  by 
adding  hydrochloric  acid  and  potassium  iodide  to  the  manga- 
nous  acid  and  titrating  the  liberated  iodine, 

H2MnO3+4HCl  =  MnCl2-h3H2O+Cl2  and  2X1+012=2X01+12- 

Hence  1  gm.-at.  1  =  8  gins.  =  5597.8  c.c.  oxygen  at  0°  C.  and 
760  mm.  pressure. 

Reagents  Required. — 1.  An  approximately  J+N  -  MnCli  solution 
obtained  by  dissolving  400  gms.of  MnCl2+4H20  in  water  and  dilut- 
ing to  1000  c.c.  The  manganese  chloride  must  be  free  from  iron. 

2.  Sodium  Hydroxide  Solution  Containing  Potassium  Iodide. — On 
account  of  the  nitrite  usually  present  in  commercial  sodium  hy- 
droxide, the  alkali  solution  is  prepared  from  sodium  carbonate  and 
calcium  hydroxide.     The  clear  liquid  is  siphoned  off  and  con- 
centrated in  a  silver  dish  until  its  specific  gravity  is  1.35.    In  100  c.c. 
of  this  solution,  10  gms.  of  potassium  iodide  are  dissolved. 

A  portion  of  the  alkaline  potassium  iodide  solution  on  being 
acidified  with  hydrochloric  acid  should  not  immediately  turn 
starch  paste  blue,  and,  furthermore,  large  amounts  of  carbonate 
must  not  be  present. 

N 

3.  —  Sodium  Thiosulphate  Solution. 

Procedure. — A  glass-stoppered  flask  of  about  250-c.c.  capacity 
is  taken  and  its  exact  capacity  is  determined  by  weighing  it  first 
empty  and  then  filled  with  water  at  17.5°  0.  If  the  water  to  be 
analyzed  is  saturated  with  air,  it  is  simply  poured  into  the  flask, 
otherwise  the  water  is  conducted  through  it  for  ten  minutes. 
Then,  by  means  of  a  pipette  reaching  to  the  bottom  of  the  flask, 
1  c.c.  of  the  alkaline  potassium  iodide  solution  is  introduced  and 
immediately  afterwards  1  c.c.  of  the  manganese  chloride  solution. 
The  flask  is  closed,  shaken,  and  allowed  to  stand  until  the  precip- 
itate has  settled.  Then,  by  means  of  the  long-stemmed  pipette, 
about  3  c.c.  of  concentrated  hydrochloric  acid  are  introduced  and 
the  contents  of  the  flask  once  more  shaken.  The  precipitate  dis- 
solves readily  with  liberation  of  iodide  and  the  latter  is  titrated 
with  sodium  thiosulphate  in  the  usual  way. 

Remark. — The  results  obtained  by  this  method  agree  closely 
with  those  obtained  by  boiling  the  water  as  described  on  p.  739. 


762  GAS   ANALYSIS. 

Carbon  Monoxide,  CO.     Mol.  Wt.  28. 

Density  =0.96702    (Air  =  l).     Weight  of   1  liter  1.2502  gms. 
Molar  Volume  =  22.397  liters.     Critical  temperature  =-  136°  C. 

Preparation.  —  Some  concentrated  sulphuric  acid  is  heated  in  a 
fractionating  flask  to  a  temperature  of  140°  to  160°  C.  upon  an  oil 
bath,  and  formic  acid  (sp.  gr.  1.2)  is  allowed  to  drop  into  it: 

HCOOH  = 


In  order  to  free  the  escaping  gas  from  water  and  acid  vapors,  it  is 
conducted  first  through  a  Liebig  condenser,  which  leads  to  an 
empty  flask  -to  receive  the  condensed  water,  and  from  thence  into 
a  concentrated  caustic  potash  solution. 

This  method  *  yields  about  60  liters  of  carbon  monoxide  in 
half  an  hour,  using  about  500  c.c.  of  concentrated  sulphuric  acid. 
The  method  of  Wade  and  P  anting,  f  according  to  which  very  pure 
carbon  monoxide  can  be  prepared  by  allowing  concentrated 
sulphuric  'acid  to  drop  upon  potassium  cyanide,  is  not,  according 
to  Aimer,  a  suitable  process  for  preparing  large  quantities  of  the 
gas;  because  considerable  potassium  cyanide  becomes  enveloped 
in  pyrosulphuric  acid  during  the  reaction,  so  that  there  is  con- 
siderable danger  involved  in  working  with  the  residues. 

By  the  action  of  hot  concentrated  sulphuric  acid  upon  oxalic 
acid,  it  is  very  easy  to  prepare  a  mixture  of  equal  volumes  carbon 
monoxide  and  carbon  dioxide;  on  account  of  the  large  amount 
of  the  latter,  however,  this  method  is  less  satisfactory  than  that 
of  Aimer. 

The  gas  is  only  very  slightly  soluble  in  water; 


Temperature.                              0                 Temperature.  /? 

0° 0 .03537  30° 0 .01998 

5° 0 .03149  35°. .. 0 .01877 

10° 0 .02816  40° 0 .01775 

15° 0.02543  45° 0.01690 

20° 0.02319  50° 0.01615 

25° 0.02142  55° 0.01548 

*  W.  Allner,  Inorg.  Dissert.  Karlsruhe,  1905. 

t  J.  Chem.  Soc.,  73,  255. 

J  L.  W.  Winkler,  Berichte,  34,  1414  (1901). 


CARBON  MONOXIDE.  763 

In  alcohol  the  gas  is  about  ten  times  more  soluble  than  it 
is  in  water. 

Its  determination  is  effected  either  by  absorption  or  by  com- 
bustion. 

Absorbents. — Ammoniacal  Cuprous  Chloride.  200  gms.  of  com- 
mercial cuprous  chloride  are  shaken  in  a  closed  flask  with  a  solu- 
tion of  ammonium  chloride  (250  gms.  in  750  c.c.  water),  and  to 
every  three  volumes  of  this  mixture  1  vol.  of  ammonia,  specific 
gravity  0.91,  is  added.  In  order  that  the  solution  may  remain 
active,  a  spiral  of  copper  wire  is  introduced  into  the  flask  long 
enough  to  reach  from  the  bottom  up  to  the  stopper. 

1  c.c.  of  this  solution  will  absorb  16  c.c.  of  carbon  monoxide. 

Formerly  it  was  the  almost  universal  custom  to  absorb  this 
gas  by  means  of  a  hydrochloric  acid  solution  of  cuprous  chloride, 
but  to-day  this  is  not  done  on  account  of  the  following  reasons. 
The  absorption  of  carbon  monoxide  by  means  of  cuprous  chloride 
takes  place  according  to  the. following  equation: 

Cu2Cl2  +  2CO^Cu2Cl2.2CO.* 

The  compound  Cu2Cl2.2CO  is  extremely  unstable  and  can  only 
be  formed  when  there  is  a  certain  pressure  exerted  by  the  carbon 
monoxide,  so  that  when  the  acid  solution  is  used  the  absorption  will 
never  be  quantitative.  Further,  if  a  gas  free  from  carbon  monoxide 
(nitrogen  or  hydrogen)  is  shaken  with  such  a  solution  after  it  has 
been  used  several  times,  a  part  of  the  Cu2Cl2.2CO  in  solution  will 
be  decomposed  according  to  the  above  equation  in  the  direction 
of  right  to  left,  until  the  partial  pressure  of  the  carbon  monoxide 
set  free  is  sufficient  to  restore  equilibrium.  Consequently  the 
volume  of  the  gas  will  appear  greater  after  it  has  been  treated 
with  the  cuprous  chloride  solution  than  it  was  originally. 

When  an  ammoniacal  cuprous  chloride  solution  is  employed, 
the  absorption  of  the  carbon  monoxide  is  almost  quantitative,  but 
after  such  a  solution  has  been  used  repeatedly  it  will  readily  give 
up  some  of  the  gas,  although  not  so  readily  as  is  the  case  of  the 
solution  of  cuprous  chloride  in  hydrochloric  acid  or  calcium  chlo- 

*  The  compound  has  been  isolated  in  the  solid  state,  according  to  W.  A. 
Jones  (Am.  Chem.  J.,  22,  287)  its  formula  is  CuCl2-2CO-4H2O,  but  according 
to  the  experiments  of  C.  v.  Girsewald  in  the  author's  laboratory,  the  formula 
isCu2C!2.2CO.2H2O. 


7^4  GAS  ANALYSIS. 

ride.*  It  is  advisable,  therefore,  to  adopt  the  suggestion  of  Dreh- 
schmidt,  and  first  absorb  the  greater  part  of  the  gas  by  means  of 
an  old  solution  of  cuprous  chloride,  afterwards  removing  the  last 
traces  by  means  of  a  freshly-prepared  solution,  or  one  which  has 
been  used  but  a  few  times. 

Besides  carbon  monoxide,  the  ammoniacal  cuprous  chloride 
solution  will  absorb  acetylene,  ethylene,  etc.,  so  that  these  gases 
must  be  removed  previously  by  means  of  fuming  sulphuric  acid  or 
bromine  water. 

By  long  shaking  with  concentrated  nitric  acid  (specific  gravity 
1.5),  carbon  monoxide  is  completely  oxidized  to  carbon  dioxide, 
and  the  latter  can  be  removed  by  treatment  with  potassium 
hydroxide  solution.! 

Determination  of  Carbon  Monoxide  by  Combustion  with 
Air  or  Oxygen. 

The  following  reaction  shows  how  carbon  monoxide  may  be 
determined  by  combustion: 

CO  +  O   =  C02. 

2  vols.       1  vol.        2  vols. 

From  the  reaction  we  can  make  the  following  deductions: 

1.  The  difference  in  the  volume  of  the  gas  mixture  before  and 
after  the  combustion  is  for  2  vols.  CO;   3  —  2=1  and  for  1  vol. 
CO  =  J.     This   difference  is  designated  as  the   contraction.     The 
contraction  caused  by  the  combustion  of  carbon  monoxide  is,  there- 
fore, equal  to  half  the  original  volume  of  CO. 

2.  The  volume  of  the  carbon  dioxide  formed  is  equal  to  the  volume 
of  the  carbon  monoxide  originally  present.     If,   then,   the   carbon 
dioxide   is   determined   by  absorption    with  caustic  potash,  the 
volume  of  the  carbon  monoxide  is  at  once  obtained,  provided  no 
other  combustible  gas  containing  carbon  is  present  at  the  same 
time. 


*  Cuprous  chloride  is  soluble  in  a  concentrated  solution  of  calcium  chlo- 
ride.    1  c.c.  of  this  solution  absorbs  12  to  15  c.c.  of  CO. 
t  Treadwell  and  Stokes,  Berichte,  21,  p.  3131. 


CARBON  MONOXIDE.  765 

3.  For  the  corr.bustion  of  2  vols.  of  CO,  1  vol.  of  oxygen  is  neces- 
sary, and  consequently  the  amount  of  oxygen  consumed  is  equal  to 
half  the  volume  of  the  carbon  monoxide. 


Methods  of  Effecting  the  Combustion. 

The  combustion  of  the  carbon  monoxide  can  be  carried  out  in 
several  different  ways: 

1.  By  explosion. 

2.  By  conducting  the  gas  over  glowing  palladium  or  platinum. 

3.  By  conducting  the  gas  over  copper  oxide. 

1.  Combustion  by  Explosion.  —  The  gas  is  mixed  with  a 
sufficient  amount  of  air  in  a  measuring  vessel,  such  as  is 
shown  in  Fig.  105,  and  the  latter  is  connected  by  means  of 
the  capillary  E  with  the  Hempel's  explosion  pipette  -shown 
in  Fig.  110.  The  gas  is  completely  driven  over  into  the  latter 

so  that  the  capillary  is  entirely 
filled  with  mercury,  the  stop-cocks 
of  the  capillary  and  of  the  explo- 
sion pipette  are  both  closed,  and 
an  electric  spark  is  made  to  pass  be- 
tween the  two  platinum  points  which 
are  fused  into  the  glass  walls  of  the 
pipette;  this  immediately  causes 
an  explosion  to  take  place.  After- 
wards the  gas  is  once  more  driven 
FIG.  110.  ,  .  . 

back   into    the   measunng   burette, 

and    its   volume   again   determined.     The    difference   in   volume 
before  and  after  the  explosion  represents  the  contraction. 

This  most  excellent  method  can  in  some  cases  lead  to  erroneous 
results.  In  practice,  it  is  almost  always  a  question  of  determining 
the  amount  of  combustible  gas  in  a  mixture  containing  nitrogen 
obtained  after  treatment  with  the  different  absorbents.  If  the 
amount  of  combustible  gas  present  is  too  small  in  proportion  to  the 
amount  of  non-combustible  gas,  there  will  be  no  combustion  what= 
soever;  while  on  the  other  hand,  if  this  relation  is  too  large,  a  part 
of  the  nitrogen  will  be  burnt  to  nitric  acid  (hydrogen  is  usually 


76:)  GAS  ANALYSIS. 

present).  According  to  Bunsen,  the  combustion  is  complete  when 
30  parts  of  combustible  gas  are  present  for  each  100  parts  of  non- 
combustible  gas.  Consequently,  if  the  explosion  method  is  to  be 
used  for  the  analysis,  the  approximate  composition  of  the  gas  must 
be  known. 

2.  Combustion  by  Conducting  the  Gas  over  Glowing  Palladium. — 
This  is  the  most  certain  of  all  methods  for  effecting  the  com- 
bustion, because  it  is  entirely  independent  of  the  proportion 
of  combustible  gas  present,*  and  there  is  no  danger  of  any  of  the 
nitrogen  being  oxidized.  The  combustion  is  best  effected,  as 
proposed  by  Drehschmidt,  by  passing  the  gas  through  a  thick- 
walled  platinum  capillary  tube  containing  three  palladium  wires. 
The  platinum  capillary  (Fig.  105,  V)  is  placed  between  the  gas 
burette  and  the  Drehschmidt  pipette  S  (Fig.  105),  and  it  is  heated 
by  means  of  the  non-luminous  flame  of  a  Teclu  burner.  The 
gas  is  repeatedly  passed  through  the  glowing  capillary  until  there 
is  no  further  diminution  in  volume,  showing  the  combustion  to 
be  complete.  There  is  no  danger  to  be  feared  from  explosions 
even  when  pure  detonating  gas  is  passed  through  the  platinum 
tube,  and  by  this  method  CO,  H,  and  CH4  are  completely  oxidized. 
In  the  analysis  of  gases  containing  only  small  amounts  of  the 
above  gases  (e.g.  exhaust  gases  from  gas-motors)  the  so-called 
fractional  combustion  is  employed.  By  this  means  either  hydrogen 
and  carbon  monoxide  are  oxidized  while  methane  is  not,  or  car- 
bon monoxide  is  alone  burned. 

Fractional  Combustion.  —  If,  according  to  Haber,f  an  abso- 
lutely dry  gas  mixture,  consisting  of  considerable  nitrogen 
and  oxygen  with  little  carbon  monoxide,  hydrogen,  and 
methane,  is  slowly  conducted  (at  the  rate  of  about  700- 
800  c.c.  per  hour)  through  a  glass  U  tube  3  mm.  in  diam- 
eter which  contains  a  palladium  wire  55  cm.  long,  folded  into 
three  lengths  of  about  18  cm.,  then  if  the  temperature  of  boiling 
sulphur  is  maintained,  the  hydrogen  and  carbon  monoxide  will 
be  completely  burned,  while  methane  will  escape  from  the  tube 


*  It  is  only  necessary  to  make  sure  that  a  large  excess  of  oxygen  is  pres- 
ent (cf.  Hempel,  Zeitschr.  f.  anorg.  Chem.,  XXXI  (1902),  p.  447. 
f  Loc.  cit. 


CARBON  MONOXIDE.  767 

in  an  unchanged  condition.  By  connecting  the  U  tube  with  a 
weighed  calcium  chloride  tube  and  then  with  two  weighed  soda- 
lime  tubes  (see  p.  380)  the  increase  in  the  weight  of  the  former 
will  show  the  amount  of  water  formed  from  the  hydrogen,  and 
the  gain  in  weight  shown  by  the  soda-lime  tubes  corresponds  to 
the  amount  of  carbon  dioxide  formed  from  the  carbon  monoxide. 
If,  after  passing  through  the  soda-lime  tubes,  the  gas  is  passed 
through  a  combustion-tube  filled  with  platinized  asbestos,  or 
copper  oxide,  which  is  heated  to  a  dark-red  heat,  the  methane 
is  quantitatively  burned  to  water  and  carbon  dioxide;  the  former 
is  absorbed  in  a  calcium  chloride  tube  and  the  latter  in  two  soda- 
lime  tubes,  all  three  tubes  being  weighed  before  the  gas  is  passed 
through  them.  In  this  way  a  check  is  obtained  upon  the  accuracy 
of  the  determination,  for  the  proportion  of  carbon  to  hydrogen 
found  should  be  1:4. 

The  combustion  of  carbon  monoxide  alone  from  a  mixture 
of  this  gas  with  hydrogen,  methane,  and  air  can  be  effected  satis- 
factorily as  follows: 

After  the  gas  has  been  freed  from  CO2,  unsaturated  hydrocarbons, 
and  aqueous  vapor,  it  is  conducted  through  a  U  tube  *  containing 
60-70  gms.  of  pure  iodine  pentoxide  f  heated  to  160°  C.  ;  by  this 
means  the  carbon  monoxide  is  alone  oxidized  with  liberation  of 
iodine  according  to  the  equation 


If  the  gas  is  now  conducted  through  two  Peligot  tubes  containing 
potassium  iodide  solution,  the  iodine  will  be  absorbed   and   can 

N 
be  titrated  at  the  end  of  the  experiment  with  —  sodium  thiosulphate 

solution. 

N 
1  c.c.  —  Na^Og  solution  corresponds  to  5.6  c.c.  CO,  measured 

under  standard  conditions. 

*  The  U  tube  is  heated  in  a  small  paraffin  bath. 

t  Iodine  pentoxide  is  prepared  by  heating  iodic  acid  in  a  current  of  dry 
air  at  180°  until  the  water  is  completely  removed. 

J  Nicloux,  Compt.  rend.,  126,  p.  746,  and  Kinnicutt,  Journ.  of  the  Am. 
Chem.  Soc.,  XXII,  p.  14. 


768  GAS   ANA'  YS1S. 

If,  after  the  carbon  dioxide  and  water  have  once  more  been 
removed  from  the  gas,  it  is  passed  through  a  combustion-tube 
half  filled  with  copper  oxide  and  half  with  platinized  asbestos, 
both  heated  to  dark  redness,  the  hydrogen  and  methane  will 
be  completely  burned  to  water  and  carbon  dioxide,  which 
can  be  absorbed  and  weighed  as  before.  From  the  amounts  of 
each,  the  hydrogen  and  methane  present  in  the  gas  can  be  calcu- 
lated. 


Qualitative  Detection  of  Traces  of  Carbon  Monoxide  in  the  Air. 

If  blood  be  diluted  with  water  until  the  solution  shows  only  a 
slight  red  color,  it  will  give  a  characteristic  absorption  spectrum; 
two  dark  absorption  bands  appear  between  the  D  and  E  lines. 
If  to  this  dilute  blood  solution  a  few  drops  of  a  concentrated^ 
freshly-prepared  ammonium  sulphide  are  added,  the  dark  bands 
disappear,  and  instead  a  single  broad  band  will  appear  at  a  place 
between  the  positions  of  the  previous  bands.  Blood  containing 
carbon  monoxide  behaves  quite  differently.  When  the  latter  gas  is 
present,  the  blood  takes  on  a  rose  color  and  the  solution  gives 
almost  the  same  absorption  spectrum  as  pure  blood  (the  bands 
shift  slightly  toward  the  violet)  but  in  this  case  the  two  bands 
do  not  disappear  on  the  addition  of  ammonium  sulphide. 

To  detect  traces  of  carbon  monoxide  in  the  air,  Vogel  directs 
that  a  100-c.c.  bottle,  filled  with  water,  be  emptied  in  the  room 
containing  the  gas,  and  that  2  to  3  c.c.  of  blood,  highly  diluted 
with  water,  and  showing  only  a  very  faint  red  color  (although 
still  giving  the  blood  spectrum  in  a  column  as  thick  as  a  test-tube) 
be  poured  into  the  bottle  and  shaken  for  some  minutes.  To  the 
solution  a  few  drops  of  ammonium  sulphide  solution  are  added 
and  the  liquid  is  examined  by  means  of  the  spectroscope.  If 
the  two  bands  are  now  visible,  carbon  monoxide  is  present.  Ac- 
cording to  Vogel  as  little  as  0.25  per  cent,  of  CO  can  be  detected 
in  this  way. 

Hempel  has  improved  this  method  to  a  marked  degree.  He 
found  that  it  was  not  possible  to  completely  remove  small 
amounts  of  carbon  monoxide  by  shaking  with  the  dilute  solution 


CARBON  MONOXIDE.  769- 

of  blood,  and  furthermore  concentrated  blood  'solutions  could  not 
be  used  because  they  foam  so  much.  By  using  a  living  animal,  its 
lungs  furnish  a  better  means  of  absorption,  for  the  gas  then 
comes  in  contact  with  undiluted  blood.  A  mouse  is  placed 
between  two  funnels  which  are  joined  together  by  means  of  a 
broad  band  of  thin  rubber  and  the  ga>  to  be  tested  is  passed 
through  the  funnels  at  a  speed  of  ten  liters  per  hour.  At  the 
end  of  two  or  three  hours  the  mouse  is  killed  by  immersing 
the  funnels  in  water  and  a  few  drops  of  its  blood  are  taken  from 
the  region  near  the  heart.  In  this  way  Hempel  was  able  to  detect 
with  certainty  as  little  as  0.032  per  cent.  CO.  With  such  small 
amounts  of  CO  the  live  mouse  showed  no  symptoms  of  poisoning; ; 
this  was  first  apparent  when  0.06  per  cent,  of  the  gas  was 
present.  In  the  latter  case  after  half  an  hour  the  mouse  breathed 
with  difficulty  and  lay  exhausted  on  its  side. 

Potain  and  Drouin  detect  small  amounts  of  carbon  monoxide 
by  passing  the  gas  through  a  dilute  solution  of  palladous  chloride, 
when  metallic  palladium  is  precipitated: 


20=2HCl+C02+P<L 


The  solution  is  decolorized,  or  turns  a  pale  gray,  when  large 
amounts  of  CO  are  present,  but  appears  a  light  yellow  in  color 
when  only  traces  are  present. 

In  order  to  estimate  better  the  decrease  in  color,  Potain  and 
Drouin  filter  off  the  deposited  palladium  and  compare  the  color 
of  the  filtrate. 

For  the  detection  of  small  amounts  of  carbon  monoxide,  C. 
Winkler  recommends  a  method  which,  as  the  author  has  found, 
will  often  lead  to  error.  According  to  Winkler,  the  gas  to  be  tested 
is  conducted  through  a  solution  of  cuprous  chloride  in  a  saturated 
solution  of  sodium  chloride,  afterwards  diluting  with  four  to  five 
tunes  as  much  water,  causing  the  precipitation  of  snow-white 
cuprous  chloride.  If  this  turbid  solution  is  treated  with  a  drop 
of  sodium  palladous  chloride,  a  black  precipitate  of  metallic 
palladium  is  obtained.  Unfortunately,  however,  the  palladium 
is  often  precipitated  even  in  the  absence  of  a  trace  of  carbon 


yyo  GAS  ANALYSIS. 

monoxide;  for  cuprous  chloride  itself  will  readily  reduce  salts  of 
palladium. 

It  is  true,  on  the  other  hand,  that  at  a  definite  concentration 
the  reduction  of  the  palladous  chloride  is  only  effected  by  means 
of  carbon  monoxide,  but  it  is  difficult  to  always  obtain  the  right 
conditions,  and  herein  lies  the  inaccuracy  of  the  method.  If  the 
solution  be  too  concentrated  with  respect  to  sodium  chloride,  even 
large  amounts  of  carbon  monoxide  will  fail  to  precipitate  a  trace 
of  palladium,  because  in  that  case  the  solution  contains  not 
only  copper  but  also  palladium  in  the  form  of  complex  sodium 
salts : 

[Cu2ClJNa2  and  [PdClJNa,. 

The  sodium  palladous  chloride  is  not  reduced  by  carbon  mon- 
oxide and  there  is  even  less  likelihood  of  the  two  sodium  salts  acting 
upon  one  another.  If  the  solution  be  diluted  with  water,  both 
salts  break  down  according  to  the  equations 

[Cu2ClJNa2  <=>  2NaCl+Cu2Cl,, 
pdC!JNa24=±  2NaCl+ PdCl2, 

and  only  when  the  palladium  is  in  the  ionic  condition  is  it  capable 
of  entering  into  the  reaction.  The  fact  that  the  reduction  of  the 
palladous  chloride  is  effected  by  means  of  CO  at  a  concentration 
at  which  Cu2Cl2  is  incapable  of  causing  any  reduction  is  easy  to 
understand,  for  the  gas,  CO,  comes  in  contact  more  readily  with  a 
sufficient  number  of  palladium  ions  than  does  the  difficultly  soluble 
cuprous  chloride. 

Hydrogen,  H.     Mol.  Wt.  2.016. 

Density  =0.06960  *  (Air  =  1).     Weight  of  1  liter  =0.089978  gm. 
Molar  volume  =22.405  1.         Critical  temperature  =  —238°  C. 

Hydrogen  is  practically  insoluble  in  water. 

*  Lord  Rayleigh,  Proc.  Roy.  Soc.,  53,  1134  (1893). 


HYDROGEN.  771 

ABSORPTION  COEFFICIENTS  OF  HYDROGEN  FOR  WATER.* 

Temperature.                            £  Temperature.                          y9 

0°  ..........  /.  0.02148  30°  ............  0.01699 

5°  ............  0.02044  35°  ............  0.01666 

10°  ............  0  .01955  40°  ............  0  .01644 

15°  ............  0.01883  45°  .........  0.01624 

20°  ............  0.01819  50°  ............  0.01608 

25°  ............  0  .01754  55°  ............  0  .01604 

The  usual  way  for  determining  this  gas  by  absorption  is  by 
means  of  metallic  palladium,  f  but  in  the  majority  of  cases  it  is 
determined  by  combustion  with  oxygen  and  observing  the  con- 

traction: 

H2     +     O     =     H2O 

2  vols.  1  vol.  0  vol. 

It  is  evident  that  by  the  combustion  of  two  volumes  of  hydro- 
gen, three  volumes  of  gas  will  disappear  (the  water  formed  occupies 
a  negligible  volume).  The  contraction,  therefore,  is  equal  to  f  the 
volume  of  the  hydrogen  consumed.  If  the  contraction  is  denoted 
by  Vc  and  the  volume  of  the  hydrogen  by  Vs,  then 


and  consequently 

VH^VC. 

In  many  cases  the  weight  of  the  water  formed  is  determined 
by  absorbing  the  latter  in  weighed  calcium  chloride  tubes,  and 
from  the  gain  in  weight  the  volume  of  hydrogen  is  computed  as 

follows: 

18.02:  22,405  =  p:z, 


x  =  —  L7Xp  =  1243.  6  X/>  c.c.  hydrogen  under  standard  conditions. 

*  L.  W.  Winkler,  Berichte,  24,  99  (1891). 

f  The  absorption  can  also  be  accomplished  very  satisfactorily  by  means 
of  a  one  per  cent,  solution  of  palladous  chloride.  Campbell  and  Hart,  Am. 
Chem.  J.,  18,  294.—  [Translator.] 


772  GAS  ANALYSIS. 

Combustion  of  Hydrogen,  according  to  Cl.  Winkler. 

The  following  method  is  employed  frequently  in  technical 
analyses  for  the  separation  of  hydrogen  from  methane. 

A  mixture  of  hydrogen  and  air  is  conducted  over  gently-ignited 
palladium-asbestos,  by  which  means  the  hydrogen  is  quantita- 
tively burned  to  water  and  the  methane  is  not  affected.  Fig.  Ill 


A 


FIG.  111. 

represents  the  apparatus  required.  A  is  the  eudiometer  and  is 
connected  by  means  of  the  capillary  E,  in  which  is  found  a  short 
fibre  of  palladium-asbestos,  with  a  Hempel  pipette  filled  with  water. 
The  capillary,  E,  is  heated  by  means  of  the  small  flame  F,  at 
the  place  where  the  palladium-asbestos  rests,  to  a  temperature  of 
about  300  to  400°,  but  not  hot  enough  to  soften  the  glass.  After 
the  gas,  which  is  mixed  with  air,*  has  been  passed  back  and  forth 


*  If  oxygen  is  used  instead  of  air,  some  of  the  methane  is  sure  to  be 
oxidized.     Cf.  O.  Brunck,  Zeit.  f.  angew.  Chem.,  1903,  p.  195. 


HYDROGEN.  773 

through  the  capillary  three  times,  the  combustion  is  complete  If 
the  above-specified  temperature  is  not  exceeded,  no  trace  of  meta- 
ane  will  be  burned  and  the  hydrogen  determination  will  be  accu. 
rate.  It  is,  however,  difficult  to  regulate  this  temperature  closely 
enough  to  prevent  the  combustion  of  some  methane  unless,  as 
recommended  by  Haber,  the  tube  is  heated  by  means  of  sulphur 
vapor;  the  results  are  usually  from  0.5  to  1  per  cent,  too  high, 

Preparation  of  Palladium-asbestos. — Three  gms.  of  sodium 
palladous  chloride  are  dissolved  in  as  little  water  as  possible, 
3  c.c.  of  a  cold  saturated  solution  of  sodium  formate  are  added 
and  enough  sodium  carbonate  solution  to  make  the  solution 
alkaline.  Then  about  1  gm.  of  soft,  long-fibred  asbestos  is 
added,  which  sucks  up  the  whole  of  the  liquid,  and  the 
mixture  is  dried  on  the  water  bath;  by  this  means  finely- 
divided  palladium  is  deposited  uniformly  through  the  asbestos: 

Na,PdCl4 + HCOONa  =  SXaCl  4-  HCi  +  CO,  +  Pd. 

The  hydrochloric  acid  formed  by  the  above  reaction  is  neutral- 
ized by  the  sodium  carbonate.  In  acid  solutions  formic  acid 
hardly  reduced  palladous  chloride  at  all. 

After  the  asbestos  has  thoroughly  dried,  the  mass  is  softened 
with  hot  water,  placed  in  a  funnel  and  washed  with  hot  water 
until  the  soluble  salt  is  completely  removed.  It  is  then  dried 
once  more  and  preserved  in  a  well-stoppered  bottle. 

The  palladium-asbestos  fibre  is  introduced  into  the  capillary 
tube  as  follows:  The  fibre  is  rolled  between  the  fingers  to  a  little 
round  wad,  the  latter  is  placed  in  the  opening  of  the  unbent  capil- 
lary tubing  and  by  gentle  tapping  upon  the  table  it  is  made  to  pass 
along  to  the  centre  of  the  tube.  The  latter  is  then  bent  as  shown 
in  the  figure. 

Remark. — Inasmuch  as  the  palladium-asbestos  is  likely  to 
become  shoved  into  the  capillary,  it  is  perhaps  more  satisfactory 
to  use  instead  a  palladium  wire  which  is  wound  into  a  spiral.* 

*  Private  communication  from  Dr.  Leutold  of  Hamburg. 


774  GAS   ANALYSIS. 

Methane,  CH4.     Mol.  Wt.  16.03. 

Density  =0.55297.     (Air  =  1.)     Weight  of  1  liter  =0.71488  gms. 
Molar  volume  =22.43  1.     Critical  temperature  =  —  82°  C. 
Preparation.  —  Methane  is  conveniently  prepared  by  a  process 
analogous  to  that  used  in  making  ethylene  *   (cf.  p.  751).    A 
mixture  of  equal  parts  methyl  iodide  and  alcohol  (sp.  gr.  0.805)  is 
allowed  to  act  upon  a  zinc-copper  couple  which  has  been  washed 
with  alcohol. 

2CH3I  +  2Zn  +  2HOH  =  ZnI2  +  Zn  (OH)  2  +  2CH4. 

\ 

The  zinc-copper  couple  is  obtained  by  pouring  a  2  per  cent. 
copper  sulphate  solution  four  times  over  granulated  zinc,  then 
washing  with  water,  and  finally  with  alcohol. 

By  allowing  the  mixture  of  methyl  iodide  and  alcohol  to  drop 
upon  the  copper-coated  zinc,  a  steady  stream  of  methane  is 
obtained  at  the  ordinary  temperature.  The  gas  is  purified  by 
shaking  it  with  fuming  sulphuric  acid,  and  then  with  caustic 
potash  solution.  It  then  contains  nearly  99  per  cent  of  CH4  and 
about  1  per  cent,  of  nitrogen. 

Methane,  also  called  marsh-gas  or  fire  damp,  is  only  slightly 
soluble  in  water. 

ABSORPTION  COEFFICIENTS  OF  METHANE  FOR  WATER.f 

Temperature.                            0  Temperature.  rt0£co 

0°               ____  0.05563  30°  ............  0.02762 

5°                      .  0  .04805  35°  ............  0  .02546 

10°'  '                   .  0.04177  40°  ............  0.02369 

15°"                   .  0.03690  45°  ............  0.02238 

20°                      .  0.03308  50°  ............  0.02134 

25°  ............  0.03006  55°  ............  0.02038 

In  alcohol,  the  gas  is  about  ten  times  as  soluble  as  it  is  in  water. 
Inasmuch  as  no  satisfactory  absorbent  for  methane  is  known, 
it  is  always  determined  by  combustion. 

From  the  equation  representing  the  combustion, 


CO2+2H20, 

2  vols.  +  4  vols.      2  vols.     0  vol. 

we  can  make  the  following  deductions: 

1.  Contraction.  —  The    contraction    caused   by   the   combustion 
of  methane  is  equal  to  twice  its  original  volume. 

2.  Carbon  Dioxide.  —  By  the  combustion  of  methane  an  equal 
volume  of  carbon  dioxide  is  produced. 

3.  Oxygen  Consumed.  —  For  the  combustion  of  one  volume  of 
methane  two  volumes  of  oxygen  are  necessary. 

*  Gladstone  and  Tribe,  J.  Chem.  Soc.,  45,  154. 
t  L.  W.  Winkler,  Berichte,  34,  1419  (1901). 


ILLUMINATING  AND  PRODUCER  GASES.  775 


ANALYSIS  OF  ILLUMINATING  AND  PRODUCER  GASES. 

The  analysis  of  all  such  gases  is  best  performed  either  by  the 
method  of  Hempel  *  or  that  of  Drehschmidt.f 


Hempel's  Method. 

Hempel's  apparatus  is  shown  in  Fig.  105, p. 743.  It  consists  of  a 
eudiometer,  W,  divided  into  \  c.c.  and  connected  by  means 
of  rubber  tubing  with  the  levelling-bulb  K.  The  eudiometer  is 
also  connected  with  the  compensation-tube  D  and  the  latter  is 
connected  with  a  manometer  C;  both  the  tubes  W  and  D  are  sur- 
rounded by  a  cylinder  containing  water. 

Calibration  of  the  Apparatus. — First  of  all  the  manometer-tube  is 
filled  with  mercury  by  raising  the  levelling-bulb  K  with  the  stop- 
cock p  in  the  position  shown  in  Fig.  105,  so  that  there  is  an  open 
connection  between  W  and  c ;  the  mercury  is  allowed  to  pass  over 
into  C  until  the  mark  mm  is  reached.  The  volume  of  the  manom- 
eter-tube from  the  mark  m  to  the  point  a  (Fig.  105)  is  now  deter- 
mined as  follows: 

By  carefully  lowering  the  bulb  K  the  mercury  is  drawn  over 
into  C  exactly  to  the  point  a  when  the  stop-cock  p  is  closed.  A 
little  air  is  allowed  to  enter  into  the  eudiometer  through  the  right- 
hand  capillary  tube  above  p  (the  tube  E  should  be  withdrawn  as 
in  Fig.  112),  the  levelling-bulb  displaced  upon  a  solid  support  at 
about  the  same  height  as  the  mercury  in  W,  and  with  the  stop- 
cock p  still  open  the  position  of  the  mercury  in  W  is  read.  The 
stop-cock  is  closed,  K  is  raised  a  little  and  p  is  turned  to  the 
position  shown  in  Fig.  105.  By  raising  K  still  higher,  the  air 
is  driven  over  into  the  manometer-tube  C  until  the  mercury  has 
exactly  reached  the  mark  m,  when  the  stop-cock  A  (Fig.  105)  is 
closed.  The  exact  position  of  the  mercury  is  then  adjusted  by 
turning  the  stop-cock  p  one  way  or  the  other,  and  the  position 
of  the  mercury  in  W  is  once  more  read.  The  difference  between  the 

*  Gasanalytische  Methoden  (1900),  p.  48  ff. 
f  Berichte,  21,  p.  3242  (1888). 


776 


GAS  ANALYSIS. 


two  readings  represents  the  volume  of  the  tube  between  the  marka 
m  and  a,  an  amount  which  must  be  added  to  all  subsequent 
readings. 

A  drop  of  water  is  now  introduced  at  ct  by  means  of  a  fina 


FIG.  112. 


pipette,  into  the  compensation-tube  D  and  the  end  of  the  tube  c 
is  either  fused  together,  or  closed  with  a  cork  stopper  and  made 
air-tight  with  sealing  wax. 

Procedure  for  the  Analysis. — If  the  analysis  is  to  be  carried 
out  on  the  spot,  a  large  sample  of  the  gas  is  collected  in  a  Dreh- 
schmidt  pipette  (Fig.  105  S).  To  accomplish  this  the  capillary 
tube  Ef  is  connected  by  means  cf  rubber  tubing  with  the  source 


ILLUMINATING  AND  PRODUCER   GASES.  777 

of  the  gas,  and  the  stop-cock  M  is  turned  so  that  the  tube 
Ef  is  in  connection  with  the  bulb  of  the  pipette,  the  levelling-bulb 
being  in  a  low  position  and  the  stop-cock  s  left  open.  The  pipette 
is  entirely  filled  with  the  gas,  then  the  stop-cock  M  is  turned 
so  that  it  communicates  with  the  outer  air,  and  the  gas  is  com- 
pletely expelled  from  the  pipette.  The  gas  is  in  this  way  drawn 
in  and  out  of  the  pipette  at  least  three  times  in  order  to  make  sure 
that  all  foreign  gas  (air)  is  removed  from  the  rubber  tubing.  The 
sample  of  gas  is  then  taken  and  the  two  stop-cocks  M  and  s  are 
closed. 

In  order  to  bring  the  gas  to  be  tested  from  the  Drehschmidt 
pipette  into  the  eudiometer,  the  two  instruments  are  connected 
by  means  of  the  capillary  Ef  (imagine  the  capillary  E  in  Fig.  105 
to  be  replaced  by  E'}  and  the  rubber  connections  are  firmly 
wired  to  the  glass.  The  stop-cock  M  is  turned  to  the  posi- 
tion shown  in  Fig.  105,  the  levelling-bulb  K  is  raised  (after 
previously  causing  the  mercury  in  the  manometer-tube  to  reach 
to  the  point  a)  and  the  burette  is  entirely  filled  with  mercury 
until  the  latter  begins  to  flow  from  out  of  the  tip  of  the  key  at 
M ,  when  the  cocks  A  and  p  are  closed.  The  cock  M  is  then 
turned  so  that  the  pipette  S  and  the  burette  W  are  in  con- 
nection, K'  is  raised,  s  opened,  K  lowered,  and  both  p  and  A  are 
opened. 

After  about  40  c.c.  of  the  gas  have  passed  over  into  the 
eudiometer,  the  cocks  A  and  M  are  closed,  the  key  of  the  stop- 
cock M  (which  must  be  entirely  filled  with  mercury)  is  dipped 
into  a  beaker  containing  mercury,  and  the  gas  in  the  capillary  is 
sucked  into  W  by  lowering  K  and  opening  A  and  p.  As  soon  as 
the  capillary  E  is  entirely  filled  with  mercury,  A,  p  and  finally  M 
are  closed. 

The  volume  of  the  gas  in  W  is  now  determined  as  follows: 
A  is  opened  and  K  raised  so  that  the  mercury  in  the  bulb  is  a 
little  higher  than  it  is  in  W.  After  this  p  is  opened  and 
the  gas  is  driven  over  towards  C  until  the  mercury  in  both 
arms  of  the  manometer-tube  is  at  about  the  same  height, 
when  A  is  immediately  closed.  The  last  fine  adjustment  of  the 
mercury  levels  within  the  tubes  is  made  by  closing  or  opening 


778  GAS  ANALYSIS 

the  screw-cock  Q;*  the  volume  is  now  read,  and  to  the  reading 
the  correction  corresponding  to  the  volume  between  the  marks 
M  and  a  is  added. 

From  this  point  begins  the  analysis. 

i.  Determination  of  Carbon  Dioxide. 

With  the  stop-cock  p  closed,  the  cock  M  is  turned  as 
shown  in  Fig.  105  the  Drehschmidt  pipette  is  removed  and 
replaced  by  a  second,  clean  pipette  completely  filled  with 
mercury.  On  connecting  the  stop-cock  M  with  the  rubber 
connector  of  the  capillary  E',  it  should  be  in  the  position 
shown  in  the  drawing.  By  this  means  the  mercury  in  the 
rubber  tubing  can  flow  out  through  the  key.  After  wiring 
the  rubber  tightly  to  the  glass,  from  3  to  5  c.c.  of  caustic 
potash  solution  (1:2)  are  introduced  through  the  key  into  the 
pipette  M  and  the  alkali  in  the  capillary  is  washed  out  with 
about  2  c.c.  of  distilled  water  and  then  with  a  little  mercury; 
after  this  the  gas  itself  is  driven  over  into  the  pipette.  When 
the  mercury  has  filled  the  whole  capillary,  both  to  the  right  and 
left  of  M,  then  A,  p,  and  M  are  closed.  The  bulb  Kf  is  raised  so 
that  extra  pressure  is  placed  upon  the  gas  in  the  pipette  and  s  is 
closed.  The  pipette  is  now  gently  shaken  for  three  minutes  with- 
out disconnecting  it  from  the  eudiometer,  after  which  the  gas  is 
returned  to  W  as  follows:  M,  p,  and  A  are  opened,  K  is  lowered,  Kf 
raised,  and  s  opened.  When  almost  all  of  the  gas  has  been  driven 
out  of  the  pipette,  M,  p,  A,  and  Q  are  closed,  the  levelling-bulb 
is  placed  on  the  table  below,  and  Kf  is  placed  upon  the  support 
(missing  from  Fig.  105,  but  shown  in  Fig.  112)  upon  which  the  pi- 
pette itself  rests.  M,  p,  A,  and  s  are  now  opened  and  Q  screwed  up 
a  little  so  that  the  gas  is  very  slowly  sucked  into  the  burette. 
As  soon  as  the  caustic  potash  solution  has  reached  M  the  latter 
is  closed.  The  gas  remaining  in  the  capillary  to  the  left  of 
M  is  now  removed  by  sucking  mercury  through  the  key  of  M 
into  W.  Finally  the  volume  of  the  unabsorbed  gas  is  read  in  the 

*  The  reading  is  best  made  with  the  help  of  a  small  telescope,  the  ocular 
of  which  is  provided  with  cross-hairs.  For  this  purpose  the  telescope  con- 
nected with  a  Bunsen  spectroscope  is  suitable. 


ILLUMINATING   AND  PRODUCER   GASES.  779 

same  way  as  before.     The  difference  between  the  two  readings 
represents  the  amount  of  CO2. 

2.  Determination  of  the  Heavy  Hydrocarbons. 

The  pipette  containing  the  caustic  potash  solution  is  removed 
and  replaced  by  another  containing  fuming  sulphuric  acid.*  The 
gas  is  driven  over  into  the  latter,  shaken  with  the  acid  for  three 
minutes,!  and  the  pipette  emptied  in  precisely  the  same  way  as 
before.  The  gas  is  now  returned  to  the  pipette  containing  the 
caustic  potash  in  order  to  remove  the  acid  vapors,  and  finally  trans- 
ferred to  the  burette  W  and  its  volume  read.  The  difference 
before  and  after  the  treatment  with  fuming  sulphuric  acid  repre- 
sents the  sum  of  the  heavy  hydrocarbons  (C2H4,  C6H6,  C2H2,  etc.). 
It  is  not  usually  customary  to  attempt  to  separate  the  benzene 
from  the  ethylene. 

3.  Determination  of  Oxygen. 

This  part  of  the  analysis  is  carried  out  in  exactly  the  same  way 
as  the  determination  of  the  CO2,  except  that  in  this  case  the  absorp- 
tion pipette  contains  an  alkaline  solution  of  pyrogallol  (cf.  pp. 
758-9). 

4.  Determination  of  Carbon  Monoxide. 

The  determination  of  carbon  monoxide  may  be  effected  either 
by  absorption  with  ammoniacal  cuprous  chloride  or  by  simul- 
taneous combustion  with  hydrogen  and  methane. 

For  the  absorption  method,  the  procedure  is  the  same  as  in 
the  case  of  the  determination  of  the  heavy  hydrocarbons,  i.e.,  the 
absorption  is  effected  in  a  pipette  containing  only  ammoniacal 
cuprous  chloride  (no  mercury).  The  gas  is  shaken  for  three 
minutes  with  a  solution  of  cuprous  chloride  which  has  already 
been  used  frequently,  and  then  the  same  length  of  time  with  a  fresh, 

*  In  this  pipette  the  bulb-tube  K'  is  fused  on  to  the  absorption-bulb, 
so  that  it  is  a  little  higher  than  the  latter,  in  the  same  way  as  in  the  Hempel 
pipette  (Fig.  115).  Mercury  is  acted  upon  by  fuming  sulphuric  acid. 

f  From  the  experience  of  the  Massachusetts  Gas  Inspectors  it  would  seem 
as  if  more  time  were  necessary  for  the  complete  absorption  of  the  heavy 
hydrocarbons — perhaps  thirty  minutes  instead  of  three. — [Translator.] 


78°  GAS  ANALYSIS. 

or  little  used,  solution  (cf.  pp.  763-4).  B3fore  reading  the  volume 
of  the  unabsorbed  gas  it  must  be  freed  from  ammonia  vapors, 
which  is  accomplished  by  shaking  with  hydrochloric  acid  (1*2) 
in  a  Drehschmidt  pipette. 

5.  Determination  of  Hydrogen  and  Methane. 

After  the  removal  of  the  carbon  monoxide,  the  gas  may  con- 
sist of  hydrogen,  methane,  and  nitrogen.  An  excess  of  oxygen 
is  added  to  this  mixture  (with  illuminating-gas  twice  its  volume 
is  added,  while  with  Dowson,  water,  and  producer  gas  only  a 
little  more  than  half  as  much  oxygen  is  necessary).  The  eudiom- 
eter W  is  connected  with  a  Drehschmidt  pipette  entirely  filled 
with  pure  mercury  *  by  means  of  a  Drehschmidt  platinum  capil- 
lary (Fig.  105,  V},  and  the  latter  is  heated  to  bright  redness 
with  the  non-luminous  flame  of  a  Teclu  burner,  taking  care 
that  the  inner  flame  mantle  does  not  come  in  contact  with  the 
platinum.  The  gas  mixture  is  conducted  three  times  in  a  slow 
stream  through  the  hot  platinum  tube,  but  taking  care  that  no 
mercury  enters  the  latter.  The  volume  of  the  unconsumed  gas 
is  then  measured  without  removing  the  platinum  capillary,  and 
the  carbon  dioxide  is  determined  by  introducing  some  caustic 
potash  into  the  pipette  and  then  shaking  the  gas  with  it;  after 
three  minutes'  shaking,  the  unabsorbed  gas  is  returned  to  the  eudi- 
ometer, closing  the  stop-cock  M  as  soon  as  the  caustic  potash 
solution  reaches  it. 

Calculation  of  Hydrogen  and  Methane. 

Assume  V  c.c.  of  gas  to  be  taken  'for  the  analysis.  The 
residue  remaining  after  the  absorption  of  the  CO2,  CnH2n,  O,  and 
CO  was  mixed  with  oxygen  and  burned.  The  contraction  pro- 
duced was  Vc  and  the  CO3  formed  amounted  to  VK. 

We  saw  on  p.  774  that  the  volume  of  the  methane  is  equal 

*  There  must  be  no  trace  of  caustic  potash  in  the  pipette,  because  in 
that  case  CO2  would  be  absorbed  and  an  inaccurate  result  would  be  obtained. 
To  make  sure  that  all  the  alkali  is  removed,  the  pipette  is  washed  first  with 
water,  then  with  hydrochloric  acid,  and  finally  with  water  once  more. 


ILLUMINATING  AND  PRODUCER  GASES.  781 

to  the  volume  of  the  CO2  formed,  VK,  and  in  per  cent.  ; 


x=  -r?100  =  per  cent.  CH4. 

Since  by  the  combustion  of  one  volume  of  CH4  two  volumes 
of  gas  disappear,  it  is  evident  that  by  the  combustion  of  VK  c.c. 
of  CH4  the  contraction  will  amount  to  2V  K. 

If  the  latter  value  be  subtracted  from  the  total  contraction  V  c  , 
the  difference  represents  the  contraction  caused  by  the  combus- 
tion of  the  hydrogen  present  (Vc  —  2F*)  and  two-thirds  of  the  lat- 
ter represents  the  amount  of  hydrogen, 


2(VC-2VK) 

~~       H' 


and  in  per  cent.: 


200(rc-2Fx) 
x=  -  ^~  -  -  =per  cent.  H. 


Determination  of  Carbon  Monoxide,   Methane,  and  Hydrogen 
by  Combustion. 

After  the  absorption  of  the  CO2,  CnH2n,  and  O,  the  residual 
gas  consists  of  CO,  CH4,  H,  and  N.  To  it  a  measured  volume  of 
oxygen  *  is  added,  the  mixture  burned,  and  both  the  contraction, 
Vc.  and  the  carbon  dioxide  formed,  VK,  are  estimated.  After  this 
the  unused  oxygen  is  determined  by  absorption  with  alkaline 
pyrogallol  solution.  If  the  excess  of  oxygen  is  subtracted  from 
the  amount  originally  added,  the  difference  will  give  the  amount 
of  oxygen  necessary  for  the  combustion.  V0. 

*  The  purity  of  the  oxygen  must  be  tested  before  the  analysis,  because 
the  commercial  product  almost  always  contains  nitrogen.  For  the  analysis  a 
measured  volume  of  nitrogen  is  added  to  a  definite  amount  of  oxygen,  as 
otherwise  the  amount  of  the  residual  gas  might  be  too  small  to  fill  the 
manometer-tube  between  the  marks  a  and  ra  (Fig.  105).  The  nitrogen 
is  prepared  by  allowing  air  to  stand  over  phosphorus  in  a  Hempel  pipette. 
(Cf.  p.  759). 


GAS  ANALYSIS. 


If  the  amount  of  CO  is'  denoted  by  x,  the  CH4  by  y,  ancl  finally 
the  hydrogen  by  z,  we  have  the  following  three  independent  equa- 
tions: 


2.  VK  = 

3.  Vo 

and  from  these  equations  we  find  that 


*  According  to  A.  Wohl  (Berichte,  1904,  433)  the  results  are  not  quit3 
accurate  when  obtained  in  this  way  because  the  molecular  volume  does  not 
always  equal  the  theoretical  value  of  22.41  liters.  Nernst,  in  his  book  on 
Theoretical  Chemistry,  gives  the  following  molecular  volumes: 

For  1  gm.-mol.  of  the  gas,  or,  referred  to  oxygen. 

H2  =  22.431.  H2  =  1.0017 

O2  =  22.391.  O2  =  1.0000 

CO  =  22.  391.  CO  =  1.0000 

CH4  =  22.441.  CH4  =  1.0020 

CO2  =  22.261.  CO2  =0.9939 

Taking  these  values  into  consideration,  A.  Wohl  obtains  for  x  CO,  y  CH4, 
and  z  H2,  the  following  formulas: 

Z  =0.3329  Fc  -  Fo  +  1.3394  Ffc, 

y  =  -  0.3336  Vc  +  1.0020  F0-0.  3340  Ffc  ; 

z  =  1.0005Fc-1.0017Fo-0.0060Ffc. 

F.  Haber  (Thermodynamik  techn.  Gasreaktionen,  p.  289)  sees  no  reason 
for  modifying  the  Bunsen  formulas  in  this  way,  for  when  a  combustion 
analysis  is  carried  out  by  explosion,  the  volume  of  gas  after  the  explosion 
is  so  poor  in  carbon  dioxide  that  the  partial  pressure  of  the  latter  does  not  vary 
much  from  that  of  an  ideal  gas,  and,  therefore,  follows  Avogadro's  Rule. 

It  is  quite  another  matter  in  the  case  of  mixtures  rich  in  carbon  dioxide, 
as  often  occur  in  gas-  volumetric  analyses.  In  that  case  the  weight  of  carbon 
dioxide  (or  of  carbonate)  is  computed  from  the  volume  of  the  gas  and  accurate 
values  are  obtained  by  using  the  observed  molecular  volume  of  22.26  for 
this  gas  (see  p.  386). 

The  necessity  of  using  the  observed  molecular  volume  instead  of  the 
theoretical  value  has  been  shown  by  Tread  well  and  Christie  (Z.  angew.  Chem., 
1905,  1930)  for  chlorine.  With  other  vapors  (NH3,  HC1,  SO2,  N2O)  the 
observed  molecular  volume  should  be  used  unquestionably. 


ILLUMINATING   AND   PRODUCER   GASES.  783 

In  order  to  illustrate  the  accuracy  of  the  method,  the  results 
obtained  in  the  analysis  of  the  gas  from  a  Dowson  gas  generator 
with  the  help  of  the  Deville  tube  (Fig.  100,  cf.  p.  732)  will  be  given. 
Two  samples  of  the  gas  were  taken,  one  35  cm.  and  the  other 
45  cm.  above  the  grate.  The  height  of  the  coal  layer  in  the 
producer  amounted  to  45  cm. 

DOWSON  GAS. 

Sample  I  (35  cm.  above  the  grate). 

I. 

CO,  =  8.54 
CnH2W=  0.30 
O  =  0.36 
CO  =  20.79 
CH4  -  1.32 
H  =21.84 
N  =  46.85 


100.00*  100.00  100.00 

The  above  analysis  was  performed  by  Korbuly  in  the  author's 
laboratory,  and  the  carbon  monoxide  was  determined  by  absorp- 
tion in  ammoniacal  cuprous  chloride,  but  in  the  following 
analysis  this  gas  was  determined,  as  described  above,  by  simul- 
taneous combustion  with  hydrogen  and  methane 


DOWSOX    GAS. 

Sample  II  (45  cm.  above  the  grate). 

I.  II. 

CO2     =     8.58  8.55 

CnH2n=     0.48  0.48 

O        =     0.17  0.26 

CO      =   20.79  20.59 

CH4    =     0.43  0.43 

H        =   19.31  19.22 

N        =   50.24  50.47 


100.00       100.00       100.00 


*  These  analyses  add   up  to  exactly  100  per  cent,  simply  because  the 
nitrogen  is  determined  by  difference. — [Translator.] 


784  GAS   ANALYSIS. 

Obviously"  the  above  results  are  perfectly  satisfactory;  it 
is  worth  mentioning,  however,  that  according  to  the  former  method 
(absorption  of  the  CO  and  combustion  of  the  residue)  the  value 
obtained  for  the  methane  is  almost  invariably  somewhat  higher, 
and  that  for  hydrogen  a  trifle  lower  than  according  to  the  second 
method.  To  illustrate  this,  the  results  of  a  third  analysis  *  will 
be  given,  which  was  also  made  by  Korbuly  in  the  sample  of  gas 
taken  35  cm.  above  the  grate 


Sample  I  (Dowson  Gas,  35  cm.  above  the  grate). 

CO  determined  CO  determined 

by  absorption.  by  combustion. 

CO2  8.51f  8.43 

CnH2n=     0.30  0.33 

O        =     0.31  0.27 

CO      =  20.80  20.91 

CH4     =     1.29  0.79 

H        =  22.05  23.38 

N        =  46.74  45.89 


100.00  100.00 

Of  the  two  methods,  the  author  decidedly  prefers  the  latter. 

Analysis  according  to  H.  Drehschmidt.t 

The  apparatus  of  Drehschmidt,  like  that  of  Hempel,  con- 
sists of  the  gas-burette  B  and  the  compensation-tube  C,  both 
of  which  are  contained  in  a*  cylinder  filled  with  water  (Fig. 
113). 

Through  the  stop-cocks  a  and  6,  B  and  C  are  connected  by 
means  of  capillary  glass  tubing  in  which  a  drop  of  a  colored 
solution  (indigo  and  sulphuric  acid)  is  placed;  in  order  to  deter- 
mine the  position  of  the  latter,  the  capillary  is  provided  with 


*  The  gas  came  from  the  same  tube  as  in  the  case  of  the  other  analyses. 
The  gas  was  removed  from  the  tube,  as  described  on  pp.  731-2. 
t  This  is  the  analysis  given  on  p.  783. 
J  Berichte,  21  (1888),  p.  3242. 


ILLUMINATING  AND  PRODUCER   GASES.  785 

a  millimeter  graduation.  The  three-way  cock  a  can  be  turned 
so  that  C  connects  with  the  outer  air  or  with  the  capillary, 
or  so  that  the  capillary  is  in  connection  with  the  air;  it  has  an 


FIG.  113. 

opening  through  the  top  of  the  key.  The  cock  b  has  a  right- 
angled  boring  like  H,  Fig.  105.  The  burette  is  divided  into  milli- 
meters and  must  be  calibrated  with  mercury  before  using.  The 
apparatus  is  used  in  the  same  way  as  described  under  the  Hempel 
method,  p.  775. 


786  GAS  ANALYSIS. 

TECHNICAL  GAS  ANALYSIS. 
Method  of  Hempel. 

The  apparatus  necessary  is  depicted  in  Fig.  114.  It  consists  of 
a  long  measuring-tube  ending  at  the  top  in  a  thick-walled  capillary 
tube  and  connected  at  the  bottom  by  means  of  rubber  tubing 
about  a  meter  long  with  the  levelling  tube. 

The  gas  is  confined  over  water  which  has  been  saturated  with 
the  gas  to  be  examined,  and  the  absorption  is  effected  in  HempePs 
absorption  pipettes  such  as  are  shown  in  Figs.  115,  116,  117,  and 
118.*  Fig.  100  represents  a  simple  pipette  for  liquid  absorbents, 
while  Fig.  101  shows  a  compound  absorption  pipette.  The  latter  is 
used  for  solutions  which  undergo  change  on  exposure  to  the  air,  e.g., 
an  alkaline  solution  of  pyrogallol,  or  an  ammoniacal  cuprous  chloride 
solution.  The  liquid  in  the  two  right-hand  bulbs  serves  to  protect 
the  solutions  on  the  left.  Fig.  117  shows  the  pipette  used  for 
fuming  sulphuric  acid.  The  small  bulb  is  filled  by  the  glass- 
blower  with  glass  beads,  which  serve  to  give  to  the  sulphuric  acid 
the  largest  possible  surface,  so  that  the  absorption  is  effected  much 
more  readily.  Fig.  118  is  a  pipette  used  for  solid  absorbents,  such 
as  phosphorus,  etc.  In  order  to  fill  it  with  phosphorus,  the  pipette 
is  placed  upside  down,  the  cylindrical  part  is  filled  with  distilled 
\\ater,  and  small  sticks  of  colorless  phosphorus  are  introduced. 
Afterfilling  the  pipette,  the  rubber  stopper  is  inserted,  the  apparatus 
is  placed  right  side  up,  water  is  poured  into  the  bulb,  and  any 
air-bubbles  in  the  cylindrical  part  of  the  pipette  are  removed  by 
blowing  through  the  bulb  until  the  water  flows  out  from  the  top 
of  the  left-hand  capillary,  which  is  then  closed  by  means  of  rubber 
tubing  and  a  pinch-cock. 

Analysis  of  Illuminating-gas. 

First  of  all  the  confining  liquid  is  prepared  by  conducting  the 
gas  through  distilled  water  in  a  wash-bottle  for  several  minutes 
with  constant  shaking. 

*  These  wooden  pipette  stands  are  no  longer  much  used;  iron  ones  are 
preferred. — [Translator.] 


ANALYSIS   OF  ILLUMINATING   GAS. 


787 


The  gas-burette  is  filled  entirely  full  with  this  liquid  and 
then  the  upper  rubber  tubing  is  closed  with  a  pinch-cock.  In 
order  to  fill  the  burette  with  gas,  the  receiver  is  connected  with 


FIG.  114. 


FIG.  116. 


the  burette  by  means  of  a  piece  of  rubber  tubing  through  which 
the  gas  has  been  flowing  for  two  or  three  minutes,  the  levelling-tube 
is  lowered,  the  pinch-cock  opened  and  a  little  more  than  100  c.c. 
of  the  gas  are  allowed  to  flow  into  the  burette.  The  upper  cock 
is  now  closed,  the  levelling-tube  raised  until  the  lower  meniscus 
of  the  confining  liquid  is  exactly  at  the  100-c.c.  mark,  when  the 
rubber  between  the  levelling-tube  and  the  burette  is  closed  near 
the  burette  with  a  pinch-cock.  The  apparatus  is  allowed  to 
stand  until  the  water  no  longer  rises  in  the  burette;  this  requires 


788  GAS   ANALYSIS. 

two  or  three  minutes.  When  the  water  is  stationary,  the  lower 
pinch-cock  is  carefully  opened  (for  there  is  extra  pressure  in  the 
burette),  which  causes  the  water-level  to  sink.  When  the  100-c.c. 
mark  is  again  reached,  this  cock  is  closed,  the  upper  pinch-cock 
is  opened  an  instant  in  order  to  allow  the  excess  of  gas  to  escape 
and  then  immediately  closed.  Then,  to  make  sure  that  the  burette 
contains  exactly  100  c.c.  of  the  gas,  the  lower  pinch-cock  is  opened 
and  after  bringing  the  water  in  the  levelling-tube  to  the  same  height 
as  in  the  burette,  the  reading  is  taken;  the  lowest  point  of  the 
meniscus  should  coincide  exactly  with  the  100-c.c.  mark  of  the 
burette.  Finally  the  lower  pinch-cock  is  closed. 

i.  Determination  of  Carbon  Dioxide. 

The  burette  is  connected  with  a  pipette  containing  caustic 
potash  solution  by  means  of  a  capillary  filled  with  water,  as  shown 
in  Fig.  114, the  levelling-tube  is  raised,  first  the  lower  pinch-cock 
and  then  the  upper  one  *  is  opened  and  the  gas  is  driven  over  into 
the  pipette.  The  confining  liquid  should  now  fill  the  entire  capil- 
lary. The  upper  pinch-cock  is  closed,  the  pipette  taken  up  and 
shaken  for  three  minutes,  f  and  the  gas  is  returned  to  the  burette, 
taking  care  that  none  of  the  alkali  enters  with  it. 

The  liquid  in  the  levelling-tube  is  brought  to  the  same  level 
as  that  in  the  burette;  the  lower  pinch-cock  is  closed  and  after 
the  water  has  completely  drained  from  the  sides  of  the  tube,  the 
volume  of  the  unabsorbed  gas  is  read. 

2.  Determination  of  the  Heavy  Hydrocarbons,  CnH2n. 

The  burette  is  connected  by  means  of  a  dry,  empty  capillary 
with  sulphuric  acid  pipette  (Fig.  117)  and  the  gas  is  passed  back 
and  forth  four  times,  taking  care  that  no  water  enters  the  pipette 
and  that  the  sulphuric  acid  does  not  reach  the  rubber  connection. 

Before  the  experiment  the  position  of  the  sulphuric  acid  is 

*  In  the  figure  this  pinch-cock  is  lacking. 

t  The  absorption  takes  place  more  rapidly  with  one  of  Hempel's  new 
pipettes,  which  is  similar  to  the  one  shown  in  Fig.  117,  except  that  the  right- 
hand  bulb  is  replaced  by  a  movable  levelling-bulb,  as  in  Fig.  105.  The  latter 
is  filled  with  mercury,  upon  which  the  liquid  absorbent  floats.  For  the 
absorption  of  CO2,  it  is  only  necessary  to  pass  the  gas  back  and  forth  once. 


DETERMINATION  OF  OXYGEN. 


789 


marked  upon  the  milk-glass  plate  back  of  the  pipette  and  at  the 
end  of  the  experiment  the  acid  must  come  to  the  same  mark.  The 
gas  in  the  burette  is  now  contaminated  with  acid  vapors  which 


FIG.  117. 


FIG.  118. 


are  removed  by  passing  it  into  the  potash  pipette,  afterwards 
returning  it  to  the  burette. 

3.  Determination  of  Oxygen. 

This  can  be  effected  by  shaking  the  gas  in  the  compound  pipette 
with  alkaline  pyrogallol  solution,  but  far  preferably  by  means  of 
phosphorus.  In  the  latter  case,  the  gas  is  driven  over  into  the 
phosphorus  pipette  and  allowed  to  remain  there  until  the  white 
vapors  disappear;  this  usually  requires  but  three  or  four  minutes 
(cf.  p.  759;.  If  no  white  vapors  can  be  detected,  this  shows  con- 
clusively that  the  absorption  of  the  heavy  hydrocarbons  was 
incomplete  (cf.  p.  759).  In  such  a  case,  the  gas  must  be  again 
treated  with  sulphuric  acid  and  afterwards  with  phosphorus. 
If  no  white  fumes  are  then  formed, -no  oxygen  is  present,  a  case 
which  practically  never  occurs,  for  in  the  determination  of 
the  hydrocarbons  a  little  air  containing  oxygen  always  reaches 
the  gas  from  the  small  capillary. 

4.  Determination  of  Carbon  Monoxide. 

The  gas  is  shaken  three  minutes  with  an  old  solution  of 
ammoniacal  cuprous  chloride  and  then  the  same  length  of  tune 
with  a  fresh  solution.  (See  pages  763,  779.) 


790  GAS  ANALYSIS. 

5.  Determination  of  Hydrogen  and  Methane. 

After  the  absorption  of  the  carbon  monoxide  the  residual  gas  is 
placed  in  the  hydrochloric  acid  pipette,  while  the  burette  is 
washed  out  with  hydrochloric  acid  in  order  to  remove  traces  of 
alkali,  and  then  filled  with  distilled  water. 

About  15  to  16  c.c.  of  the  gas  in  the  hydrochloric  acid  pipette 
are  transferred  to  the  burette,  and  after  reading  its  volume  it  is 
driven  over  into  an  explosion  pipette  containing  mercury  (Fig.  1 10) . 
100  c.c.  of  air  (containing  20.9  c.c.  of  oxygen)  are  accurately  meas- 
ured off  in  the  burette  and  added  to  the  contents  of  the  explosion 
pipette.  The  latter  is  then  closed  by  means  of  a  pinch-cock, 
the  contents  of  the  pipette  are  mixed  by  shaking,  the  levelling- 
tube  is  lowered  so  that  the  gas  is  placed  under  reduced  pressure, 
and  the  glass  stop-cock  of  the  pipette  is  closed.  The  platinum 
wires  which  are  fused  in  the  upper  part  of  the  bulb  are  now  con- 
nected with  the  poles  of  a  small  induction  coil  so  that  sparks  pass 
between  the  platinum  points  within  the  pipette.  The  explosion 
at  once  occurs  with  a  flash  without  ever  breaking  the  pipette. 
The  gas  is  returned  to  the  burette.  It  would  seem  natural  to 
read  the  volume  of  the  gas  and  then  determine  the  amount  of 
carbon  dioxide  formed,  the  latter  being  a  measure  of  the  amount 
of  methane  burned.  This  is  not  advisable,  however,  because 
the  gas  in  the  burette  is  confined  over  water  which  absorbs  ap- 
preciable quantities  of  carbon  dioxide.*  Consequently  without 
reading  the  volume  of  the  gas,  it  is  transferred  to  the  potash  pipette, 
the  carbon  dioxide  removed,  and  the  volume  of  the  gas  then  read ; 
this  gives  the  contraction  Vc.  Finally,  the  amount  of  unused 
oxygen  is  determined  by  means  of  absorption  with  phosphorus. 
If  the  excess  of  oxygen  is  subtracted  from  the  total  amount  added 
(20.9  c.c.),  the  amount  of  oxygen  required  for  the  combustion 
is  determined  (V0),  so  that  we  have  two  equations  from  which  the 
amount  of  hydrogen  and  methane  can  be  computed. 

*  Subsequent  experiments  have  shown  that  the  error  caused  by  absorption 
of  CO2  by  the  water  hi  so  slight,  during  the  short  time  of  waiting,  that 
it.  ib  better  to  determine  the  CO2  with  caustic  potash  after  the  explosion, 
as  Hempel  also  recommended.  The  results  thus  obtained  are  usually  more 
concordant  than  those  by  the  method  described  in  the  text.  (See  p.  792.) 


ANALYSIS  OF  ZURICH  ILLUMINATING-GAS.  791 

If  we  represent  by  x  the  volume  of  the  hydrogen,  and  by  y  the 
volume  of  the  methane  we  have 


2.  V0 
and  from  these  equations  we  find 

x=$Vc-2V0, 


The  values  thus  obtained  are  referred  to  the  total  gas  residue 
and  in  this  way.  the  amount  of  hydrogen  and  methane  present 
in  the  illuminating  gas  is  determined. 

Great  accuracy  is  naturally  not  to  be  expected  by  such  an 
analysis,  but  the  procedure  is  very  satisfactory  for  an  approxi- 
mate estimation.  In  order  to  illustrate  this  point,  the  results  of 
analyses  made  by  two  different  students  in  the  author's  labora- 
tory at  the  same  time  will  be  given. 

Analysis   of   Zurich   Illuminating-gas   by  HempePs 
Technical  Method. 

I.  n. 

Gas  taken  ...............   100  c.c.  100  c.c. 

->1.8%C02  -»1.8%  CO, 

After  removal  of  CO2  ......     98  .  2  98  .  2 


"          "         "  CnH^  ...     94.6  94.6 

-*0.6%  O  -*0.6%  O 

"          "         "  O  .......     94.0  94.0 

-»8.6%  CO  ->8.8%  CO 

"CO  ......     85.4  85.2 

For  the  H   and  CH4  deter- 
mination   were  taken    of 

gas  ....................     16.0  15.6 

-fair  ....................   116.0  115.6 


After  the  explosion.  .......     86  .  0  85.8 

—  »5  .  2  excess  oxygen  —  >5  .  6  excels 

oxyge* 
"     removal  of  excess  of  O     80.8  80.2 

Fo=20.9-5.2=15.7.  F0=20.9-5.6=15.3. 


792 


GAS   ANALYSIS 


If  the  values  of  Vc  and  Vo  are  inserted  in  the    above    equations,  we 
have: 

Hydrogen   x=8.6  :c=9.1 

Methane      y=5.7  y=5A 


and  in  per  cent. : 


s=45.9%  H 
2/=30.42%CH4 


z=49.7%  H 
2/=29.5%  CH4 


SUMMARY    OF    THE    TWO    ANALYSES. 


I. 

II. 

Difference. 

CO2 

1.8 

1.8 

0.0 

CnH  n 

3.6 

3.6 

0.0 

o 

0.6 

0.6 

0.0 

CO 

8.6 

8.8 

0.2 

H 

45.9 

49.7 

3.8 

CH4 

30.4 

29.5 

0.9 

N 

9.1 

6.0 

3.1 

From  the  results  obtained,  it  is  obvious  that  in  each  case  the 
values  obtained  by  absorption  agree  closely;  on  the  other  hand, 
the  two  determinations  of  hydrogen  differ  by  almost  4  per  cent. 
while  that  of  methane  shows  a  divergence  of  nearly  1  per  cent. 

It  is  possible  to  obtain  a  much  closer  agreement  than  the 
above  in  the  determination  of  hydrogen  and  methane,  but  the 
analysis  is  inaccurate  on  account  of  the  fact  that  only  one-fifth 
of  the  residual  gas  is  taken  for  the  explosion;  thus  every  error 
is  multiplied  by  five. 

As  was  mentioned  in  the  foot-note  on  page  790,  the  gas  residue 
may  be  analyzed  as  under  (a)  with  the  exception  that  the  COu 
obtained  by  combustion  is  measured.  Then,  if  the  volume  of  the 
hydrogen  =x  and  that  of  the  methane  =y,  the 

contraction  =  Vc  =  f  z  4-  2y. 


from  which  the  hydrogen  (x)  can  be  computed  as  follows  : 


/IN A  LYSIS  OF  ZURICH  ILLUMINATING-GAS.  793 

As  an  example  of  this  kind  of  an  analysis,  two  of  the  author's 
students  analyzed  independently  a  sample  of  illuminating  gas  from 
Montbeliard. 

I  II 

Taken  100  c.c.  100  c.c. 

-CO2               97.4  c.c.  =  2.6%  CO2  97.2  c.c.  =  2.8%  CO2 

- C»H2ft            92 . 7  c.c.  =  4.7%  CnHjn  92 . 4  c.c.  =  4 . 8%  CnH2» 

-O2                 92.4c.c.  =  0.3%O2  92.0c.c.=0.4%O2 

-CO                83.1  c.c. -9. 3%  CO  83.0  c.c.  =  9.0%  CO2 
Of  the  residual  gas  there 

was  taken  for  H  and 

CH4     determination  + 

air                                   15. 4  c.c.  15. 6  c.c. 

115. 4  c.c.  115. 6  c.c. 

After  the  explosion  90.6  c.c.  =  24.8  =  Vc  90.2  c.c.  =  25.4  Vc 

-CO2  84.5  c.c.  =  6.1  =  Vfc=CH4       83.8  c.c.  =  6.4  =  Vk  = 

CH4 

-O2  80.3  c.c.  =  4. 2  =  excess oxy-   79.8  c.c.  =  4.0  =  excess 

gen  of  O2 

If  the  values  of  Vc  and  Vk  are  inserted  in  the  above  equa- 
tions: 

Hydrogen z=8.4  =  45.3  per  cent  H2  x  =  8.4    44.7  per  cent  H2 

Methane */  =  6.1  =  32.9       «        CH4         t/  =  6.4     34.0        "        CH4 

SUMMARY   OF   THE   TWO  ANALYSES 

I.  II.  Difference. 


C02= 

2.6 

2.8 

0.2 

iH,n  = 

4.7 

4.8 

0.1 

02= 

0.3 

0.4 

0.1 

CO2= 

9.3 

9.0 

0.3 

H2=             45.3  44.7  0.6 

CH4=             32.9  34.0  1.1 

H2=               4.9  4.3  0.6 

100.0  100.0 


794  GAS   ANALYSIS 

Much  better  results  are  obtained  by  the 

(b)  Method  of  W inkier-Dennis. 

In  this  method,  the  entire  gas  residue  is  transferred  to  a 
Hempel  pipette  containing  mercury  and  connected  with  a 
leveling  bulb  (Fig.  119).  Through  the  rubber  stopper  at  the 
bottom  two  steel  needles  are  inserted  (knitting  needles),  the 
longer  of  which  is  enveloped  throughout  its  whole  length  by  a 
glass  tube,  and  the  upper  end  is  connected,  at  about  three-quarters 


FIG.  119. 

the  height  of  the  cylindrical  part  of  the  pipette,  with  a  thin 
platinum  spiral. 

.     The  pipette  is  now  connected  with  a  Hempel  burette  containing 
100  c.c.  of  oxygen  *  over  water,  a  low  pressure  is  produced  in  the 

*  The  oyxgen  used  for  experiments  in  gas  analysis  should  preferably  be 
prepared  in  the  laboratory  by  heating  potassium  chlorate  in  a  small  retort, 
which  is  prepared  by  blowing  a  bulb  (of  about  20  c.c.  capacity)  at  the  end 
of  a  narrow  piece  of  glass  tubing;  after  introducing  about  5  gms.  of  potassium 
chlorate,  the  tubing  is  bent  to  a  right  angle  close  to  the  bulb.  The  end  of 
the  tube  is  connected  with  a  short  piece  of  rubber  tubing  and  the  bulb  heated 
over  a  free  flame.  As  soon  as  oxygen  begins  to  come  off  freely  (lighting 
a  glowing  splinter)  the  rubber  tubing  from  the  retort  is  connected  with  a 
Drehschmidt  absorption  pipette,  which  contains  a  little  caustic  potash 
solution  and  is  filled  with  mercury  (cf.  Fig.  105,  p.  743).  The  oxygen  is  not 


ANALYSIS  OF  ZURICH  ILLUMINATING-GAS. 


795 


oxygen  burette  by  lowering  the  leveling  tube  and  then  closing 
the  rubber  tubing  with  a  screw-cock,  after  which  the  leveling 
tube  is  placed  in  a  high  position.  The  bottom  ends  of  the  two 
needles  of  the  pipette  are  now  connected  with  the  wires  of  a 
small  storage  battery  of  such  a  strength  that  the  platinum  spiral  is 
heated  to  dull  redness.  By  lowering  the  leveling  bulb,  a  slightly 
lower  pressure  is  produced  in  the  pipette,  and  by  opening  the 


FIG.  120. 

two  upper  screw-cocks  between  the  pipette  and  the  oxygen  burette, 
and  gradually  opening  the  lower  screw-cock  on  the  burette,  a 
very  slow  stream  of  oxygen  is  conducted  into  the  pipette.  Since 
a  large  excess  of  the  gas  residue  is  present  at  the  start,  the  com- 
bustion takes  place  quietly;  explosions  never  occur.  During  the 

introduced  at  once  into  the  pipette,  but  is  allowed  to  pass  through  the  cock 
M  into  the  air.  After  about  a  minute,  one  can  assume  that  the  air  from 
the  retort  and  rubber  tubing  has  been  entirely  replaced  by  oyxgen.  The 
leveling  bulb  K  of  the  piptte  is  lowered,  the  cock  s  opened,  and  the  cock  M 
turned  90°  so  that  the  pipette  fills  with  oxygen.  When  the  filling  is  accom- 
plished, M  is  closed  and  the  retort  removed.  By  shaking  the  pipette,  any 
carbon  dioxide  formed  by  the  burning  of  dust,  etc.,  is  absorbed. 


790  GAS  ANALYSIS. 

combustion  the  platinum  spiral  begins  to  glow  more  brightly; 
to  prevent  its  melting,  a  resistance  *  must  be  placed  in  the  circuit 
by  means  of  which  the  strength  of  current,  and  thus  the  glowing 
of  the  platinum,  may  be  regulated  as  desired. 

As  .soon  as  all  the  oxygen  is  in  the  pipette,  the  spiral  is  allowed 
to  glow  two  or  three  minutes  longer,  the  electric  current  is  then 
stopped,  and  the  gas  allowed  to  remain  in  the  pipette  for  fifteen 
minutes  so  that  it  will  assume  the  room  temperature.  It  is  then 
transferred  to  a  Hempel  burette  and  its  volume  measured;  the 
carbon  dioxide  is  determined  in  the  usual  manner. 

To  illustrate  the  accuracy  of  this  technical  method,  the  follow- 
ing three  analyses  were  carried  out  independently  by  three  of  the 
author's  students. 

ANALYSIS    OF    ZURICH    ILLUMINATING-GAS    ON    JULY    14,    1909. 

C02 

\^n,Xl27i 

02 
CO 
CH4 
H2 

N2. 

100.0%         100.0%         100.0% 

Remark. — By  this  method  it  is  possible  to  burn  pure  acetylene 
without  any  explosion.  The  oxygen,  however,  must  not  be 
conducted,  as  above,  into  the  acetylene  because  in  that  case  the 
combustion  of  the  acetylene  will  be  incomplete  and  considerable 
carbon  will  deposit.  If  the  oxygen  is  first  placed  in  the  combustion 
pipette,  the  platinum  wire  brought  to  glowing,  and  then  the 
acetylene  introduced,  the  combustion  takes  place  nicely  without 
deposition  of  any  carbon. 

The  Winkler-Dennis  pipette  is  open  to  the  objection  that  the 
rubber  stopper  eventually  leaks;  for  this  reason  the  author 
prefers  the  form  of  apparatus  devised  by  his  assistant,  M.  Bretsch- 
ger,  as  shown  in  Fig.  120. 

*  The  resistan  e  mentioned  on  page  178  is  suitable  to  use  here. 


I 

II 

III 

2.0% 

2.2% 

1.9% 

4.4 

4.4 

4.6 

0.7 

0.5 

0.6 

9.2 

9.2 

9.3 

27.6 

28.2 

27.9 

49.8 

49.4 

49.3 

6.3 

6.1 

6.4 

ORSATS  APPARATUS. 


'97 


Instead  of  burning  the  gas  residue  according  to  the  Winkler- 
Dennis  method,  it  may  be  conducted  over  glowing  cupric  oxide.* 

Orsat's  Apparatus. 

For  the  analysis  of  flue  gases,  Orsat  has  constructed  the  appa- 
ratus shown  in  Fig.  121.     It  consists  of  the  100  c.c.  measuring-tube 


a.. 


FIG.  121. 

B  surrounded  by  a  cylinder  containing  water,  and  connected  on 
the  one  hand  with  three  Orsat  tubes  by  means  of  the  cocks  /, 
II,  and  ///,  and  the  other  hand  with  the  outer  air  through  the 
stop-cock  h.  The  Orsat  tube  ///  contains  caustic  potash,  II  alka- 
line pyrogallol  solution,  and  I  ammoniacal  cuprous  chloride 
solution. 

*  Jager,  J.  Gasbeleuchtung,  1898,  764.    G.  v.  Knorre,  Chem.  Ztg.,  1909,  717. 


798  GAS  ANALYSIS. 

Manipulation. — By  raising  the  leveling-bottle  N  and  open- 
ing the  stop-cock  h,  the  measuring -tube  B  is  filled  with  water. 
As  soon  as  the  water  is  above  the  mark  in  the  widened  part  of 
the  measuring-tube,  the  rubber  tubing  between  the  levelling-bottle 
and  the  measuring-tube  is  closed  by  means  of  a  pinch-cock,  a  is 
connected  with  the  source  of  the  gas,  and  the  gas  is  sucked  into 
the  measuring-tube  by  lowering  the  levelling-bottle  and  opening 
the  pinch-cock.  The  U  tube  on  the  outside  of  the  apparatus  is 
filled  with  glass-wool  and  serves  as  a  filter;  any  smoke  being 
removed  from  the  gas  to  be  examined.  The  sample  thus  col- 
lected is  naturally  contaminated  with  the  air  from  the  rubber 
tubing,  the  U  tube,  and  the  capillary,  which  must  be  removed.  The 
cock  h  serves  for  this  purpose  and  is  provided  with  a  T  boring. 
The  cock  is  turned  so  that  the  burette  communicates  with  the 
outer  air  through  a  small  tube  (not  shown  in  the  illustration)  and 
the  gas  is  expelled  by  raising  the  bottle  N.  This  process  of 
filling  and  emptying  is  repeated  three  times,  and  the  fourth  filling 
of  the  tube  B  is  taken  for  the  analysis.  The  gas  in  the  burette  is 
brought  to  the  0  mark,  and  it  is  placed  under  atmospheric  pres- 
sure by  quickly  opening  and  then  closing  h.  After  this  the  gas 
is  driven  over  into  the  potash-bulb  and  back  again  to  the  meas- 
uring-tube several  times,  until  there  is  no  further  absorption,  after 
which  the  volume  of  the  gas  is  again  read.  In  the  same  way 
the  gas  is  successively  passed  into  the  pyrogallol  and  the  cuprous 
chloride  tubes,  thus  obtaining  the  amount  of  CO2,  0,  and  CO  in 
the  gas. 

Bunte's  Apparatus. 

This  apparatus,  shown  in  Fig.  122,  differs  from  those  previously 
described,  inasmuch  as  the  absorption  takes  place  in  the  measuring 
vessel  itself,  whereas  in  the  other  cases  the  absorption  takes  place 
in  the  pipettes. 

The  Bunte  burette  has  a  capacity  of  about  110  to  115  c.c. 
between  a  and  b;  a  is  a  three-way  cock,  while  b  is  bored  only  once. 

Manipulation. — The  burette  is  connected  with  the  levelling- 
bottle  N,  as  shown  in  the  illustration,  a  and  6  are  opened,  and  the 
water  is  allowed  to  run  up  to  the  mark  in  the  funnel  above  a. 
The  key  of  the  stop-cock  a  is  connected  with  the  source  of 
the  gas,  N  is  lowered,  a  turned  to  the  proper  position,  and 


BUNTE'S  APPARATUS. 


799 


the  gas  is  sucked  into  the  burette.  After  about  101  to 
103  c.c.  of  the  gas  have  entered  the  burette,  a  and  b  are 
closed,  N  is  raised,  and  by  opening  b  the  gas  in  the  burette  is 
compressed  until  the  confining  liquid  has  exactly  reached  the 
zero  mark.  The  cock  a  is  now  cautiously  opened,  when  some  of 
the  gas  in  the  burette  will  escape  through  the  water  in  the  funnel. 
The  gas  in  the  burette  is  now  under  a  pressure  equal  to  that  of  the 
atmosphere  plus  the  pressure  from  the  column  of  water  in  the 
funnel,  and  all  subsequent  measurements  are  taken  under  the 
same  conditions. 

Absorptions. — In  order  to  introduce  the  different  absorbents 
into  the  burette,  its  lower  end  is  connected  by  means  of  the  rub- 
ber tubing  h  with  the  bottle  F  containing  a  little  water,  the  water 
having  been  blown  up  into  the  rubber  tubing.  The  cock  b  is 
opened,  as  is  the  screw-cock  at  h,  and 
the  water  in  the  burette  is  allowed  to 
run  out  until  it  exactly  reaches  the  cock 
6,  which  is  then  closed.  The  absorbent 
is  placed  in  a  small  dish,  the  lower 
tip  of  the  burette  is  introduced  into  the 
liquid,  and  the  cock  b  is  opened.  Inas- 
much as  the  gas  in  the  burette  is  under 
less  than  atmospheric  pressure,  the  ab- 
sorbent is  sucked  up  into  the  burette. 
The  cock  b  is  now  closed,  the  burette 
grasped  above  a  and  below  6  (in  order 
not  to  warm  the  gas),  and  its  contents 
well  shaken,  after  which  the  burette  is 
a°;ain  dipped  into  the  absorbent  in  the 
dish  and  a  little  more  of  the  latter 
drawn  up  into  the  burette.  This  process 
is  repeated  until  no  more  of  the  ab- 
sorbent is  sucked  up  into  the  burette. 
It  would  now  be  incorrect  to  read  the 
volume  of  the  unabsorbed  gas,  for  it 
is  under  quite  a  different  pressure  than 
at  the  beginning  of  the  analysis;  namely, 
the  atmospheric  pressure  less  the  pressure  of  the  column  of 


FIG.  122. 


800  GAS  ANALYSIS. 

liquid  remaining  in  the  burette  with  the  cock  b  open.  Further- 
more the  vapor  tension  of  the  liquid  in  the  burette  is  different 
from  that  of  the  water  originally  present.  In  order  to  obtain 
the  original  conditions,  the  burette  is  connected  with  the 
bottle  F,  which  now  only  contains  enough  water  to  fill  the 
rubber  tubing  and  the  glass  tube,  and  the  absorbent  is  sucked 
from  the  burette  into  the  bottle  until  the  upper  level  of  the 
liquid  reaches  the  cock  b*  The  end  of  the  burette  is  then 
dipped  into  a  dish  containing  water,  which  rises  into  the  burette 
on  opening  b.  The  latter  is  closed  and  water  is  allowed  to  run 
into  the  burette  from  the  funnel  until  the  original  pressure  is 
established,  when  the  volume  of  the  gas  is  once  more  read.  The 
difference  gives  at  once  the  per  cent,  of  absorbed  gas. 

By  means  of  this  excellent  method  the  carbon  dioxide  can  be 
removed  by  caustic  potash,  heavy  hydrocarbons  by  bromine 
water,  oxygen  by  alkaline  pyrogallol  solution,  and  carbon 
monoxide  by  cuprous  chloride. 

ANALYSIS  OF  GASES  WHICH  ARE  ABSORBED  CONSIDERABLY 

BY  WATER. 

Under  this  heading  belong 

N2O,  SO2,  H2S,  Cl,  SiF4,  HF,  NH3,  etc. 

Nitrous  Oxide,  N2O.     Mol.  Wt.  44.02. 

Density  =  1 .5297  t  (Air  =  1) .     Weight  of  1  liter  =  1 .9766  gms. 
Molar    volume  =22.26  1.     Critical  temperature  =  +36°  C. 
This  gas  is  best  prepared  according  to  the  method  of  Victor 
Meyer,  J   by  allowing  sodium  nitrite  to  act  upon  a  concentrated 
solution  of  a  salt  of  hydroxylamine : 

NH2OH  •  HC1  +  NaNO2  =  NaCl  +  2H2O  +  N2O. 

*  The  absorbent  is  now  by  no   means   exhausted,   so  that  it  is  returned 
to  the  proper  bottle,  and  can  be  used  for  several  other  determinations. 
t  Lord  Rayleigh,  Proc.  Roy.  Soc.,  74,  181  (1904). 
J  Ann.  Chem.  Pharm.,  157,  141. 


BUNT  PS  APPARATUS.  80 1 

It  is  best  to  proceed  as  follows: 

A  concentrated,  aqueous  solution  of  sodium  nitrite  is  added 
drop  by  drop  from  a  separately  funnel,  with  constant  cooling, 
to  a  concentrated  solution  of  hydroxylamine  hydrochloride,  which 
is  contained  in  a  small  evolution  flask;  in  this  way  the  gas 
evolved  is  pure  and  escapes  in  a  regular  stream.  It  is  not  advis- 
able to  proceed  in  the  opposite  way,  namely,  to  add  the  hydroxyl- 
amine solution  to  a  concentrated  nitrite  solution,  for  in  the  latter 
case  the  decomposition  is  likely  to  take  place  with  explosive  vio- 
lence; it  is  still  less  advisable  to  add  one  of  the  reagents  in  the 
solid  form.  In  a  very  dilute  condition  the  solutions  scarcely  act 
upon  one  another. 

Nitrous  oxide  is  never  pure  when  it  is  prepared  by  heating 
ammonium  nitrate;  it  is  always  contaminated  with  nitrogen  and 
nitric  oxide,  but  the  latter  may  be  removed  by  washing  the  gas 
with  a  solution  of  ferrous  sulphate. 

According  to  L.  Pollak  the  solubility  of  nitrous  oxide  between 
0°  and  22°  C.  is  expressed  by  the  formula 

3  =1.13719  -0.042265  •*  + 0.000610 -*2, 

while  according  to  Bunsen  its  solubilty  is  greater,  being  expressed 
by  the  formula 

£  =  1 .3052  -  0.045362  - 1  -f  0.0006843  •  t\ 

The  gas  is  absorbed  to  a  much  greater  extent  by  alcohol  than 
by  water.  According  to  Pollak,  the  absorption  coefficient  for 
alcohol  is 

£  =  3.22804-0.04915-*+0.00023-*2, 

wnile  according  to  Bunsen  it  is  somewhat  greater: 
£=4.17805-0.069816-J+0.000609-*2. 

The  determination  of  nitrous  oxide  can  be  effected  with  accu- 
racy by  combustion,  and  this  may  be  carried  out  in  two  different 

ways : 


802  GAS  ANALYSIS. 

1.  According  to  Bunsen,  by  exploding  with  hydrogen,  or  accord- 
ing to  Knorre,  by  means  of  the  Drehschmidt  capillary.     The  con- 
traction produced  is  equal  to  the  original  volume  of  the  nitrous 
oxide : 

N20     +     H2     =     H20     +     N2. 

2  vols.  2  vols.  0  vol.  2  vols. 

2.  According    to    Pollak,    by    combustion    with    pure    carbon 
monoxide,  either  by  explosion  or  with  the  help  of  the  Drehschmidt 
capillary;  the  volume  of  the  CO2  formed,  which  is  measured,  is 
equal  to  the  volume  of  the  nitrous  oxide: 

N20     +     CO     =     CO2     +     N2. 

2  vols.  2  vols.  2  vols.  2  rols. 

There  is  no  contraction  in  this  case. 

Nitric  Oxide,  NO.     Mol.  Wt.  30.01. 

Density  =1.0366,*  (Air  =  l).     Weight  of  1  liter  =  1.3402  gms. 
Molar    volume  =22.39  1.     Critical  temperature  =  -94°  C. 

Preparation  of  Pure  Nitric  Oxide. 

The  best  way  to  prepare  pure  nitric  oxide  is  the  method  of  A. 
De venter, t  in  which  a  solution  of  potassium  ferrocyanide  and 
potassium  or  sodium  nitrite  is  acidified  with  acetic  acid  and 
shaken : 

2K4Fe  (CN)  6  +  2KNO2  +  4HC2H3O2  =  4KC2H3O2 +2K3Fe  (CN)  6 

+  2H2O  +  2NO. 

According  to  Emich  {  a  very  pure  gas  is  obtained  by  shaking 
a  nitrate  with  concentrated  sulphuric  acid  in  a  nitrometer  con- 
taining mercury. 

*  Computed  from  observations  of  Gray  (1905),  Guye  and  Davila  (1906). 
t  Berichte,  26,  589  (1893). 
t  Monatshefte,  13,  73  (1892). 


NITRIC  OXIDE.  803 

ABSORPTION    COEFFICIENTS    OF    NITRIC    OXIDE    FOR    WATER.* 

Tempetature.  /9  Temperature.  /? 

0°  0.07381  30°  0.04004 

5  0.06461  35  0. 03734 

10  0.05709  40  0.03507 

15  0.05147  45  0.03311 

20  0.04706  50  0.03152 

25  0.04323  55  0.03040 

Although  nitric  oxide  is  only  slightly  soluble  in  water,  its 
determination  will  be  discussed  at  this  place  because  this  gas 
frequently  occurs  with  nitrous  oxide,  and  must  therefore  be 
determined  at  the  same  time. 

Xitric  oxide  may  be  determined  by  absorption  with  a  con- 
centrated solution  of  ferrous  sulphate  or  an  acid  solution  of  potas- 
sium permanganate,  likewise,  according  to  E.  Divers,f  by  an  alka- 
line solution  of  sodium  sulphite  (40  gms.  Na2SO3+4  gms.  KOH 
in  200  c.c.  H20)  with  the  formation  of  Na2N2O2SO3.t  It  is 
better,  however,  to  carry  out  a  combustion  by  the  method  of 
Knorre  and  Arndt,  §  in  which  the  gas  is  mixed  with  hydrogen  and 
very  slowly  passed  through  a  Drehschmidt's  platinum  capillary 
heated  to  bright  redness.  Under  these  conditions  the  nitric  oxide 
is  quantitatively  burned  according  to  the  equation. 

2NO     +     2H2     =     2H20     +     N2. 

4  vols.  4  vols.  0  vol.  2  vols. 

The  contraction  produced  by  the  combustion  of  one  volume 
of  nitric  oxide  is  equal,  therefore,  to  f  the  original  volume  of  the  gas. 

Remark. — If  the  gas  mixture  is  passed  too  quickly  through 
a  platinum  capillary  heated  to  bright  redness,  or  slowly  through 
a  less  strongly  heated  platinum  capillary,  an  appreciable  amount 
of  ammonia  is  formed  and  the  results  obtained  are  inaccurate. 

By  explosion  with  hydrogen  it  is  not  possible  to  burn  NO 

*  L.  W.  Winkler,  Berichte,  34,  1414  (1901). 

f  Journ.  Science  Coll.  Imp.  University;  Tokio,  Vol.  XI  (1893),  p.  11. 

J  Nitric  oxide  is  only  partially  absorbed  by  an  alkaline  solution  of  pyro- 
gallol,  where  alkali  nitrite,  N2O,  and  X2  are  formed.  (C.  Oppenheim,  Berichte, 
36,  1744  (1903). 

§  Berichte,  21  (1889),  p.  2136. 


804  GAS  ANALYSIS. 

when  it  is  pure;  when  it  is  mixed  with  considerable  nitrous  oxide, 
violent  explosions  take  place,  yet  the  combustion  of  the  NO  is 
even  then  not  quantitative. 

The  gas  may  be  determined,  however,  by  combustion  with 
carbon  monoxide  in  the  Drehschmidt  capillary. 

According  to  Henry  a  mixture  of  carbon  monoxide  and  nitric 
oxide  is  not  explosive.  On  the  other  hand,  according  to  Pollak, 
by  conducting  a  mixture  of  these  gases  through  a  Drehschmidt 
platinum  capillary  heated  to  bright  redness,  the  combustion  is 
quantitative  if  at  the  same  time  the  carbon  dioxide  formed  is 
removed  by  means  of  caustic  potash;  *  otherwise  the  oxidation 
is  not  quantitative.  According  to  the  equation 

2NO+2CO  =  2CO2+N2 

4  vols.     4  vols.        0  vol.        2  vols. 

the  contraction  produced  is  equal  to  f  the  volume  of  the  nitric 
oxide. 

Remark.  —  If  considerable  nitrous  oxide  is  present  at  the  same 
time,  the  combustion  in  the  Drehschmidt  capillary  takes  place 
quantitatively  without  the  removal  of  the  carbon  dioxide.  In 
this  case  the  contraction  is  ?  the  volume  of  the  nitric  oxide. 

Analysis  of  a  Mixture  of  Nitrous  and  Nitric  Oxides. 
I.  Combustion  with  Hydrogen. 

The  gas  is  mixed  with  an  excess  of  hydrogen  and  oxidized 
according  to  Knorre  in  the  Drehschmidt  platinum  capillary  heated 
to  bright  redness.  If  the  volume  of  the  N2O  =  x  and  that  of  the 
N0  =  ?/,  we  have: 

N20.     NO. 

1.  x+y  =  V 

2.  z-f2/=|Vc  (contraction) 

from  which  can  be  calculated: 

z  =  3V-2V6 


*  The  mercury  in  the  Drehschmidt  tube  is  covered  with  caustic  potash 
solution,  by  which  means  the  CO2  is  absorbed  immediately  after  its  formation. 


DETERMINATION  OF  NITROUS   OXIDE,  ETC.  805 

n.  Combustion  with  Carbon  Monoxide. 

The  gas  mixture  is  treated  with  an  excess  of  carbon  monoxide 
and  burned  in  the  red-hot  platinum  capillary;  the  contraction, 
Vc,  and  the  carbon  monoxide,  Vk,  are  both  determined: 

N20.     NO. 
x  +     y  =  Vk 

to  -  Vc 

from  which  we  can  compute: 

x  =  Vk-2Ve 


Determination  of  Nitrous  Oxide,  Nitric  Oxide,  and  Nitrogen  in 
the  Presence  of  One  Another. 

I.  By  Combustion  ivith  Hydrogen  in  a  Drehschmidt  Capillary. 

After  noting  the  contraction  formed  by  the  combustion  with 
hydrogen,  an  excess  of  oxygen  is  added  to  the  gas  residue  and  the 
mixture  is  burned  in  the  Drehschmidt  capillary;  two-thirds  of  the 
contraction  which  now  takes  place  is  equal  to  the  amount  of  unused 
hydrogen  in  the  first  oxidation.  If  this  quantity  is  deducted 
from  the  amount  of  hydrogen  originally  added,  the  difference,  Vw, 
represents  the  amount  of  hydrogen  necessary. 

We  have  now: 

N2O.     NO.     N. 

1.  x  +     y  +  z  =   V 

2.  x  +  ly  =   Vc 

3.  x  +     y          =  Vw 

from  which  we  can  compute 

x=3Vw-2Va 
y=2(V,-Vw) 


806  G4S   ANA  LYSIS. 

II.  By  Combustion  with  Carbon  Monoxide  in  the  Drehschmidt 

Capillary. 
We  have: 

N20.    NO.     N. 
x  +     y  +  z  =  V 

\y          =VC  (contraction) 
x  +     y          = 

from  which  it  follows  : 


z=V-Vk. 

Determination  of  Nitrous  Oxide,  Nitric  Oxide,  and  Nitrogen  in 
the  Presence  of  Carbon  Dioxide. 

The  accurate  determination  of  nitrous  oxide  in  the  presence 
of  carbon  dioxide  offers  certain  difficulties.  It  is  not  possible 
to  determine  the  former  by  combustion  with  hydrogen  in  the 
Drehschmidt  capillary,  because  when  the  carbon  dioxide  is  present 
it  takes  part  to  some  extent  in  the  combustion, 

C02+H2=H20+CO, 

and  the  previous  absorption  of  the  carbon  dioxide  by  means  of  a 
large  quantity  of  caustic  potash  is  equally  unsatisfactory,  because 
a  considerable  amount  of  nitrous  oxide  will  be  absorbed  by  the 
reagent.  The  only  way  which  can  be  recommended  to  effect 
this  determination  consists  in  absorbing  the  carbon  dioxide  by 
means  of  the  smallest  possible  quantity  of  caustic  potash,  in  which 
case  the  error  introduced  by  the  absorption  of  the  nitrous  oxide 
is  reduced  to  a  minimum  ;  the  residual  gas  is  examined  as  described 
above. 

Nitrogen,  Mol.  Wt.  28.02. 

Density  =  0.9673  (Air=l).     Weight    of    1    liter  =  1.2505    gms. 
Molar  volume  22.41  liters.      Critical  temperature  =  —  149°  C. 

Pure  nitrogen  is  best  prepared  by  heating  a  concentrated 
solution  of  potassium  nitrate  and  ammonium  chloride,  present 
in  amounts  proportional  to  their  molecular  weights,  and  then 


NITROGEN.  807 

conducting  the  escaping  gas  over  glowing  copper  to  reduce  traces 
of  nitric  oxide. 

Nitrogen  is  but  slightly  soluble  in  water.     According  to  L. 
Winkler,* 

ABSORPTION    COEFFICIENTS    OF    NITROGEN    FOR    WATER. 


Temperature. 

0° 

0.02348 

Temperature. 
30° 

.3 
3.01340 

5 

0.02081 

35 

0.01254 

10 

0.01857 

40 

0.01183 

15 

0.01682 

45 

0.01129 

20 

0.01542 

50 

0.01087 

25 

0.01432 

55 

0.01051 

Nitrogen  cannot  be  determined  by  any  of  the  ordinary  methods 
of  gas  analysis.  It  is  always  estimated  by  determining  all  the 
other  constituents  present  in  a  mixture  and  subtracting  the  sum 
of  the  percentages  found  from  100. 

Technical  preparations  o.  nitrogen,  prepared  from  the  air, 
always  consist  of  nitrogen  and  small  amounts  of  rarer  elements. 
According  to  Cavendish  these  latter  may  be  obtained  by  adding 
oxygen  and  allowing  a  strong  electric  spark  to  pass  through 
the  mixture.  In  this  way  the  nitrogen  is  completely  oxidized  to 
nitric  acid,  which  can  be  removed  by  means  of  caustic  potash 
solution.  Then,  by  absorbing  out  the  oxygen,  the  rarer  gases  are 
obtained.  A  still  better  process  is  that  of  Hempel,  in  which  the 
nitrogen  is  absorbed  by  passing  the  gas  over  a  glowing  mixture 
of  1  gm.  magnesium,  o  gm.  freshly  burnt  lime,  and  0.25  gms. 
sodium.  The  rare  gases  are  not  absorbed  by  this  treatment. 

According  to  Bunsen,  there  is  no  combustion  of  nitrogen 
when  detonating  gas  explodes  in  the  presence  of  air,  provided  net 
more  than  30  volumes  of  combustible  gas  are  present  for  each  100 
volumes  of  non-combustible  gas.  There  is  no  oxidation  of  nitrogen 
during  a  combustion  of  a  gas  mixture  which  is  passed  through  a 
Drehschmidt  platinum  capillary. 

*  Barichte,  24,  3506  (1891). 


8o8  GAS  ANALYSIS. 


Analysis  of  Gases  by  Titration  of  the  Absorbed   Constituents. 

If  a  mixture  of  gases  contains  several  constituents,  of  which 
two  are  removed  by  the  same  absorbent,  and  one  of  these  can 
be  determined  by  titration,  it  is  a  matter  of  no  difficulty  to 
determine  the  amount  of  each.  The  diminution  in  volume  after 
treatment  with  the  absorbent  represents  the  amount  of  the  two 
constituents,  the  titration  value  represents  the  amount  of  one  of 
them,  and  the  difference  shows  the  amount  of  the  other.  Such 
problems  can  be  solved  in  a  variety  of  ways,  and  only  a,  few 
examples  will  be  mentioned. 

Chlorine,  Cl.     Mol.   Wt.  =  70.92. 

Density  =2.488  (Air  =  1) .*    Weight  of  1  liter  =3.2164  gms. 
Molar  volume  =  22.049  liters.     Critical  temperature  =  + 146°  C. 

Determination  of  Carbon  Dioxide  in  Electrolytic  Chlorine.f 

The  author  has  used  the  apparatus  shown  in  Fig.  123  with  the 
best  success  for  this  purpose. 

The  absolutely  dry  eudiometer,  B,  the  capacity  of  which 
between  the  two  stop-cocks  is  accurately  known,  and  for  con- 
venience may  be  100  c.c.,  is  filled  through  the  lower  cock, 
after  the  gas  has  been  dried  by  passing  it  through  a  long 
calcium  chloride  tube.J  After  five  or  ten  minutes  it  is  safe 
to  assume  that  the  air  has  been  completely  replaced  by  the 
gas.  The  lower  three-way  cock  is  now  closed  and  then  the 
upper  one.  The  temperature  and  barometric  pressure  are 
both  noted  at  this  point. 

The  tip  of  the  burette  is  connected  by  rubber  tubing  with 
the  reservoir  N,  the  three-way  cock  is  turned  so  that  the 
reservoir  communicates  with  the  outer  air,  and  then  the  lower 

*  Leduc,  Compt.  rend.,  116,  968  (1893)  and  Treadwell  and  Christie,  Z 
angew.  Chem.,  47,  446  (1905). 

t  Treadwell  and  Christie,  Z.  angew.  Chem.,  47,  1930  (1905). 

J  If  the  burette  and  gas  are  not  perfectly  dry,  some  chlorine  will  be 
absorbed  by  the  water.  This  will  not  affect  the  gas  reading,  but  will  be 
harmful  in  the  subsequent  titration. 


CHLORINE. 


809 


tip  of  the  burette  and  the  stop-cock  are  thoroughly  washed, 
after  which  the  latter  is  closed.  A  solution  of  potassium 
arsenite  is  prepared  by  dissolving  4.95  gms. 
As2O3  in  dilute  potassium  hydroxide,  adding 
dilute  sulphuric  aoid  until  the  solution  is  neu- 
tral to  phenolphthalein  and  then  diluting  to 
1  liter.*  100  c.c.  of  this  solution  are  placed  in 
N  and  any  air  in  the  rubber  tubing  is  expelled 
by  pinching  it  with  the  thumb  and  finger. 
By  raising  N  and  opening  the  stop-cock,  a 
little  of  the  arsenite  solution  is  made  to  flow 
into  the  burette,  which  is  inclined  from  side 
to  side  in  such  a  way  that  the  walls  are 
thoroughly  wet  with  the  arsenite  solution. 
The  chlorine  is  slowly  absorbed,  as  is  evident 
from  the  fact  that  the  solution  slowly  rises 
in  the  burette.  As  soon  as  there  is  no 
further  absorption,  the  lower  stop-cock  is 
closed  and  the  solution  in  the  burette  is 
made  to  flow  back  and  forth  in  the  burette 
several  times,  by  inverting  the  burette  and 
then  turning  it  back  again.  After  one  or 
two  minutes  all  of  the  chlorine  will  have 
been  absorbed.  Then  in  order  to  absorb  all  the  carbon  dioxide 
present,  the  tube  N  is  lowered,  10  c.c.  of  potassium  hydroxide 
solution  (1:1)  are  poured  in  the  funnel,  and  carefully  made 
to  flow  into  the  burette.  The  stop-cock  is  again  closed  and  the 
alkali  solution  poured  back  and  forth  in  the  burette. 

After  the  liquid  in  the  burette  and  in  the  leveling  tube  has  been 
brought  to  the  same  height,  the  reading  is  taken.  The  unabsorbed 
gas  residue  on  being  deducted  from  the  original  volume  of  the  gas 
gives  the  volume  of  the  chlorine  plus  that  of  the  carbon  dioxide. 
For  the  determination  of  the  chlorine,  the  contents  of  the  burette 
leveling  tube,  N,  are  emptied  into  a  large  Erlenmeyer  flask  and 
the  stop-cock  is  turned  to  the  position  shown  in  the  drawing,  so 


FIG.  123. 


*  An   ordinary  solution   of  arsenite  prepared  with  sodium  bicarbonate 
cannot  be  used  here. 


8 10  GAS  ANALYSIS. 

that  the  liquid  in  the  rubber  tubing  can  flow  out.  The  tubing  is 
then  removed  from  the  burette  and  washed  out  with  distilled 
water  which  is  also  allowed  to  run  into  N.  The  contents  of  the 
burette  are  added  and  the  burette  itself  is  rinsed  with  distilled 
water. 

The  contents  of  the  Erlenmeyer  flask  are  treated  with  two 
drops  of  phenolphthalem  solution  and  then  with  hydrochloric  acid 
until  the  red  color  j  ust  disappears,  60  c .  c .  of  sodium  bicarbonate  solu- 
tion are  added  (35  gms.  in  1000  c.c.  water),  a  little  starch  solution, 

N  . 

and  the  excess  of  the  arsenious  acid  is  titrated  with  —  iodine  solu- 
tion. 

It  will  be  assumed  that  n  c.c.  are  used  in  the  tit  rat  ion.  The 
ratio  of  the  arsenite  solution  to  the  iodine  is  then  established 
in  the  same  way  as  in  the  above  titration.  100  c.c.  of  arsenite 
solution  are  placed  in  an  Erlenmeyer  flask,  10  c.c.  of  caustic-potash 
solution  (1:2)  are  added,  two  drops  of  phenolphthalem,  hydro- 
chloric acid  to  decolorization,  and  then  60  c.c.  of  sodium  bicar- 
bonate solution.  The  solution  is  then  diluted  to  the  same 
volume  as  that  of  the  original  experiment  and  titrated  with 
tenth-normal  iodine.  Hereby  n'  c.c.  are  required.  The  difference 
n'  —  n  multiplied  by  1.102  *  gives  the  number  of  cubic  centimeters 
of  chlorine  gas  at  0°  C.  and  760  mm.  pressure.  In  other  words, 

F'0=(n'-rOX  1.102. 

As,  however,  the  original  gas  was  measured  at  the  temperature 
t°  C.  and  under  the  pressure  B  mm.,  it  follows^  according  to  page 
666,  that  r-£-273 

0     760- (273+0' 

*  This  number  is  derived  from  the  observation  that  the  density  of  chlorine 
is  2.488  at  20°,  70.92 

0.001293X2.488 

Therefore,  35.46  gms.  of  chlorine  at  0°  and  760  mm.  occupy  a  volume 
of  11,020  c.c.  In  the  paper  just  cited,  the  value  22,039.2  c.c.  is  taken  for 
the  molecular  volume  of  the  chlorine  and  0.001293  for  the  density  of  air. 

In  the  analyses  of  gases  rich  hi  chlorine,  correct  values  are  obtained  by 
using  the  observed  molecular  volume  of  chlorine,  which  is  22.049  liters,  and 
according  to  the  experiments  of  N.  Busvold  in  the  author's  laboratory,  correct 
results  are  also  obtained  with  this  value  in  the  analysis  of  gases  containing 
little  chlorine. 


CHLORINE.  8  n 

from  which  can  be  computed 

F,_W  760(273+Q 
.6.273 

If  V  is  the  original  volume  of  the  gas  used  and  R  that  of 
the  residual  gas  in  the  burette,  then 

C12+C02+ Residue  =  V 
Residue  =  R 


ci2+co2=y-# 

-012=7! 


and  in  per  cent. 

[F 
x= • — '        =  per  cent.  CO2. 

Remark. — In  the  first  edition  of  this  book  the  mixture  of 
chlorine  and  carbon  dioxide  was  absorbed  by  means  of  5  per 
cent,  caustic  soda  solution  and  the  solution  titrated  with  arsenious 
acid. 

This  method  is  not  entirely  correct,  however,  because  it  is 
based  upon  the  assumption  that  the  chlorine  is  absorbed  by  the 
alkali  in  accordance  with  the  equation, 

2NaOH  +  C12  =  NaCl  +  NaCIO  +  H  2O, 

whereas  in  reality  there  is  always  some  chlorate  formed    and 
escapes  the  tit  ration.*     Offerhaus,f  therefore,  uses  two  burettes 

*  The  error  here  is  practically  constant  and  amounts  to  0.77  per  cent, 
chlorine.  The  work  can  be  carried  out  according  to  the  original  method, 
adding,  for  the  sake  of  accuracy,  0.7  per  cent,  to  the  value  of  chlorine  found. 
Cf .  O.  Steiner,  loc.  cit.,  and  Treadwell  and  Christie,  loc  cit. 

If  the  weight  of  chlorine  foui^J  by  titration  is  used  hi  connection  with  the 
theoretical  molecular  volume,  a  considerable  error  will  be  introduced  for,  in 
spite  of  the  formation  of  chlorate,  about  0.9  per  cent,  too  much  chlorine  will 
be  found  in  electrolytic  chlorine. 

tCl.  Winkler  (Industriegase  II,  318)  and  Offerhaus  (Z.  angew.  Chem., 
1903,  1033),  also  Lunge-Berl,  Chem.  techn.  Untersuchungsmethoden,  6th 
edition,  Vol.  I,  p.  582,  and  O.  Steiner,  Z.  Elektrochemie,  1904,  327. 


8i2  GAS  ANALYSIS. 

for  the  determination,  absorbing  the  chlorine  and  carbon  dioxide 
in  one  by  means  of  caustic-potash  solution,  and  absorbing  the 
chlorine  in  another  sample  of  gas  by  means  of  potassium  iodide 
and  the  titrating  with  tenth-normal  thiosulphate  solution. 

This  is,  however,  an  unnecessary  complication,  for  besides 
requiring  an  additional  burette  it  involves  the  use  of  more  of  the 
expensive  potassium  iodide.  It  is  possible,  however,  to  carry 
out  the  analysis  in  one  burette  by  absorbing  first  the  chlorine 
with  10  per  cent,  potassium-iodide  solution,  and  then  intro- 
ducing caustic  potash  solution  from  the  top  of  the  burette. 
Hereby  the  carbon  dioxide  is  absorbed  and  the  iodine  is  trans- 
formed into  iodide  and  iodate  (the  solution  becomes  nearly 
colorless)  : 

C12  +  2KI=2KC1  +  I2| 
3I2  +  6KOH=5KI  +  KIO3+3H2C. 

In  order  to  determine  the  amount  of  iodine  originally  set 
free,  the  contents  of  the  burette  are  allowed  to  flow  into  a 
potassium  iodide  solution  which  is  acid  with  hydrochloric  acid: 


and  the  liberated  iodine  is  titrated  with  tenth-normal  sodium 
thiosulphate  solution. 

The  method  has  no  advantage  over  that  described  above  and 
is  not  quite  as  accurate. 

Recently  Schloetter  has  described  another  method  for  the 
examination  of  electrolytic  chlorine  gas.  The  chlorine  is  absorbed 
by  means  of  hydrazine  sulphate,  whereby  two  volumes  of  chlorine 
set  free  one  volume  of  nitrogen.  The  carbon  dioxide  is  then 
absorbed  by  means  of  caustic  soda  solution. 

P.  Ferchland*  determines  the  chlorine  by  absorption  with 
mercury  in  the  residual  gas  after  the  CO2  has  been  absorbed  with 
caustic  potash.  This  last  method,  according  to  the  experiments 
of  Busvold,f  gives  good  results;  it  is  to  be  recommended  especially 
for  the  analysis  of  chlorine  gas  from  the  Deacon  process. 

Examination  of  the  Unabsorbed  Gas  Residue.  —  Usually  the 
residual  gas  is  too  small  in  amount  (as  in  the  above  case)  to 

*  Z.  Elektrochem.,  13,  114. 

t  Inaug.  Dissert.  Zurich,  1909,  also  P.  Philosophoff.  Chem.  Ztg.,  1907,  959. 


CHLORINE. 


examine  quantitatively,  so  that  for  this  part  of  the  analysis  a 
larger  sample  of  the  gas  is  taken.  The  author  has  used  the 
apparatus  shown  in  Fig.  124  for  this  .purpose  with  good  results 


FIG.  124. 

The  thick-walled  filter-bottle  A  has  a  capacity  of  about  1.5  liters. 
It  contains  about  500  c.c.  of  strong  caustic  potash  solution  and 
the  absorption-tube  with  stop-cock  H  is  fastened  air-tight  within  it. 
Manipulation. — First  of  all,  the  absorption-tube  is  entirely 
filled  with  the  caustic  potash  solution  by  suction  through  H,  finally 
closing  the  latter.  The  patent  cock  is  then  turned  to  the  position 
//,  and  by  suction  through  the  left  side-arm,  the  glass  tube 
is  filled  with  the  alkali  up  to  the  cock.  The  latter  is  then  turned 
to  the  position  I,  the  left  side-arm  is  connected,  by  means  of  a 

*  This  apparatus  has  been  used  often  by  the  author  in  the  study  of  elec- 
trolytic chlorine  gas  and  was  described  in  the  first  edition  of  this  text-book. 
Since  then  a  similar  apparatus  has  been  recommended  by  Thiele  and  Deckert, 
Z.  angew.  Chem.,  20,  437  (1907). 


814  GAS   ANALYSIS. 

shqrt  pieqe  of  rubber  tubing  and  a  long  piece  of  glass  tubing,  with 
the  sour.ce  of  the  gas  and  several  liters  of  gas  are  drawn  through 
this  tube  by  connecting  the  right  side-arm  with  an  aspirator. 
As  soon  as  it  is  safe  to  assume  that  all  of  the  air  has  been  driven 
out  from  the  tubing,  the  cock  is  turned  to  the  position  //,  the 
aspirator  is  connected  at  a  with  the  flask  A  in  which  a  slight  vacuum 
is  produced,  whereby  the  gas  begins  to  collect  in  the  absorption- 
tube.  Chlorine  and  carbon  dioxide  are  completely  absorbed, 
while  the  residual  gas  collects  in  the  upper  part  of  the  absorption- 
tube.  The  gas  is  allowed  to  enter  the  tube  until  from  50  to  70 
c.c.  of  the  gas  residue  are  obtained;  the  cock  /  is  then  closed, 
the  aspirator  removed,  and  the  gas  driven  over  into  a  HempePs 
gas-burette  and  analyzed  according  to  the  methods  already  de- 
scribed. 

00.9  c.c.  of  the  gas  residue  from  the  above-mentioned  elec- 
trolytically  prepared  chlorine  *  gave : 

Oxygen  =40.71  f  02  =  66.9 

Carbon  monoxide  =  2.6  \  and  in  per  cent,  j  CO=     4.3 
Nitrogen  =17.6  J  IN    =  28.8 


60.9  100.0 

At  the  carbon  electrode  (the  anode)  not  only  chlorine  but  also 
a  small  amount  of  oxygen  is  liberated.  The  latter  attacks  the 
carbon  of  the  electrode,  forming  carbon  monoxide,  the  greater 
part  of  which  in  turn  combines  with  the  chlorine,  forming  phosgene 
gas,  COC12,  but  the  latter  is  decomposed  by  water  with  the  forma- 
tion of  CO2  and  HC1: 

COC12 + H2O  =  C02 + 2HC1. 

This  accounts  for  the  presence  of  the  CO2  and  CO  in  chlorine  which 
has  been  prepared  electrolytically. 

Hydrochloric  Acid  HC1.    Mol.  Wt.  36.47. 

Density  =  1.2686    (Air  =  l).f   Weight  of  one  liter  =  1.6400  gms. 
Molar  volume =22.24  liters.     Critical  temperature  =  +52°  C. 
Hydrochloric  acid  is  determined  in  gas  mixtures  by  absorbing 
with  standardized  alkali. 

*  Consisting  of  99.0  per  cent.  C12  and  0.6  per  cent.  CO2. 

t  Leduc,  Compt.  rend.,  125,  571  (1897),  found  the  density  of  hydrogen  to 
be  1.2692  and  Busvold,  Inaug.  Dessert.  Zurich,  1910,  obtained  the  value 
1.2680.  The  above  figure  is  the  mean  of  these  two  determinations. 


SULPHUR  DIOXIDE.  815 

Sulphur  Dioxide,  Mol.  Wt.  64.07. 

Density  =2.2639  (Air  =  l).*    Weight  of  one  liter  =2.9267  gms. 
Molar  volume  =  2  1.89  liters.     Critical  temperature  =  +155°  C. 

For  the  determination  of  sulphur  dioxide  from  pyrite  burners, 
F.  Reich  recommends  that  the  gas  should  be  sucked  by  means 

N 
of  an  aspirator  through  a  measured  amount  of  ^  iodine  solution, 

colored  blue  with  starch,  until  the  latter  is  decolorized.  The 
amount  of  the  gas  is  equal  to  the  quantity  of  water  which  has 
flowed  from  the  aspirator  +  the  volume  of  the  absorbed  SO2. 

N 
For  example,  10  c.c.  of  ^  iodine  solution  were  decolorized 

after  V  c.c.  of  water  had  flowed  from  the  aspirator;  the  gas  was 
at  t°  C.  and  760  mm.  pressure.  Since  in  the  absorption  of  the 
S(>2  by  the  iodine  the  following  reaction  takes  place, 


it  is  evident  that  the  amount  of  862  absorbed,  measured  dry  at 

X 
0°  and  760  mm.  pressure,  will  be  10.95  c.c.,  for  1  c.c.  ~  iodine 

solution  corresponds  to  1.095  c.c.  862. 

It  follows,  then,  that  the  volume  of  gas  taken  for  the  analysis 
equals 

V-  (B-w).  273 


760.(273  +  0 
and  from  this  the  per  cent,  of  S(>2  in  the  gas  can  be  calculated: 

Vi:  10.95  =  100z 
x  —  -y—  =  per  cent.  SC>2. 

Other  examples  of  gas  analyses  in  which  the  absorbed  con- 
stituent is  estimated  by  titration  are  found  in  the  determination 
*  Leduc,  Compt.  rend.,  117,  219  (1893). 


8i6 


GAS  ANALYSIS. 


of  the  hydrogen  sulphide  in  gas  mixtures  (see  below)  and  in  the 
determination  of  carbonic  acid  in  the  atmosphere  by  the  method 
of  Pettenkofer  (cf.  p.  593). 

Hydrogen  Sulphide,  H2S;  Mol.  Wt.  =34.09. 
Density  =1.1895  (Air  =  l).*     Weight  of  one  liter  =  1.5378  gms. 
Molar  volume  =  22. 16  liters.     Critical  temperature  =  100°  C. 

Determination  of  Hydrogen  Sulphide  in  Gas  Mixtures. 

Hydrogen  sulphide,  when  present  in  the  gases  escaping  from 
mineral  springs,  is  estimated  as  follows: 

A  large  funnel,  of  from  2  to  3  liters  capacity,  is  lowered  into 
the  spring  and  held  in  place  by  means  of  a  wooden  frame  B 
weighted  with  stones,  s  (Fig.  125).  The  rubber  tubing  d  is 


FIG.  125. 


removed  from  the  flask  a,  the  stop-cock  h  is  opened,  and  the 
funnel  T  is  filled,  by  means  of  suction,  with  water  up  to  the 
stopper,  and  then  h  is  closed.  As  soon  as  the  water  in  the  funnel 
has  been  replaced  by  the  ascending  bubbles  of  gas,  the  flask  a 
is  connected  on  the  one  side  with  the  stop-cock  tube  h  and  on 
the  other  side  directly  with  the  aspirator  A  by  means  of  a  long 
rubber  tubing.  Suction  is  then  started  by  opening  H  and  is 
*  Leduc.  Compt.  rend.,  125,  571  (1897). 


HYDROGEN  SULPHIDE.  817 

continued  with  h  open  until  the  water  in  the  funnel  T  again 
reaches  the  stopper,  when  h  is  closed.  The  funnel  is  allowed 
to  fill  with  gas  again,  and  this  is  eventually  removed  through  a 
by  means  of  suction.  This  operation  is  repeated  twice  more. 
In  this  way  the  neck  of  the  funnel,  the  glass  tube  h,  the  rubber 
tubing  d,  and  the  flask  a,  all  have  the  air  originally  present  in 
them  replaced  by  gas  from  the  spring;  a  few  drops  of  water 
are  carried  along  mechanically  into  a.  Ten  c.c.  of  hundredth- 
normal  iodine  solution  are  then  introduced  into  the  ten-bulb 
tube  b  and  10  c.c.  of  hundredth-normal  thiosulphate  solution  are 
placed  in  the  tube  c.  The  flask  a  is  now  quickly  connected  with 
b  by  means  of  a  short  piece  of  rubber  tubing  /,  and  c  is  con- 
nected with  the  aspirator  A  by  means  of  a  longer  piece  of  tubing. 
Meanwhile  the  funnel  T  is  again  filling  with  gas.  A  measuring 
cylinder  C  is  placed  under  the  outlet  tube  of  the  bottle  .4,  H  is 
opened,  and  then  the  stop-cock  h  is  cautiously  turned.  The  gas 
is  allowed  to  bubble  slowly  through  b  until  the  iodine  solution 
becomes  light  yellow,  but  is  not  decolorized.  H  is  now  closed 
and  after  about  two  minutes  h  also.  The  contents  of  c  are  poured 
into  b,  starch  is  added,  and  the  excess  of  the  thiosulphate  is 
titrated  with  hundredth-normal  iodine  solution  (cf.  page  650). 
The  number  of  cubic  centimeters,  n,  of  iodine  required  for  the 
titration,  represents  the  amount  of  iodine  which  reacted  origin- 
ally with  the  hydrogen  sulphide.  The  position  of  the  water  in 
the  graduate  is  noted  (V  c.c.),  the  temperature  of  the  room 
t°,  and  the  barometer  reading  B]  w  is  the  tension  of  aqueous 
vapor  at  the  temperature  t°. 

In  computing  the  amount  of  hydrogen  sulphide  in  the  gases 
escaping  from  the  spring,  it  is  to  be  remembered  that  the  volume 
of  gas  which  is  taken  for  the  analysis  is  equal  to  the  amount  of 
water  which  has  flowed  into  c  plus  the  volume  of  hydrogen 
sulphide  which  has  been  absorbed  by  the  iodine  in  6  during  the 
experiment.  Inasmuch,  however,  as  the  amount  of  the  latter 
is  small  in  comparison  with  the  total  amount  of  gas  taken  for 
analysis,  it  may  be  neglected  here.  Furthermore,  it  is  neces- 
sary to  call  attention  to  the  fact  that  the  volume  of  gas  escaping 
from  the  spring  is  at  a  different  temperature  than  that  of  the 
analysis;  all  the  volumes,  therefore,  should  be  reduced  to  corre- 


8i8  GAS  ANALYSIS. 

spond  to  the  temperature  at  the  spring.  The  amount  of  hydrogen 
sulphide  present  per  liter  in  the  spring  gases  at  the  temperature 
t°  and  the  barometric  pressure  B  is 

308.4^p±|=c.c.H2S  per  liter* 

Determination  of  Ethylene,  according  to  Haber. 

The  principle  of  this  method  was  discussed  on  p.  752.  The 
determination  is  effected  in  the  Bunte  burette  (cf.  p.  799,  Fig. 
122). 

First,  the  contents  of  the  lower  portion  of  the  burette  from 
the  lowest  scale  division  to  the  cock  is  determined  by  weighing 
the  water  drawn  from  between  these  points,  after  allowing  the 
burette  to  drain.  Then  about  90  c.c.  of  the  gas  to  be  examined 
are  placed  in  the  burette  and  the  thermometer  and  barometer 
readings  are  taken.  Then,  exactly,  as  described  on  p.  799,  the 
liquid  is  sucked  down  to  the  stop-cock, f  a  little  bromine  water 
is  poured  into  a  small  evaporating-dish,  about  10  c.c.  of  the 
liquid  are  allowed  to  rise  into  the  burette,  and  in  order  to  wash 
the  bromine  water  from  the  tip  into  the  burette,  2  or  3  c.c.  of 
water  are  added. 

The  walls  of  the  burette  are  now  thoroughly  wet  with  the 
bromine  water  by  suitably  turning  and  inclining  the  tube,  and 
in  this  way  the  ethylene  is  quickly  absorbed.  In  order  to  de-% 
termine  the  excess  of  bromine,  a  strong  solution  of  potassium 
iodide  is  allowed  to  rise  into  the  burette,  and  the  contents  of 
the  latter  are  vigorously  shaken.  The  liquid  is  then  run  out 
into  an  Erlenmeyer  flask,  the  burette  is  carefully  washed  out 

N 
with  water  and  the  deposited  iodine  is  titrated  with  ~  sodium 

thiosulphate  solution.  The  titre  of  the  bromine  water  added 
is  next  determined  by  pouring  a  little  into  a  porcelain  dish, 

*  In  this  formula  the  temperature  of  the  spring  does  not  come  into  con- 
sideration because  the  gas  is  at  the  laboratory  temperature  when  measured. 

t  After  about  one  minute  liquid  will  collect  above  the  stop-cock,  owing  to 
the  drainage  of  the  liquid  from  the  sides  of  the  burette;  this  is  removed  before 
adding  the  bromine. 


DETERMINATION  OF  ETHYLENE.  819 

pipetting  off  10  c.c.  of  it,  allowing  this  amount  to  run  into  a 
solution  of  potassium  iodide  and  titrating  the  liberated  iodine  with 

N 

— -  sodium  thiosulphate  solution. 

The  method  of  calculating  the  results  will  be  illustrated  best 
by  means  of  a  single  example. 

Example. — A  gas  consisting  of  90  volumes  of  air  and  XO  volumes 
of  ethylene  was  used  for  the  analysis. 

Taken  for  analysis,  91.2  c.c.  of  the  mixture. 

Temperature,  18.3°  C. 

Barometer  reading,  725  mm. 

Tension  of  aqueous  vapor  at  18.3°  C.  =  15.6  mm.  mercury. 

Volume  of  the  ungraduated  portion  of  the  burette. . .     6.10  c.c. 
Reading  of  the  bromine  water  in  the  graduated  part. .   10.00  " 

Bromine  water  used 16.10  c.c. 

Titre  of  the  bromine  water: 

N 
10  c.c.  of  the  bromine  water  correspond  to  12.0  c.c.  -^  sodium 

thiosulphate   solution,  so    that  16.10  c.c.  of   bromine  water  are 

N 
equivalent  to  19.32  c.c.  of  —  sodium  thiosulphate. 

We  have  now: 

N 
16.1  c.c.  bromine  water —19.32  c.c.  —  solution. 

16.1  c.c.  bromine  water  +  ethylene  =12.23    "     " 

The  ethylene  corresponds  to 7.09    "    "        " 

Since  the  absorption  of  the  ethylene  by  the  bromine  water 
takes  place  according  to  the  equation 

it  follows  that 

"N" 
2Br=2I=20.000  c.c.  ~  sodium  thiosulphate  solution =22, 270* 

N 
c.c.  ethylene,  and  since  1  c.c.  ^  sodium  thiosulphate  corresponds 

*Cf.  page  751. 


820  GAS  ANALYSIS. 

to    1.100  c.c.  C2H4,    the  7.09  c.c.  of  —  solution  used  represent 

7.09X1.100  =  7.94  c.c.  C2H4  at  0°  C.  and  760  mm.  pressure,  or  9.10 
c.c.  C2H4  at  18.3°  C.  and  725  mm.,  measured  moist. 
The  gas  consists,  therefore,  of: 

C'H<  =  9-!i  and  in  per  cent.  JC2H4=10.0  percent. 
Air  =82.1)  (Air   =90.0     "      " 

91.2  100.0  per  cent. 

This  method  is  especially  suited  for  the  determination  of  ethylene 
present  with  benzene  in  illuminating-gas.  In  one  sample  the 
sum  of  the  two  gases  is  determined  by  absorption  with  fuming 
sulphuric  acid  or  bromine  water,  and  in  a  second  sample  the 
ethylene  is  determined  as  described  above. 

This  method  is  suitable  for  determining  ethylene  mixed  with 
benzene  vapors  in  illuminating  gas.  In  one  sample  the  sum  of 
the  two  is  obtained  by  absorbing  with  fuming  sulphuric  acid 
or  bromine  and  in  a  second  sample  the  ethylene  is  determined 
as  above. 

Remark. — Instead  of  using  bromine  water,  which  changes  its 
strength  so  rapidly,  the  author  uses  a  tenth-normal  solution  of 
potassium  bromate;  on  acidifying  an  equivalent  quantity  of 
bromine  is  obtained. 

The  experiment  is  carried  out  as  follows :  Exactly  as  described 
above,  90  c.c.  of  the  gas  are  led  into  the  Bunte  burette,  the  water 
withdrawn  till  the  lower  cock  is  reached,  and  then  some  potas- 
sium bromate  solution  is  placed  in  a  small  porcelain  dish  and  about 
10  c.c.  of  it  is  sucked  up  into  the  burette  and  the  volume  deter- 
mined. Then,  after  wiping  off  the  lower  capillary,  an  excess  of 
concentrated  potassium  bromide  solution  and  finally  an  excess 
of  dilute  hydrochloric  acid  is  introduced.  After  shaking  eight 
minutes,  all  the  ethylene  will  be  brominated.  At  the  end  of  this 
time,  10  per  cent,  potassium  iodide  solution  is  allowed  to  enter 
the  burette,  the  contents  shaken,  and  emptied  into  an  Erlenmeyer 
flask.  The  iodine  thus  liberated  is  titrated  with  tenth-normal 
sodium  thiosulphate  solution.  The  calculation  is  carried  out  as 
before.  Using  this  modification  of  Haber's  method,  the  author's 


DETERMINATION  OF  ETHYLENE.  821 

assistant,  M.  Bretschger,  analyzed  a  mixture  containing  air  and 
known  quantities  of  ethylene  with  the  following  results: 

Ethylene  taken.  Found. 

1  5.20%  5.23% 

2  5.20  5.16 

3  5.20  5.15 

4  3.90  3.88 

5  3.70  3.70 
•       6  3.70  3.68 


Determination  of  Ethylene  in  the  Presence  of  Acetylene. 

Since  acetylene  is  not  attacked  in  the  cold  by  either  bromine 
or  iodine,  whereas  ethylene  is  readily  attacked,  the  author  has 
caused  his  assistant  to  carry  out  a  number  of  experiments  in  thus 
analyzing  gas  mixtures. 

In  one  sample  of  the  mixture,  the  sum  of  the  ethylene-acetylene 
was  determined  by  absorption  with  fuming  sulphuric  acid  and  in 
another  sample  the  ethylene  was  determined  by  the  above 
bromate-bromide  method.  The  accuracy  is  attested  by  the 
following  analyses  of  M.  Bretschger: 

Ethylene  taken.      Ethylene  found.         Acetylene  found. 

4.38%  4.33%  0.61% 

4.17  4.23  0.32 

9.96  9.71  0.35 

9.83  9.84  0.43 

7.85  7.73  4.65 

7.83  7.82  4.60 

2.69  2.80  19.18 

2.64  2.74  19.60 


822  GAS  ANALYSIS. 

GAS-VOLUMETRIC  METHODS. 

If  in  consequence  of  a  chemical  reaction  a  gas  is  evolved,  from 
the  volume  of  the  latter  the  weight  of  the  original  substance  may 
be  determined. 

Examples  of  this  sort  of  an  analysis  were  given  under  the 
determination  of  CC>2  in  carbonates  (pp.  384,  388,  393),  the  carbon 
contents  of  iron  and  steel  (pp.  402  and  404),  and  the  NO3  in 
nitrates  (p.  456). 

At  this  place  a  few  more  important  determinations  of  the  same 
nature  will  be  described. 

Determination  of  Ammonia  in  Ammonium  Salts. 

The  following  method,  first  proposed  by  Knop*  and  later 
modified  by  P.  Wagner,  f  depends  upon  the  fact  that  ammonia 
is  oxidized  by  sodium  hypobromite  with  evolution  of  nitrogen: 

2NH3 + SNaOBr = 3H2O  +  3NaBr + N2. 

The  nitrogen  is  collected  in  an  azotometer  and  measured. 

If  the  amount  of  the  ammonia  be  calculated  from  the  volume 
of  the  nitrogen,  too  low  results  will  be  obtained,  and  this  fact 
was  formerly  explained  by  the  assumption  that  a  part  of  the 
nitrogen  was  absorbed  as  such  by  the  alkaline  bromine  solution. 
To-day,  however,  we  know  that  such  is  not  the  case.  At  ordinary 
temperatures  all  of  the  ammonia  is  not  oxidized  according  to 
the  above  equation  to  water  and  nitrogen,  but  a  small  amount 
of  ammonium  hypobromite  is  formed;  for  this  reason  too  little 
nitrogen  is  obtained  in  the  azotometer.  If,  on  the  other  hand, 
the  decomposition  takes  place  at  100°  C.,  the  reaction  goes  quanti- 
tatively according  to  the  equation.  It  is  inconvenient  to  work 
at  such  a  high  temperature,  so  that  it  is  more  practical  to  make 
a  correction  to  the  volume  of  nitrogen  obtained  at  ordinary  tem- 
peratures. 

*  Chem.  Centralbl.,  1860,  p.  243. 

t  Zeitschr.  f.  anal.  Chem.,  XIII  (1874),  p.  383;  XV  (1876),  p.  250. 


DETERMINATION  OF  ETHYLENE.  823 

Reagents  and  apparatus  required  : 

1.  An   ammonium   chloride   solution,   obtained  by  dissolving 
S.3544  gms.  of  the  pure  sublimed  salt  in  water  and  diluting  to 
500  c.c. 

10  c.c.  of  this  solution  evolve  at  0°  C.  and  760  mm.  pressure 
35  c.c,  of  nitrogen  if  the  reaction  takes  place  according  to  the 
equation. 

2.  Sodium   hypobromite   solution.     100   gms.   of   sodium   hy- 
droxide  are  dissolved   hi  water,  diluted  to  1250  c.c.  and    after 
cooling  by  placing  the  flask  hi   cold  water,  25  c.c.  of  bromine 
are  added,  the  contents  of  the  flask  vigorously  shaken  and  again 
cooled. 

This  solution  must  be  preserved  hi  a  stoppered  bottle  and 
protected  from  the  action  of  light. 

3.  An  azotometer.     Instead  of  the  azotometer  of  Wagner,* 
Lunge's  Universal  Apparatus  (Fig.  61,  6,  p.  387),  or  any  such  meas- 
uring instrument  may  be  used. 

Procedure. — Ten  c.c.  of  the  standard  ammonium  chloride  solu- 
tion are  placed  in  the  small  Wagner  decomposition-bottle  (Fig. 
61,  a,  p.  387)  while  40  to  50  c.c.  of  the  hypobromite  solution 
are  poured  into  the  glass  L  (which  is  fused  to  the  bottom  of 
the  bottle  H}.  The  bottle  is  then  connected  with  the  measuring- 
tube  J.,f  which  is  entirely  filled  with  mercury,  b  opened  and  the 
levell ing-tube  B  lowered.  The  bottle  H  is  inclined  so  that  some 
of  the  hypobromite  solution  comes  in  contact  with  the  solution 
of  ammonium  chloride  and  the  two  liquids  are  mixed  by  gentle 
shaking.  A  lively  evolution  of  nitrogen  at  once  takes  place  and 
the  liquid  becomes  heated.  As  soon  as  the  action  ceases,  more  of 
the  hypobromite  solution  is  allowed  to  act  upon  the  ammonium 
salt  and  the  process  is  repeated  until  finally  all  of  the  hypobromite 
is  in  the  outer  part  of  H.  As  soon  as  no  more  gas  is  evolved 
by  shaking,  the  decomposition-bottle  is  placed  hi  water  at  the 
room  temperature  and  after  allowing  it  to  stand  ten  minutes, 
the  volume  of  the  nitrogen  is  read  under  the  conditions  de- 
scribed on  p.  389  The  volume  of  nitrogen  at  0°  and  760  mm. 


*  Loc.  cit. 

t  The  contents  of  the  decomposition-bottle  are  previously  cooled  to  the 
room  temperature  before  the  cock  b  is  connected  with  it. 


824  GAS  ANALYSIS. 

thus  found  will  be  smaller  than  the  theoretical  value  of  35  c.c., 
but  it  corresponds  to  the  amount  of  ammonia  contained  in  10  c.c., 
of  the  ammonium  chloride  solution,  i.e.,  0.05320  gm.  NH3. 

A  number  of  such  determinations  are  carried  out  and  the  mean 
of  the  results  obtained  is  taken  for  the  correct  value. 

After  this,  some  of  the  ammonium  salt  to  be  analyzed  is  weighed 
out,  dissolved  in  water,  and  diluted  so  that  10  c.c.  of  the  solution 
will  contain  approximately  the  same  amount  of  ammonia  as  in 
the  case  of  the  standard  solution.  Then  if,  for  example,  from 
a  gms.  of  an  ammonium  salt,  V  c.c.  of  nitrogen  at  0°  C.  and  760 
mm.  pressure  were  found,  we  have: 

V:  7=0.05320  :x 

7iX  0.05320 

~T~ 
and  in  per  cent.: 

FiX  5.320 

—  -  =per  cent. 


Remark.  —  The  results  obtained  by  this  method  agree  exactly 
with  those  obtained  by  the  distillation  method  described  on  p.  560. 
Only  with  substances  containing  sulphocyanates  are  the  results 
obtained  too  high;  in  this  case  the  sulphocyanate  is  decomposed 
by  the  alkaline  hypobromite  solution  with  evolution  of  nitrogen 
and  carbon  monoxide.  f 

Consequently,  the  above  method  affords  uncertain  results  in 
the  analysis  of  the  ammonia  in  gas  liquors. 

Urea  is  decomposed  by  the  alkaline  hypobromite  solution 
according  to  the  equation: 


*  Lunge  (Lunge-Berl,  Chem.  techn.  Untersuchungsmethoden,  6th  edition, 
Vol.  I,  p.  155)  does  not  standardize  against  the  solution  of  ammonium  chloride 
of  known  strength,  but  adds  2.2  per  cent,  more  ammonia  to  correspond  to 
the  loss  of  nitrogen.  Then  FX0.001558  =  gm.  ammonia. 

f  Donath  and  Pollak,  Zeitschr.  f.  angew.  Chem.,  1897,  p.  555. 


DETERMINATION  OF  NITROUS  AND  NITRIC  ACIDS.         825 


so  that  it  can  be  determined  in  the  same  way  as  ammonium  salts, 
the  carbon  dioxide  produced  by  the  decomposition  being  kept  back 
by  means  of  caustic  soda  solution. 

Determination  of  Nitrous  and  Nitric  Acids. 

Principle.  —  If  a  solution  of  a  nitrite  or  nitrate  be  shaken 
with  mercury  and  an  excess  of  sulphuric  acid,  all  of  the  nitrogen 
is  set  free  as  nitric  oxide: 

2HNO2+2Hg+  H2SO4=2H2O+  Hg2SO4+2NO, 
2HN03  +  6Hg  +  3H2S04  =  4H2O  +  3Hg2SO4  +  2NO. 

From  the  volume  of  the  nitric  oxide,  the  weight  of  the  nitrate 
or  nitrite  is  computed. 

The  analysis  is  best  performed  hi  a  Lunge  nitrometcr.f  The 
latter  is  a  Bunte  burette,  in  which  the  lower  stop-cock  is  lacking  and 
the  lower  end  of  which  is  connected  with  a  levelling-tube  containing 
mercury.  By  raising  the  latter,  the  nitrometer  (which  need  not 
be  graduated)  is  entirely  filled  with  mercury  and  the  two-way 
cock  under  the  funnel  is  closed.  Then  a  weighed  amount  of  the 
substance  dissolved  in  a  little  water  is  placed  in  the  funnel,  the 
levelling-tube  lowered,  and  the  solution  introduced  into  the 
nitrometer  by  carefully  opening  the  cock,  the  funnel  being  finally 
washed  out  four  times  with  two  or  three  c.c.  of  concentrated 
sulphuric  acid.  The  decomposition-tube  is  now  taken  out  of 
the  frame,  it  is  placed  several  times  in  a  nearly  horizontal  position 
and  then  quickly  changed  to  a  vertical  position.  By  this  means 
the  mercury  becomes  intimately  mixed  with  the  acid  and  the 
decomposition  at  once  begins.  The  shaking  is  continued  one  or 
two  minutes  until  there  seems  to  be  no  further  increase  in  the 
volume  of  the  liberated  gas.  The  decomposition  vessel  is  then 

*  This  reaction  does  not  take  place  as  completely  as  with  ammonium  salts. 
Lunge  finds  in  the  determination  of  urea  in  urine  that  the  nitrogen  deficit  is 
9  per  cent.  If,  therefore,  the  volume  of  nitrogen  after  being  reduced  to  0° 
and  760  mm.  is  multiplied  by  2.952,  the  correct  urea  value  is  obtained. 

f  Berichte,  1890,  p.  440,  and  Zeitschr.  f.  angew.  Chem.,  1890,  p.  139. 

J  See  also  Lunge-Berl,  Chem.  techn.  Untersuchungsmethoden,  6th  edition, 
Vol.  I,  p.  156. 


826  GAS  ANALYSIS. 

connected  by  means  of  a  short  piece  of  rubber  tubing  with  the 
gas-burette  filled  with  mercury,  the  nitric  oxide  is  transferred 
to  the  latter,  and  its  volume  read  after  reducing  it  to  the  standard 
conditions  by  means  of  the  gas-compensation  tube.  (Cf.  p.  387, 
Fig.  61,  6.) 

If  in  an  analysis  a  gms.  of  a  nitrate  were  taken  and  VQ  c.c. 
of  NO  were  obtained,  we  have: 

(N03) 
22,391  c.c.:62.01  =  Fo:z 

70X  62.01 
X=     22391 
and  in  per  cent. : 

6201      Fo  =  ()  2769  x—=  per  cent.  N03. 


22391      a  a 

Remark. — For  the  analysis  of  "nitrose,"  *  the  author  knows  of 
no  method  which  affords  such  exact  results. 

For  the  determination  of  nitrous  acid  in  the  presence  of 
nitric  acid  by  a  gas-volumetric  method,  P.  Gerlingerf  treats 
the  neutral  solution  of  the  two  salts  with  a  concentrated  solution 
of  ammonium  chloride,  whereby  the  following  reaction  takes 
place : 

NH4C1 +KN02  =  2H20  +  KC1  +  N2. 

Half  of  the  nitrogen  evolved,  therefore,  comes  from  the  nitrous 
acid  present.  For  the  details  of  this  determination,  the  original 
article  should  be  consulted. 

Hydrogen  Peroxide  Methods. 

Hydrogen  peroxide  is  oxidized  by  means  of  a  number  of 
different  substances,  giving  up  all  of  the  oxygen;  twice  as  much 
oxygen  is  evolved  as  is  obtained  from  the  oxidizing  agent: 

XO  +  H2O2  =  X  +  H20  +  O2. 

*  Cf.  Vol.  I. 

fZ.  angew.  Chem.,  1901,  1250.  See  also  J.  Gaihlot,  J.  Pharm.  Chim., 
1900,  6th  Series,  Vol.  XII,  p.  9. 


STANDARDIZATION  OF  PERMANGANATE  SOLUTIONS.       827 

Since,  however,  hydrogen  peroxide  itself  slowly  decomposes 
on  standing  (the  decomposition  being  measurable  on  shaking 
and  quite  considerable  by  shaking  in  the  presence  of  solid  sub- 
tances  (sand,  etc.)),  it  follows  that  in  the  following  methods  no 
great  excess  of  hydrogen  peroxide  should  be  used,  and  long-con- 
tinued shaking  must  be  avoided. 


Standardization  of  Permanganate  Solutions. 

The  determination  is  best  made  according  to  Lunge  in  a  gas 
volumeter  (p.  387,  Fig.  61).  In  order  to  obtain  correct  results, 
however,  it  is  absolutely  necessary  that  no  excels  of  hydrogen 
peroxide  be  present.  Consequently  it  is  necessary  to  determine 
by  means  of  a  preliminary  experiment  the  exact  value  of  the 
permanganate  solution  in  terms  of  the  H2O2  solution  used  (cf. 
p.  626).  Then  a  measured  amount  of  the  latter  is  placed  in  the 
outside  part  of  the  Wagner  decomposition-bottle  (Fig.  61a,  p.  387), 
and  30  c.c.  of  dilute  sulphuric  acid  (1:5)  are  added.  After  this, 
the  exact  amount  of  hydrogen  peroxide  required  for  the  decom- 
position of  the  permanganate  is  introduced  into  the  inner  part  o~ 
of  the  bottle  and  the  latter  is  connected  with  the  measuring-tube, 
which  is  filled  with  mercury,  the  cock  b  being  removed  for  the 
time  being,  but  it  is  replaced  at  the  end  of  two  or  three  minutes 
and  turned  to  the  position  shown  in  the  figure. 

The  two  liquids  are  then  mixed,  taking  care  to  hold  the  decom- 
position-flask so  that  its  contents  will  not  be  warmed  by  the  heat 
of  the  hand,  inclining  it  to  an  angle  of  about  90°  with  the  vertical, 
and  shaking  for  exactly  one  minute.  While  the  oxygen  is  being 
evolved,  care  must  be  taken  that  the  gas  in  the  eudiometer  is 
under  reduced  pressure.  At  the  end  of  the  decomposition,  the 
gas  is  placed  under  atmospheric  pressure,  b  is  closed,  and  by  means 
of  the  compensation-tube,  the  volume  of  the  gas  is  reduced  to 
what  it  would  be  at  0°  C.  and  760  mm.  pressure,  as  described  on 
p.  388. 

One-half  of  the  observed  volume  corresponds  to  the  amount 
of  oxygen  given  up  by  the  potassium  permanganate.  This  number 
multiplied  by  0.001429  gives  the  weight  of  the  oxygen  obtained 
from  the  permanganate. 


828  GAS  ANALYSIS. 

Remark. — The  amount  of  permanganate  to  be  taken  for  the 
experiment  is  determined  by  the  size  of  the  measuring-tube.  If 

N 

the  latter  has  a   capacity  of   150  c.c.,  15  c.c.  of    a  —     solution 

o 

N 
or  40  to  50  c.c.  of  a  —^  solution  should  be  taken. 

The  hydrogen  peroxide  used  should  not  be  too  concentrated; 
it  should  be  about  a  2  per  cent,  solution.  The  active  oxygen 
present  in  a  sample  of  pyrolusite  *  may  be  determined  by  the 
same  procedure. 

Determination  of  Cerium  in  Soluble  Ceric  Salts. 

If  hydrogen  peroxide  is  added  to  an  acid  solution  of  a  soluble 
eerie  salt,  the  latter  is  reduced  with  evolution  of  oxygen: 

2CeO2 + H2O2  =  Ce2O3 + H2O  +  O2. 

The  determination  is  effected  in  precisely  the  same  way  as 
was  described  above  in  the  standardization  of  the  permanganate 
solution.  If  half  the  volume  of  liberated  oxygen  is  multiplied 
by  0.03077,  the  product  represents  the  corresponding  amount 
of  CeO2.t 

Remark. — If  a  large  excess  of  hydrogen  peroxide  is  avoided 
in  the  above  analysis,  satisfactory  results  will  be  obtained. 

For  other  determinations  of  this  sort,  consult  "  Lunge's  Alkali 
Makers'  Handbook"  and  "Hempel's  Gas- Analytical  Methods." 


Silicon  Fluoride,  SiF4.     Mol.  Wt.  =  104.3. 

Density  =  3.605  (Air=  1).     Weight  of  one  liter  =  4.660  gms. 
Molar  volume  =  22.40  liters. 


*  Lunge's  Alkali  Makers'  Handbook. 

f  Assuming  that  the  atomic  weight  of  Ce=  140.25. 


DETERMINATION  OF  FLUORINE  AS  SILICON  FLUORIDE.       829 


Determination    of  Fluorine    as    Silicon    Fluoride    (Hempel  and 

Oettel).* 

Principle. — If  a  mixture  of  calcium  fluoride  and  powdered 
quartz  is  treated  with  concentrated  sulphuric  acid  in  a  glass 
vessel,  all  of  the  fluorine  will  be  expelled  as  silicon  fluoride: 

2CaF2  +  SiO2  +  2H2SO4  =  2CaSO4  -f  2H2O  +  SiF4, 

and  this  gas  can  be  collected  and  measured. 

One  c.c.  SiF4  at  0°  and  760  mm.  pressure  corresponds  to 
0.006978  gms.  CaF2,  or  0.003395  gms.  F2. 

Procedure. — A  weighed  amount  of  the  very  finely-powdered 
substance,  which  must  not  contain  any  other  acid  that  can  be 
expelled  by  treatment  with  concentrated  sulphuric  acid,f  is 
mixed  with  3  gms.  of  ignited  finely-powdered  quartz  and  intro- 
duced into  the  dry  decomposition  flask  K  (Fig.  126).  The  latter 
is  then  evacuated  somewhat  by  twice  lowering  the  leveling-tube 
A*  with  the  stop-cock  H  open,  closing  the  cock  and  expelling 
the  air.  At  the  beginning  of  the  experiment,  the  burette  H  is 
not  connected  with  the  Orsat  tube  0.  By  raising  the  ground- 
glass  tube  R,  about  30  c.c.  of  concentrated  sulphuric  acid  are 
allowed  to  flow  into  the  flask.  This  acid  must  have  been  pre- 
viously heated  in  a  porcelain  crucible  for  some  time  at  a  tem- 
perature near  the  boiling-point,  in  order  to  detsroy  every  trace 
of  organic  matter,  and  allowed  to  cool  in  a  desiccator  over 
phosphorus  pentoxide.  The  acid  in  K  is  heated  to  boiling  with 
the  stop-cock  H  open  and  the  flask  is  frequently  shaken.  During 
the  entire  experiment,  the  mercury  level  in  the  tube  N  is  kept 
a  little  lower  than  that  of  the  mercury  in  the  measuring-tube  M.% 

At  first  the  sulphuric  acid  foams  considerably,  but  soon  ceases, 
which  is  a  sure  sign  that  the  decomposition  is  complete.  The 
flame  is  then  removed,  the  sulphuric  acid  allowed  to  cool  and 
all  the  gas  in  K  is  expelled  by  introducing  through  V  sulphuric 

*  Gasanalytische  Methoden,  III,  p.  342. 
t  Cf.  p.  479. 

J  In  order  to  keep  the  inside  of  N  perfectly  dry,  2  or  3  c.c.  of  concen- 
trated sulphuric  acid  are  placed  on  top  of  the  mercury. 


83° 


GAS  ANALYSIS. 


acid,  which  has  been  previously  heated  and  cooled  as  described 
above.  As  soon  as  the  sulphuric  acid  reaches  the  stop-cock  H, 
this  is  closed.  After  waiting  ten  minutes  more,  the  gas  is  placed 
under  atmospheric  pressure,  by  suitably  raising  A7,  and  the 
volume  and  temperature  are  noted. 

The  gas  is  now  driven  over  into  the  Orsat  tube  0,  containing 
caustic  potash  solution  (1:2).  The  silicon  tetrafluoride  is  imme- 
diately absorbed.  The  residual  gas  is  carried  back  to  the  tube  M, 
and  after  waiting  fifteen  minutes  the  volume  is  read.  The  differ- 


FIG.  126. 

ence  between  the  two  readings  gives  the  volume  of  silicon  tetra- 
fluoride. 

Remarks. — A.  Koch  tested  this  method  in  the  author's  labor- 
atory and  obtained  results  varying  from  98.97  to  102.63  per  cent, 
with  pure  calcium  fluoride.  To  obtain  this  accuracy,  however, 
it  is  necessary  to  carry  out  the  decomposition  under  approx- 
imately atmospheric  pressure.  When  working  under  a  vacuum 
the  results  were  always  too  low.  Thus  in  one  case  only  85.70 
per  cent,  of  the  theoretical  value  was  obtained. 

During  these  experiments  with  reduced  pressure,  a  white 
sublimate  forms  at  the  lower  part  of  the  condenser,  which  on 
coming  in  contact  with  the  sulphuric  acid  that  is  introduced  at 


DETERMINATION  OF   VAPOR  IN  GAS  MIXTURES.  831 

the  last,  causes  a  strong  effervescence.  Since,  however,  all  the 
gas  in  the  burette  was  replaced,  we  believed  that  the  low  results 
could  be  traced  to  the  absorption  of  silicon  fluoride  by  the  sul- 
phuric acid.  This  idea  proved  to  be  false,  for  a  measured  volume 
of  silicon  fluoride  does  not  change  when  allowed  to  stand  for 
twenty-four  hours  over  concentrated  sulphuric  acid.  The  error, 
therefore,  must  be  caused  by  the  strange  deposit  that  has  con- 
densed in  the  lower  part  of  the  condenser.  If  the  work  is  carried 
out  under  atmospheric  pressure  as  described  above,  the  white 
deposit  is  never  obtained. 

The  method  can  be  used  for  the  estimation  of  fluorine  in  the 
presence  of  carbonates.  In  this  case  the  silicon  fluoride  is 
absorbed  by  means  of  a  little  water  and  the  carbon  dioxide  by 
means  of  caustic  potash.  As,  however,  a  little  of  the  carbon 
dioxide  is  dissolved  by  the  water,  the  gas  residue  which  has  been 
freed  from  carbon  dioxide  is  shaken  with  this  water  again,  whereby 
this  dissolved  carbon  dioxide  is  removed  and  can  be  absorbed 
by  a  further  treatment  with  caustic  potash  solution.  For  further 
details,  consult  the  original  paper  by  Hempel  and  Scheffler.* 

Determination  of  Vapor  in  Gas  Mixtures. 

It  is  often  desired  to  estimate  the  weight,  and  from  this  the 
volume,  of  vapor  present  in  a  mixture  of  gases  and  vapors.  This 
will  be  illustrated  by  one  or  two  examples.  Let  it  be  assumed 
that  in  a  unit  of  volume  of  a  given  gas  mixture  there  are  r'  parts 
of  gas  and  v"  of  vapor.  If  the  whole  mixture  is  under  the 
pressure  P,  then  the  partial  pressures  of  the  gas  and  vapor  will 
be  respectively  v'P  and  v"P'.  The  following  equation  then  holds: 

P=v'P  +  v"P. 

The  total  pressure  is  therefore  equal  to  the  sum  of  the  partial 
pressures. 

If  now  the  partial  pressure  of  either  constituent  is  known,  the 
volume  of  this  constituent  can  be  found  by  dividing  by  the 
total  pressure.  If,  for  example,  v"P=w,  then 


Applications.  —  1.    Reduction  of  Volumes  of  Moist  Gases  to  a 
Dry  Condition  at  0°  C.  and  760  mm.  Pressure. 
*  Z.  anorg.  Chem.,  20,  1  (1897). 


832  GAS  ANALYSIS. 

A  gas  saturated  with  water  vapor  occupies  a  volume  vt  at 
t°  C.  and  P  mm.  pressure.  A  unit  of  volume  of  the  gas  consists 
of  vf  volumes  of  dry  gas  and  v"  volumes  of  water-vapor.  Now 
the  tension  of  aqueous  vapor  at  t°  =  w,  a  value  which  can  be 
obtained  from  the  tables  on  p.  842.  This  value,  w,  represents 
in  fact  the  partial  pressure  of  the  water-vapor.  Consequently 
P—w  is  the  partial  pressure  of  the  dry  constituents  and  the 
volume  of  the  latter,  v',  is,  as  explained  above: 


—  —  at  t°  and  P  mm.  pressure. 


This  volume  reduced  to  0°  and  760  mm.  pressure  is 

'  =        (P-w)P  P-w 

V°~P-7QQ-(l+at)     760(1  +at}' 

If  the  original  volume  of  the  gas  and  vapor  is  not  1,  but  Vt, 
then, 

y>  _      P~w       Vi 
760(1  +  at) 

Similarly,  the  volume  of  the  water-vapor  at  0°  and  760  mm. 
pressure  is 

V"  =  _  -  ___  v  ,  * 
760(1  +at) 

2.  Calculation  of  the  Moisture  in  the  Air  at  Normal  Pressure 
(760  mm.)  and  Temperature  t°. 

What  is  the  volume  of  water-  vapor  contained  in  100  c.c. 
of  moist  air  at  0°,  25°,  and  35°  C.?  According  to  the  table: 
w0  =  4.6  mm.;  w25  =  23.5  mm.;  and  w35  =  41.8  mm.  Hence  the 
volume  of  water-vapor  present  in  a  unit  of  volume  is 


_ 

760  ~~  760  '     760  ~  760       760  ~  760  ' 


*  In  this  formula  is  it  assumed  that  the  water-vapor  also  follows  Boyle's 
law,  which  is  not  strictly  true. 


DETERMINATION  OF   VAPOR  IN  GAS  MIXTURES.  833 

and  the  percentage  of  moisture  is 

0.61  per  cent  at  0°; 
3.09  "  "25°; 
5.50  "  "  35°. 

If  the  gas  is  not  saturated  with  moisture,  the  relation  of  the 
dry  gas  to  the  moist  one  is  not  so  easy  to  determine,  unless  the 
degree  of  saturation  is  known.  The  humidity  of  a  gas,  or  the 
degree  of  saturation,  expresses  the  amount  of  moisture  present 
as  compared  with  the  total  amount  which  the  gas  can  take  up 
when  perfectly  saturated  with  moisture.  Thus  if  the  humidity 
is  50  per  cent,  the  gas  could  take  up  as  much  again  moisture 
at  the  prevailing  temperature. 

If  the  degree  of  humidity  of  a  gas  is  expressed  by  r,  then  the 
volume  of  the  given  gas  at  this  temperature  and  760  mm. pressure, 
is 

V(P-r-w) 
760       ' 

and  the  volume  of  the  water-vapor  present  is 


3.  Calculation  of  the  Weight  of  Water-vapor  in  a  Given  Volume 
of  Air  which  is  Saturated  with  Moisture  at  the  Temperature  f3 
and  the  Pressure  P  mm. 

One  c.c.  of  water-vapor  weighs  0.000801  gm.  at  0°  C.  and 
760  mm.  pressure. 

One  c.c.  of  water-vapor  at  t°  and  P  mm.  pressure  occupies  a 
volume  of 

l-P 

^~~  760(1  +at)' 
and  weighs 


If  now  w=   the  vapor  pressure  of  water  at  t°  and  P  mm, 


834  GAS  4N A  LYSIS. 

pressure;  then  the  volume  of  water-vapor  present  in  the  volume 
V  of  the  gas  is,  under  these  conditions, 

w-Vt 
P    ' 

and  the  weight  of  the  water-vapor  amounts  to 


760(1  +at)  P  760(1  +at) 

If  the  weight  of  the  water-vapor  is  g,  then  the  yolume  of 
the  moist  air  is 

_g •  760(1 +«Q  ^ 
V'~~  "0.000801 -10  ' 

If  the  gas  is  not  saturated  with  water  vapor,  but  the  degree  of 
saturation  (the  humidity)  is  known,  then  the  following  formula 
gives  the  weight  of  water-vapor  present  in  the  volume  vt  of  the 
gas, 

0.000801  -r-w-vt 

9  =  — ?ftnn  -L    A —   Sm3' 
7bO(l  -rat) 

Or,  if  the  weight  of  vapor  present  in  a  given  gas  volume  is 
known,  then  from  the  last  equation  the  humidity,  r,  of  gas  may 
be  computed: 

_  gf- 760(1 +aQ 
~~  0.000801-  w-vt 

4.  Calculation  of  the  Weight  of  One  Liter  of  Air  at  0°  and  760  mm. 
when  Free  from  Moisture  and  Carbon  Dioxide. 

1  c.c.  of  pure,  dry  air  weighs  0.0012928  gm.  at  0°  and  760  mm. 
1  cc.  of  water-vapor  weighs  0.000801  gm.  at  0°  and  760  mm. 
1  cc.  of  water-vapor  is  therefore  0.62  times  as  heavy  as  1  c.c.  of  air 
1  c.c.  of  CO2  weighs  0.001977  gm.  at  0°  and  760  mm. 


*  Similarly,  the  volume  of  a  gas  saturated  with  any  other  vapor  can 
be  computed  if  the  weight  of  vapor  present  and  its  density  are  known. 


DETERMINATION  OF  VAPOR  IN  GAS  MIXTURES.  835 

Air  contains,  on  an  average,  0.03  per  cent  of  C(>2.     One    liter 
of  dry  air,  containing  the  average  amount  of  CC>2  consists  of 

999.7  c.c.air       and  weighs     999.7X0.001293  =  1.2924  gms. 
0.3   "    CO2  0.3X0.001977  =  0.0006 


i  { 


1000.0   '  1.2930  "      =a 

The  corresponding  volume  of  dry  air  and  of  water-vapor  at  0° 
and  760°  mm.  pressure  is 

760 -w     jv_ 
760        760 
and  weighs 

760 -w          w 
760  760 

or,  in  other  words,  1  liter  of  moist  air  weighs  at  0°  and  760  mm. 
pressure, 


5.  Calculation  of  the  Weights  of  One  Liter  of  Moist  Air  Con- 
taining Carbon  Dioxide  at  t°  and  P  mm.  Pressure. 

If  the  tension  of  aqueous  vapor  =  wt,  then  the  volume  of  the 
moist  gas  is 

•J2 

-p-  -\ =— ! •  =  1  at  t°  and  P  mm.  pressure, 

and  at  0°  and  760  mm. 

wt  760 -wt 


760(1  +at)     760(1  +at)' 
and,  as  shown  under  4,  this  weight, 


or 


836  GAS  ANALYSIS. 

0.46445  (P-0.38w) 

—  273.1*  -  =  the  weight  of  1  nter  of  moist  air  at  t°  and 

P  mm.  pressure. 
V  liters  of  moist  air  would  therefore  weigh: 


,_ 
9  ~ 


0.46445  (P-0.38ti?) 
273  +  t 


APPENDIX. 


838 


APPENDIX. 


SPECIFIC  GRAVITY  OF  STRONG  ACIDS  AT          IN  VACUO. 

(According  to  LUNGE,  ISLER,  NAEF,  and  MARCHLEWSKY.)* 


Specific 
Gravity 

atf 
(Vacuo). 

Per  Cent,  by  Weight. 

Specific 
Gravity 

Per  Cent,  by  Weight. 

HC1. 

HNO3 

H2SO4. 

at  > 
(Vacuo). 

HNO3. 

H2SO4. 

1.000 

0.16 

0.10 

0.09 

1.235 

37.51 

31.70 

1.005 

1.15 

1.00 

0.95 

.240 

38.27 

32.28 

1.010 

2.14 

1.90 

1.57 

.245 

39.03 

32.86 

1.015 

3.12 

2.80 

2.30 

.250 

39.80 

33.43 

1.020 

4.13 

3.70 

3.03 

.255 

40.56 

34.00 

.025 

5.15 

4.60 

3.76 

.260 

41.32 

34  .  57 

.030 

6.15 

5.50 

4.49 

.265 

42.08 

35.14 

.035 

7.15 

6.38 

5  23 

.270 

42.85 

35.71 

.040 

8.16 

7.26 

5.96 

.275 

43.62 

36.29 

.045 

9.16 

8.13 

6.67 

.280 

44.39 

36.87 

.050 

10.17 

8.99 

7.37 

.285 

45.16 

37.45 

.055 

11.18 

9.84 

8.07 

.290 

45.93 

38.03 

.060 

12.19 

10  67 

8.77 

.295 

46.70 

38.61 

.065 

13.19 

11.50 

9.47 

.300 

47.47 

39.19 

.070 

14.17 

12.32 

10.19 

.305 

48.24 

39.77 

.075 

15.16 

13.14 

10.90 

.310 

49.05 

40.35 

.080 

16.15 

13.94 

11.60 

.315 

49.88 

40.93 

.085 

17.13 

14.73 

12.30 

.320 

50.69 

41.50 

.090 

18.11 

15.52 

12.99 

.325 

51.51 

42.08 

.095 

19.06 

16.31 

13.67 

.330 

52.34 

42.66 

.100 

20.01 

17.10 

14.35 

.335 

53.17 

43.20 

.105 

20.97 

17.88 

15.03 

.340 

54.04 

43.74 

.110 

21.92 

18.66 

15.71 

.345 

54.90 

44.23 

.115 

22.86 

19.44 

16.36 

.350 

55.76 

44  .  82 

.120 

23.82 

20.22 

17.01 

.355 

56.63 

45.35 

.125 

24.78 

20.99 

17.66 

.360 

57.54 

45.88 

.130 

25.75 

21.76 

18.31 

.365 

58.45 

46.41 

.135 

26.70 

22.53 

18.96 

.370 

59.36 

46.94 

.140 

27.66 

23.30 

19.61 

.375 

60.27 

47.47 

.145 

28.61 

24.07 

20.26 

.380 

61.24 

43.00 

.150 

29.57 

24.83 

20.91 

.385 

62.21 

48.53 

.155 

30.55 

25.59 

21  .  55 

.390 

63.20 

49.06 

,160 

31.52 

26.35 

22.19 

.395 

64.22 

49.59 

.165 

32.49 

27.11 

22.83 

.400 

65.27 

50.11 

.170 

33.46 

27.87 

23.47 

.405 

66.37 

50.63 

.175 

34.42 

28.62 

24.12 

.410 

67.47 

51.15 

.180 

35.39 

29.37 

24.76 

.415 

68.60 

51.66 

.185 

36.31 

30.12 

25.40 

.420 

69.77 

52.15 

.190 

37.23 

30.87 

26.04 

.425 

70.95 

52.63 

.195 

38.16 

31.60 

26.68 

.430 

72.14 

53.11 

.200 

39.11 

32.34 

27.32 

.435 

73.35 

53.59 

.205 

33.07 

27.95 

.440 

74.64 

54.07 

.210 

33.80 

28.  53 

.445 

75.94 

54.55 

.215 

• 

34.53 

29.21 

.450 

77.24 

55.03 

.220 

35.26 

29.84 

.455 

78.56 

55.50 

.225 

36.01 

30.48 

1.460 

79-94 

55.97 

.230 

36.76 

31.11 

1.465 

81.38 

56.43 

*  Lunge-Berl,  Chem.  techn.  Untersuchungsmethoden,  6th  edition,  Vol.  I, 
Tables,  61,  39,  and  49. 


SPECIFIC  GRAVITY  OF  STRONG  ACIDS. 


839 


SPECIFIC  GRAVITY  OF  STRONG  ACIDS  AT  IN    VACUO.— Cont. 

(According  to    ISLER,  XAE?,  and  MARCHLEWSKT.) 


G^vify           Per  Cent,  by  Weight. 

15° 
at  — 

Specific 
Gravity 
15° 
at   4^- 
!    (Vacuo). 

Per  Cent, 
by 
Weight. 

Specific 
Gravity 

"f 

(Vacuo). 

Per  Cent, 
by 
Weight. 

(VaCUo)-           HN03. 

HaSO,. 

Hi;SO4. 

HzSO,. 

.470 

82.86 

56.90 

.610 

.    69.56 

.750 

81.56 

.475 

84.41 

57.37 

.615 

70.00 

.755 

82.00 

.480 

86.01 

57.83 

.620 

70.42 

.760 

82.44 

.485 

87.66 

58.28 

.625 

70.85 

.765 

83.01 

.490 

89.86 

58.74 

.630 

71.27 

.770 

83.51 

.495 

91.56 

59.22 

.635 

71.70 

.775 

84.02 

.500 

94.04 

59.70 

.640 

72.12 

.780 

84.50 

.505 

96.34 

60.18 

1.645 

72  .  55 

.785 

85.10 

.510 

98.05 

60.65 

1.650 

72.96 

.790 

85.70 

.515 

99.02 

61.12 

1.655 

73.40 

.795 

86.30 

.520 

99.62 

61.59 

.660 

73.81 

.800 

86.92 

.525 

62.06 

.665 

74  .  24 

.805 

87.60 

.530 

62.53 

.670 

74.66 

.810 

88.30 

.535 

63.00 

.675 

75  .  08 

.815 

89.16 

.540 

63.43 

.680 

75.50 

.820 

90.05 

.545 

63.85 

.685 

75  .  94 

.825 

91.00 

.550 

64.26 

.690 

76.38 

.830 

92.10 

.555 

64.67 

.695 

76.76 

.835 

93.56 

.560 

65.20 

.700 

77.17 

.840 

95.60 

.565 

65.65 

.705 

77.60 

.8405 

95.95 

.570 

66.09 

.710 

78.04 

.8410 

96.38 

.575 

66.53 

.715 

78.48 

.8415 

97.35 

.580 

66.95 

1.720 

78.92 

.8410 

98.20 

.585 

67.40 

1.725 

79.36 

.8405 

98.52 

1  .  590 

67.83 

1.730 

79.80 

.8400 

98.72 

1  .  595 

68.26 

1.735 

80.24 

.8395 

98.77 

1.600 

68.70 

1.740 

80.68 

.8390 

99.12 

1.605 

69.13 

1.745 

81.12 

.8385 

99.31 

840 


APPENDIX. 


SPECIFIC  GRAVITY  OF  POTASSIUM  AND  SODIUM  HYDROXIDE 
SOLUTIONS  AT  15°  C. 


Specific 
Gravity. 

Per  Cent. 
KOH. 

Per  Cent. 
NaOH. 

Specific 
Gravi.y. 

Per  Cent. 
KOH. 

Per  Cent. 
NaOH. 

1.007 

0.9 

0.59 

1.252 

27.0 

22.50 

1.014 

1.7 

1.20 

1.263 

23.2 

23.50 

1.022 

2.6 

1.65 

1.274 

28.9 

24  .  48 

1.029 

3.5 

2.50 

1.2  5 

29.8 

25.50 

1.037 

4.5 

3.22 

.    1.297 

30.7 

26.58 

1.045 

5.6 

3.79 

1.303 

31.8 

27.65 

1.052 

6.4 

4.50 

1.320 

32.7 

28.83 

.060 

7.4 

5.20 

.332 

33.7 

30.00 

.067 

8.2 

5.86 

.345 

34.9 

31.20 

.075 

9.2 

6.58 

.357 

35.9 

32.50 

.083 

10.1 

7.30 

.370 

36.9 

33.73 

.091 

10.9 

8.07 

.3-3 

37.8 

35.00 

.100 

12.0 

8.78 

.397 

38.9 

36.36 

.108 

12.9 

9.50 

1.410 

39.9 

37.65 

.116 

13.8 

10.30 

1.424 

40.9 

39.06 

.125 

14.8 

11.06 

1.438 

42.1 

40.47 

.134 

15.7 

11.90 

1.453 

43.4 

42.02 

.142 

16.5 

12.69 

1.468 

44,6 

43.58 

.152 

17.6 

13.50 

1.483 

45.8 

45.16 

.162 

18.6 

14.35 

1.498 

47.1 

47.73 

.171 

19.5 

15.15 

1.514 

48.3 

48.41 

.180 

20.5 

16.00 

1.530 

49.4 

50.10 

.190 

21  .4 

16.91 

1.546 

50.6 

— 

1.200 

22.4 

17.81 

1.563 

51.9 

— 

1.210 

23.3 

18.71 

1.580 

53.2 

— 

1.220 

24.2 

19.65 

1.597 

54.5 

— 

1.231 

25.1 

20.69 

1.615 

55.9 

— 

1.241 

26.1 

21.55 

1.634 

57.5 

SPECIFIC  GRAVITY  OF  AMMONIA  SOLUTIONS. 


841 


SPECIFIC  GRAVITY  OF  AMMONIA  SOLUTIONS  AT    15°  C. 
(According  to  LUNGE  and  WIERNIK.)* 


Specific  Gravity. 

Per  Cent.  NH3. 

Specific  Gravity. 

Per  Cent.  NH3. 

1.000 

0.00 

0.940 

15.63 

0.993 

0.45 

0.938 

16.22 

0.996 

0.91 

0.936 

16.82 

0.994 

1.37 

0.934 

17.42 

0.992 

1.84 

0.932 

18.03 

0.990 

2.31 

0.930 

18.64 

0.988 

2.80 

0.928 

19.25 

0.986 

3.30 

0.926 

19.87 

0.984 

3.80 

0.924 

20.49 

0.982 

4.30 

0.922 

21.12 

0.980 

4.80 

0.920 

21.75 

0.978 

5.30 

0.918 

22.39 

0.976 

5.80 

0.916 

23.03 

0.974 

6.30 

0.914 

23.68 

0.972 

6.80 

0.912 

24.33 

0.970 

7.31 

0.910 

24.99 

0.968 

7.82 

0.903 

25.65 

.0.966 

8.33 

0.906 

26.31 

0.964 

8.84 

0.904 

26.98 

0.962 

9.35 

0.902 

27.65 

0.960 

9.91 

0.900 

23.33 

0.958 

10.47 

0.898 

29.01 

0.956 

11.03 

0.896 

29.69 

0.954 

11.60 

0.894 

30.37 

0.952 

12.17 

0.892 

31.05 

0.950 

12.74 

O.S90 

31.75 

0.948 

13.31 

0.888 

32.50 

0.946 

13.88 

0.886 

33.25 

0.944 

14.46 

0.884 

34.10 

0.942 

15.04 

0.882 

34.95 

*  Lunge-Berl,  Chem.  techn.  Untersuchungsmetho  den,  6th 


p.  531. 


842  APPENDIX. 

TENSION  OF  WATER  VAPOR  ACCORDING  TO  REGNAULT. 


Degrees, 
C. 

Tension  in 
Millimeters. 

Degrees, 

Tension  in 
Millimeters. 

Degrees, 

Tension  in 
Millimeters. 

-2.0 

3.9£5 

+  2.0 

5.302 

+  6.0 

6.998 

1.9 

3.985 

2.1 

5.340 

6.1 

7.047 

1.8 

4.016 

2.2 

5.378 

6.2 

7.095 

1.7 

4.047 

2.3 

5.416 

6.3 

7.144 

1.6 

4.078 

2.4 

5.454 

6.4 

7.193 

1.5 

4.109 

2.5 

5.491 

6.5 

7.242 

1.4 

4.140 

2.6 

5.530 

6.6 

7.292 

1.3 

4.171 

2.7 

5.569 

6.7 

7.342 

1.2 

4.203 

2.8 

5.608 

6.8 

7.392 

1.1 

4.235 

2.9 

5.647 

6.9 

7.442 

1.0 

4.267 

3.0 

5.6S7 

7.0 

7.492 

0.9 

4.299 

3.1 

5.727 

7.1 

7.544 

0.8 

4.331 

3.2 

5.767 

7.2 

7.595 

0.7 

4.364 

3.3 

5.807 

7.3 

7.647 

0.6 

4.397 

3.4 

5.848 

7.4 

7.699 

0.5 

4.430 

3.5 

5.889 

7.5 

7.751 

0.4 

4.463 

3.6 

5.930 

7.6 

7.804 

0.3 

4.497 

3.7 

5.972 

7.7 

7.857 

0.2 

4.531 

3.8 

6.014 

7.8 

7.910 

0.1 

4.565 

3.9 

6.055 

7.9 

7.964 

0.0 

4.600    . 

4.0 

6.097 

8.0 

8.017 

+0.1 

4.633 

4.1 

6.140 

8.1 

8.072 

0.2 

4.667 

4.2 

6.183 

8.2 

8.126 

0.3 

4.700 

4.3 

6.226 

8.3 

8.181 

0.4 

4.733 

4.4 

6.270 

8.4 

8.236 

0.5 

4.767 

4.5 

6.313 

8.5 

8.291 

0.6 

4.801 

4.6 

6.357 

8.6 

8.347 

0.7 

4.836 

4.7 

6.401 

8.7 

8.404 

0.8 

4.871 

4.8 

6.445 

8.8 

8.461 

0.9 

4.905 

4.9 

6.490 

8.9 

8.517 

.0 

4.940 

5.0 

6.534 

9.0 

8.574 

.1 

4.975 

5.1 

6.580 

9.1 

8.632 

.2 

5.011 

5.2 

6.625 

9.2 

8.690 

.3 

5.047 

5.3 

6.671 

9.3 

8.748 

.4 

5.082 

5.4 

6.717 

9.4 

8.807 

1.5 

5.118 

5.5 

6.763 

9.5 

8.865 

1.6 

5.155 

5.6 

6.810 

9.6 

8.925 

1.7 

5.191 

5.7 

6.857 

9.7 

8.985 

1.8 

5.228 

5.8 

6.904 

9.8 

9.045 

1.9 

5.265 

5.9 

6.951 

9.9 

9.105 

TENSION  OF  WATER   YAPOR. 
TENSION  OF  WATER  VAPOR.— Continved. 


843 


Degrees,           Tension  in 
C.                Millimeters. 

Degrees. 

Tension  in 
Millimeters 

Degrees 
C 

Tension  m 
Millimeters 

+  10.0             9.165 

+  14.0 

11-  90S 

+  18.0 

15.357 

10.1 

9.227 

14.1 

11.986 

18.1 

15  454 

10.2 

9.2<8 

14.2 

12.06i 

18.2 

15.552 

10.3 

9.350 

14.3 

12.142 

18.3 

15.650 

10.4 

9.412 

14.4 

12.220 

18.4 

15.747 

10.5 

9.474 

14.5 

12.298 

18.5 

15.845 

10.6 

9.537 

14.6 

12.378 

18.6 

15.945 

10.7 

9.601 

14.7 

12.45S 

18.7 

16.045 

10.8 

9.665 

14.8 

12.538 

18.8 

16.145 

10.9 

9.728 

14.9 

12.619 

18.9 

16.246 

11.0 

9.792 

15.0 

12.699 

19.0 

16.346 

11.1 

9.857 

15.1 

12.781 

19.1 

16.449 

11.2 

9.923 

15.2 

12.864 

19.2 

16.552 

11.3 

9.989 

15.3 

12.947 

19.3 

16.655 

11.4 

10.054 

15.4 

13.029 

19.4 

16.758 

11.5 

10.120 

15.5 

13.112 

19.5 

16.861 

11.6 

10.187 

15.6 

13.197 

19.6 

16.967 

11.7 

10.255 

15.7 

13.2S1 

19.7 

17.073 

11.8 

10.322 

15.8 

13.366 

19.8 

17.179 

11.9 

10.389 

15.9 

13.451 

19.9 

17.285 

12.0 

10.457 

16.0 

13.536 

20.0 

17.391 

12.1 

10.526 

16.1 

13.623 

20.1 

17.500 

12.2 

10.596 

16.2 

13.710 

20.2 

17.608 

12.3 

10.665 

16.3 

13.797 

20.3 

17.717 

12.4 

10.734 

16.4 

13.885 

20.4 

17.826 

12.5 

10.804 

16.5 

13.972 

20.5 

17  935 

12.6 

10.875 

16.6 

14.062 

20.6 

18.047 

12.7 

10.947 

16.7 

14.151 

20.7 

18.159 

12.8 

11.019 

16.8 

14.241 

20.8 

18.271 

12.9 

11.090 

16.9 

14.331 

20.9 

18.383 

13.0 

11.162 

17.0 

14.421 

21.0 

18.495 

13.1 

11.235 

17.1 

14.513 

21.1 

18.610 

13.2 

11.309 

17.2 

14.605 

21.2 

18.724 

13.3 

11.383 

17.3 

14.697 

21.3 

18.839 

13.4 

11.456 

17.4 

14.790 

21.4 

18.954 

13.5 

11.530 

17.5 

14.882 

21.5 

19.069 

13.6 

11.605 

17.6 

14.977 

21.6 

19.187 

13.7 

11.681 

17.7 

15.072 

21.7 

19.305 

13.8 

11.757 

17.8 

15.167 

21.8 

19.423 

13.9 

11.832 

17.9 

15.262 

21.9 

19.541 

844 


APPENDIX. 
TENSION  OF  WATER  VAPOR.—  Continued. 


Degrees 
C. 

Tension  in 
Millimeters. 

Degrees, 
C. 

Tension  in 
Millimeters. 

Degrees, 
C. 

Tension  in 
Millimeters. 

+  22.0 

19.659 

+  26.0 

24.9S8 

+  30.0 

31.548 

22.1 

19.  7.-  0 

26.1 

25.133 

30.1 

31.729 

22.2 

19.901 

26.2 

25.238 

30.2 

31.911 

22.3 

20.022 

26.3 

25.433 

30.3 

32.094 

22.4 

20.143 

26.4 

25.538 

30.4 

32.278 

?2.5 

20.265 

26.5 

25.738 

30.5 

32.463 

22.6 

20.3S9 

26.6 

25.  £91 

30.6 

32.650 

22.7 

20.514 

26.7 

26.045 

30.7 

32.537 

22.8 

20.639      ;         26.8 

26.193 

30.8 

33.026 

22.9 

20.763 

26.9 

26.351 

30.9 

33.215 

23.0 

20.888 

27.0 

26.505 

31.0 

33.405 

23.1 

21.016 

27.1 

26.663 

31.1 

33  .  596 

23.2 

21  .  144 

27.2 

26.820 

31.2 

33.787 

23.3 

21.272 

27.3 

26.978 

31.3 

33.930 

23.4 

21.400 

27.4 

27.136 

31.4 

34.174 

23.5 

21.528 

27.5 

27.294 

31.5 

34.368 

23.6 

21.659 

27.6 

27.455 

31.6 

34.564 

23.7 

21.790 

27.7 

27.617 

31.7 

34.761 

23.8 

21.921 

27.8 

27.778 

31.8 

34.959 

23.9 

22.053 

27.9 

27.939 

31.9 

35.159 

24.0 

22.184 

28.0 

28.101 

32.0 

35.359 

24.1 

22.319 

28.1 

28.267 

32.1 

35.559 

24.2 

22.453 

28.2 

28.433 

32.2 

35.760 

24.3 

22.588 

28.3 

23.599 

32.3 

35.962 

24.4 

22.723 

28.4 

23.765 

32.4 

36.165 

24.5 

22.858 

28.5 

28.931 

32.5 

36.370 

24.6 

22.996 

28.6 

29.101 

32.6 

36.576 

24.7 

23.135 

28.7 

29.271 

32.7 

36.783 

24.8 

23.273 

23.8 

29.441 

32.8 

36.991 

24.9 

23.411 

28.9 

29.612 

32.9 

37.200 

25.0 

23.550 

29.0 

29  .  782 

33.0 

37.410 

25.1 

23.692 

29.1 

29.956 

33.1 

37.621 

25.2 

23.834 

29.2 

30.131 

33.2 

37.F32 

25.3 

23.976 

29.3 

30.305 

33.3 

33.045 

25.4 

24.119 

29.4 

30.479 

33.4 

38.258 

25.5 

24.261 

29.5 

30.654 

33.5 

38.473 

25.6 

24.406 

29.6 

30>33 

33.6 

38.6^9 

25.7 

24.552 

29.7 

31.011 

33.7 

38.906 

25.8 

24.697 

29.8 

31.190 

33.8 

39.124 

25.9 

24.842 

29.9 

31.369 

33.9 

39.344 

HEAT  OP  COMBUSTION  Of-   GAS. 
TENSION  OF  WATER  VAPOR.— Continued. 


845 


. 

Degrees, 
C. 

Tension 
Millimeters. 

Degrees, 
C. 

Tension 
Millimeters.    , 

Degrees, 
<J. 

Tension. 
Millimeters. 

+  34.0 

39.565 

+  34.4 

40.455 

+  34.  S 

41.364 

34.1 

39.7c6 

34.5 

40.680 

34.9 

41.595 

34.2              40.007 

34.6 

40.907 

34.3              40.230 

34.7 

41.135 

35.0 

41.827 

HEATS  OF  COMBUSTION  OF  1  LITER  OF  GAS  MEASURED  AT  0° 
AND  760  MM.  BAROMETRIC  PRESSURE. 


Gas. 

Weight  of  One 
Liter. 

Referred  to 

Gaseous  Water 
Calories. 

Liquid   Water 
Calories. 

Carbon  monoxide 

1.25016 
0.09004 
0.71488 
1.25899 
1.93660 
3.48428 
1.18080 

3,034 
2,595 
8,505 
14,018 
21,226 
(?)  33,750 
13,582 
about       900 
3,386 
1,400 
5,000 

3,034 
3,077 
9,469 
14,989 
22,720 
(?)  35,198 
14,073 
about    1,000 
3,700 

5,500 

Hydrogen 

Methane                    

Ethylene                        .... 

Propylene              

Benzene-gas              

Generator-gas 

\Vater-gas 

Dowson  gas 

Illuminating-gas  

The  values  in  the  above  table  are  based  upon  Thomsen's  measurements 
and  only  hi  the  case  of  benzene  is  the  theoretical  density  used.* 

*  Julius  Thomsen,  Thermochem.  Untersuchungen  (1882),  Vol.  II,  pp.  56, 
85,  107,  and  Vol.  IV,  p.  254. 

CO    +  O   =  CO2    +67,960  cals. 

H2     +  O  =  H2O  +68,357  cals. 

CH4  +4O  =2H2O+  CO2  +  211,930cals. 

C2H4  +  6O  =  2H2O  +  2CO2  +  333,350  cals. 

C8H6  +  9O   =  3H2O  +  3CO2  +  492,740  cals. 

CflHe  +  15O  =  3H2O  +  6CO2  + 787,950  cals. 

C2H2  +  5O  =  H2O  +  2CO2  + 310,450  cals. 


*  TABLES   FOR  CALCULATING  ANALYSES. 

DIRECTIONS  FOR  USING  THE  TABLES. 

Assume  that  it  is  desired  to  find  the  per  cent,  of  arsenic  present 
in  an  ore.  For  this  purpose  a  gms.  of  the  ore  are  taken  for  the 
analysis  and  the  arsenic  is  determined  as  Mg2As2O7;  the  ignited 
precipitate  weighed  p  gms.  The  calculation  of  the  arsenic  is 
made  as  follows: 

Mg2As2O7:As2=/>:5 

As^ 

-M&AsA     P 
and  in  per  cent. 


™        ^ 

Mg2As207 

x_  IQQ.As,     p 
Mg2As2O7    a 

The  last  equation  is  solved  for  x,  after  the  proper  values  for 
p,  a,  etc.,  have  been  inserted,  with  the  aid  of  logarithms.     From 

the  tables,  the  logarithm  of  ^  —  r—  —  is  taken,  to  it  the  logarithm 


of  p  is  added,  and  from  the  sum  the  logarithm  of  a  is  deducted. 
The  number  corresponding  to  this  difference  (found  in  the  table 
of  antilogarithms)  represents  the  desired  result  in  per  cent. 

Example.  —  0.5  gm.  of  mispickel  gave  0.4761  gin.  Mg2As20^. 
What  is  the  per  cent,  of  arsenic  in  the  ore? 

In  the  following  table  under  the  heading  Sought  we  find 
As,  and  under  Found  we  look  for  MgjAs^,  the  form  in  which 


*  The  tables  in  this  book  can  be  purchased  separately  in  flexible  cloth 
binding.     Price,  thirty-five  cents. 

847 


848  TABLES  FOR.   CALCULATION  OF  THE  ANALYSIS. 

the  arsenic  was  determined,  and  then  from  the  column  headed 
Log  we  take  the  logarithm  of  this  factor  multiplied  by  100; 

log.  f actor X 100  =1.6837 
+  log.  0.4761  (p)  =9.6777-10 

1.3614 
-  log.  0.5        (a)  =9.6990 -10 

1.6624 
Number  =  45. 96  per  cent,  arsenic. 

For  most  analytical  computations  the  accompanying  four- 
place  logarithms  are  sufficiently  accurate;  where  greater  accuracy 
is  desired  tables  of  five-  or  even  seven-place  logarithms  may  be 
used. 


INTrRNATlONAL   ATOMIC  WEIGHTS. 
International  Atomic  Weights,  1910. 


849 


Symbol 

Atomic 
Weight. 

Symbol 

Atomic 
Weight 

Aluminium  
Antimony  

Al 
Sb 

27.1 
120  2 

Molybdenum  
Xeodymum 

Mo 
Nd 

96.0 
144  3 

Argon 

A 

39  9 

Neon 

Ne 

20  0 

Arsenic 

As 

74  96 

Nickel 

Ni 

58  68 

Barium  

Ba 

137.37 

Nitrogen 

N 

14  01 

Bismuth           .  .  . 

Bi 

208  0 

Osmium 

Os 

190  9 

Boron 

B 

11  0 

'  Oxveen 

o 

16  00 

Bromine  

Br 

79.92 

Palladium 

Pd 

106  7 

Cadmium       .  . 

Cd 

112  40 

Phosphorus 

p 

31  0 

Caesium  

Cs 

132.81 

Platinum  

Pt 

195  0 

Calcium  

Ca 

40.09 

Potassium       .    . 

K 

39  10 

Carbon     

c 

12  00 

Praseodymium 

Pr 

140  6 

Cerium 

Ce 

140  25 

Radium 

Ra 

99(5    4 

Chlorine  

Cl 

35.46 

Rhodium  

Rh 

102  9 

Chromium     .    . 

Cr 

52  0 

Rubidium 

Rb 

85  45 

Cobalt  

Co 

58.97 

Ruthenium  

Ru 

101  7 

Columbium   

Cb 

93  5 

Samarium  

Sa 

150  4 

Conner 

Cu 

63  57 

Scandium 

Sc 

44   1 

Dysprosium  

Dy 

162.5 

Selenium  

Se 

79  2 

Erbium            .... 

Er 

167.4 

Silicon. 

Si 

28  3 

Europium 

Eu 

152  0 

Silver 

Ae 

107  88 

Fluorine  

F 

19.0 

Sodium  

Na 

23  00 

Gadolinium 

Gd 

157  3 

Strontium 

Sr 

87  62 

Gallium 

Ga 

69  9 

Sulphur 

s 

32  07 

Germanium 

Ge 

72  5 

Tantalum 

Ta 

181  0 

Glucinum*          .  . 

Gl 

9.1 

Tellurium  

Te 

r'7  ") 

Gold 

Au 

197.2 

Terbium 

Tb 

159  2 

Helium 

He 

4  0 

Thallium 

Tl 

204  0 

Hydrogen  

H 

1.008 

Thorium  

Th 

232  42 

Indium 

In 

114  8 

Thulium  . 

Tm 

168  5 

Iodine  

I 

126.92 

Tin  

Sn 

119  0 

Indium        

Ir 

193.1 

Titanium  

Ti 

48  1 

Iron                 .    . 

Fe 

55.85 

Tungsten 

W 

184  0 

Ivrvpton 

Kr 

83  0 

Uranium 

u 

2S8  T 

Lanthanum     

La 

139.0 

Vanadium 

V 

51  2 

Lead 

Pb 

207  10 

Xenon 

Xe 

130  7 

Lithium   

Li 

7.00 

Ytterbium 

Lutecium     

Lu 

174.0 

(  Xeoyt  terbium) 

Yb 

172  0 

Magnesium 

Mg 

24.32 

Yttrium 

Yt 

89  0 

Manganese 

Mn 

54  93 

Zinc 

Zn 

65  37 

Mercury 

Hg 

200  0 

Zirconium 

Zr 

90  6 

*  Also  called  Beryllium.  Be. 


850 


TABLE  OF  CHEMICAL  FACTORS. 


Sought. 

Found. 

Factor. 

Log.* 

Sought 

Found. 

Factor. 

Log. 

Ag 

AgCl 
AgBr 
Agl 

0.75262 
0.57444 
0.45946 

1.87658 
1.75925 
1.66224 

BaO 

BaSO4 
BaOO4 
BaSiF6 

0.65700 
0.60532 
0.54840 

1.81757 

1.78199 
1.73909 

Ag20 

AgCl 

0.80843 

1.90764 

Bi 
Bi203 

Bi203 
BiAsO4 
Bi 

0.89654 
0.59942 
1.1154 

1.95258 
1.77773 
2  .  04743 

Al 

ALA, 

A1PO4 

0.53033 
0.22195 

1.72455 
1.34625 

Br 

Ag 
AgBr 
AgCl 

0.74082 
0.42556 
0.55755 

1.86971 
1.62896 
1  .  74629 

A1203 

A1PO4 

0.41851 

1.62171 

As 

ASaS, 

As2S5 
Mg2As207 
Mg2P2Q7 

0.60911 
0.48319 
0.48269 
0.67338 

1  .  78470 
1.68412 
1.68371 

1.82826 

C 
CO3 

C02 
CO2 

0.27273 
1  .  3636 

1.43573 
2.13470 

Ca 

CaO 
CaCO3 
CaSO4 
CaF2 

0.71474 
0.40054 
0.29443 
0.51338 

1.85415 
1.60265 
1.46899 
1.71044 

As203 

AS^ 
AS& 

Mg2As2O7 
Mg2P207 

0.80405 
0.63783 
0.63724 

0.88897 

1  .  90528 
1.80471 
1.80430 

1.94887 

CaO 

CaCO3 
CaSO4 
CaF2 

0.56040 
0.41194 
0.71827 

1  .  74850 
1.61484 
1  .  85629 

AsO3 

As2S3 
As2S5 
Mg2As207 
Mg2P207 

0.99915 
0.79260 
0.79186 
1.1046 

1.99963 
1.89905 
1.89865 
2.04319 

Cd 

CdS 
CdO 
CdSO4 

0.77801 
0.87539 
0.53919 

1.89099 
1.94220 
1.73174 

As205 

£;t: 
Kb0; 

0.93414 
0.74103 
0.74034 
1.0327 

1.97041 
1.86984 
1  .  86943 
2.01397 

CdO 

CdS 
Cd 
CdSO4 

0.88877 
1.14235 
0.61591 

1.94879 
2  .  05780 
1  .  78952 

AsO4 

It 
MM*a 

1  .  1292 
0.89574 
0.89490 
1.2483 

2.05276 
1.95218 
1.95178 
2.09631 

CdS 

Cd 
CdO 
CdS04 

1.2853 
1  .  1252 
0.69300 

2.10901 
2.05121 
1.84073 

B 
B02 
BO3 
B407 

i:8: 
i:8; 

0.31428 
1.2286 
1.6857 
1.1143 

1.49732 
2.08940 
2  .  22678 
2  .  04700 

Cl 

AgCl 
Ag 

0.24738 
0.32870 

1.39337 
1.51680 

C1H 

AgCl 
Ag 

0.25442 
0.33804 

1.40555 
1  .  52898 

Ba 

BaSO4 
BaCrO4 
BaSiFe 

0.58846 
0.54217 
0.49119 

1  .  76973 
1.73414 
1.69125 

C103 

AgCl 
KC1 
NaCl 

0.58225 
1.1194 
1.4276 

1.76511 
2  .  04897 
2.15462 

*  In  this  column  the  logarithm  of  the  factor  multiplied  by  100  is  given. 
The  logarithms  are  given  to  five  decimal  places,  but  it  should  be  borne  in 
mind  that  the  fourth  decimal  place  is  in  most  cases  doubtful.  Four-place 
logarithms  are  accurate  enough  for  nearly  all  chemical  analyses.  The 
atomic  weights  for  1910  are  used  in  these  tables. 


TABLE  OF  CHEMICAL   FACTORS. 


851 


Sought. 

Found.           Factor. 

Log. 

Sought. 

Found. 

Factor. 

Log. 

C1O3K 

AgCl         0.85503 
KC1          1.6438 

1.93198 
2.21584 

SiF8 

CaF2           0.60742 

1  .  78349 

Fe 
FeO 

i$ 

0.69944 
0.89981 

1.84475 
1.95415 

C103Xa 

AgCl          0.74271 
XaCl          1.8211 

1.87082 
2.26033 

H             H,O 

0.11190 

1  .  04SS3 

C104 

AgCl 
KC1 

XaCl 

0.69388 
1.3339 
1.7013 

1.84128 
2.12514 
2.23079 

Hg 

V 

0.8494C 
0.86181 

1.92911 
1.93541 

C104K 

AgCl 
KC1 

0.96665 
1.8584 

1.98527 
2.26913 

I 

Agl 
PdI2 
AgCl 

0  .  54055 
0.70406 
0.88545 

1  .  73283 
1.84761 
1.94716 

C104Xa 

AgCl       .  0.85433 
XaCl         2  .  0948 

1.93163 
2.32114 

K 

KC1           0.52441 
K,SO4         0.44873 
KC1O4         0.2S219 
K,PtCl6        0.16091 
"Pt              0  .  401G3 

1.71967 
1.65199 
1.45054 
.20660 
.60317 

CX 

AgCX        0.19426 
Ag           0.24110 

.28839 
.38220 

CNS 

AgCXS 
Cu< 
BaSO4 

0.34396 
0.477i4 
0.24880 

.54402 
.  67891 
.39585 

KC1 

K2S04 
KC1O4 
K,PtCl« 
"Pt 

0.85569 
0.53811 
0.30684 
0.76472 

.93231 
.73087 
.48692 
.88350 

HCNS 

AgCXS 
CuCNS 
BaS04 

0.35604        55150 
0.48572      .68639 
0.25312      .40332 

K20 

KC1 
K2S04 
KC1O4 
K,PtCl, 
Pt 

0.63170    .80052 
0.54054     .73283 
0.33993     .53138 
0.  19383  1.2s742 
0.483081.68402 

Co 
CoO 

CoSO4 
Co 
CoSO4 

0.38035 
1.2713 
0.48355 

1  .  58019 
2.10426 
1.68444 

Cr           Cr2O3 
PbCr04 
BaCrO4 

0.68421 
0.16094 
0.  20523 

1.83519 
1.20667 
1.31248 

Li 

Li2SO4 
LiCl 

0.127191.10446 
0.164861.21/12 

Li2O 

Lid 

Li,SO4 

0.353271.54811 
0.272561.43545 

Cr,O3       PbCrO4 
BaCr04 

0.23522 
0.29996 

1.37148 
1.47706 

Mg 

MgO 
MgS04 
Mg2P207 

0.603  18!  1.78044 
0.20201  1.30537 
0.21847:1.33939 

CrO3         Cr2Os 
PbCr04 
BaCrO4 

1.3158 
0.30950 
0.39468 

2.11919 
1.49066 
1.59625 

MgO 

MgS04 

M&PA 

0.33491  1.52493 
0.362201.55895 

Cu           CuO 
Cu2S 
:     CuCXS 

0.79892 
0.79857 
0  .  52256 

1.90250 
1.90231 
1.71814 

Mn 

MnSO4 
MnS 
Mn304 
Mn2P2O7 

0.363781.56083 
0.631381.80029 
0.720301.85751 
0.387021.58774 

CuO 

CuzS 
CuCNS 
Cu 

0.99956 
0.65408 
1.2517 

1.99981 
1.81563 
2.09750 

MnO 

MnSO4 
MnS 
Mn3O4 
Mn2P2O7 

0.46973 
0.81529 
0.93011 
0.49975 

1.67185 
1.91131 
1.96853 
1.69876 

F 

CaF2 
CaSO4 

0.48662 
0.27908 

1.68719 

1.44.574 

8S2 


TABLE  OF  CHEMICAL  FACTORS. 


Sought. 

Found. 

Factor. 

Log. 

Sought. 

Found. 

Factor. 

Log. 

Mo 

MoO3 

0.66667 

1.82391 

NO2 

NO 

1.5332 

2.18559 

N 

NH3 

NH4C1 
(NH^PtCle 

0.82247 
0.26187 
0.06313 
0.14369 

1.91512 
1.41808 
0.80024 
1  .  15743 

NA 

NO 

1.2666 

2.10263 

p 

M&PA 

P2O5,24MoO3 
(NH4)3P04, 
12Mo03 

0.27848 
0.017232 

0.016515 

1.44479 
0.23633 

0.21787 

Na 

NaCl 
Na2SO4 

0.39343 
0.32378 

1  .  59487 
1  .  51026 

P04 

Mg2P207 

PA,24MoO3 
(NH4)3P04, 
12Mo03 

0  .  85340 
0.052807 

0.050610 

1.93115 
0.72269 

0.70423 

Na2O 
NH3 

NaCl 
Na2S04 

0.53028 
0.43640 

1  .  72450 
1  .  63989 

NH4C1 

(NH4)2PtCl6 
Pt 

0.31821 
0.07671 
0.17461 

1.50271 

0.88487 
1.24206 

PA 

M&PA 

P2O5,24MoO3 
(NH4),P04, 
12MoO3 

0.63780 
0.039466 

0.037824 

1.80469 
0.59623 

0.57777 

NH4 
NH4C1 

NH3 
NH4C1 

(NH4)2PtCl6 
Pt 

1.0592 
0.33723 
0.08130 
0.18504 

2.02497 
1  .  52793 
0.91009 
1  .  26728 

Pb 

PbO 
PbO2 
PbS 
PbSO4 
PbCrO4 
PbCl2 

0.92828 
0.86616 
0.86591 
0.68312 
0.64098- 
0.74491 

.96768 
.93760 
.93747 
.  83449 
.80684 
.87210 

NH3 

(NH4)2PtCle 
Pt 

3.1409 
0.24108 
0.54872 

2.49705 
1  .  38216 
1  .  73935 

Ni 

NiO 
NiC8H14N404 

0.78576 
0.20316 

1  .  89529 
1  .  30785 

PbO 

Pb02 
PbS 
PbSO4 
PbCrO4 
PbCl2 

0.93308 
0.93281 
0.73589 
0.69050 
0.80246 

.96992 
.  96979 
1  .  86681 
1.83916 
1  .  90442 

NiO 

Ni 
NiCbH14N404 

1.2727 
0.25856 

2.10471 
1.412£6 

N03 
N03H 

"NA^ 

NO 
NH3 
NH4C1 

(NH4),PtCl0 

C20H17N503 

2.0663 
3.6404 
1  .  1591 
0.27943 
0.63600 
0.16528 

2.31520 
2.56115 
2.06411 
1.44627 
1  .  80346 
1.21823 

S 
S02 

so, 

S04 
S04H2 
H2S 
FeS2 

BaSO4 
BaS04 
BaSO4 
BaS04 
BaS04 
BaSO4 
BaSO4 

0.13738 
0.27446 
0.34300 
0.41154 
0.42018 
0.14602 
0.25700 

.13792 
.43848 
.  53530 
.61441 
.  62343 
.  16443 
.40994 

NO 
NH3 
NH4C1 

(NH4)2PtCl6 
Pt 
C20H17N503 

2  .  0999 
3  .  6995 
1.1779 
0.28397 
0.64634 
0.16797 

2  .  32220 
2.56815 
2.07111 
1  .  45327 
1.81046 
1  .  22523 

Sb 

Sb2O4 
Sb2S3 

0.78975 
0.71418 

.  89749 
.85381 

Si 
SiO3 

SiO2         0.46931   i  1.67147 
SiO2         1.2653     |2.  10221 

NO 
NH3 
NH4C1 
(NH^PtCle 

C20H17NA 

1.7997 
3.1707 
1.0095 
0.24338 
0.55395 
0.14396 

2.25521 
2.50116 
2.00412 

1.38628 
1  .  74347 
1  .  15824 

Sn 
SnO2 

SnO2 

Sn 

0.78808 
1.2689 

1.89657 
2.10343 

Sr 

SrO 
SrCO, 
SrS04 

Sr(N03)2 

0.84559 
0.59355 
0.47700 
0.41401 

1.92716 
1  .  77346 
1.67852 
1.61701 

TABLE  Of  CHEMICAL   FACTORS. 


853 


Sought. 

Found. 

Factor. 

Log. 

Sought. 

Found. 

Factor. 

Log. 

SrO 

SrC03 
SiSO, 

Sr(X03), 

0.70194 

0.56410 
0.48961 

1.84630 
1.75136 
1.68985 

W 

WO3 

WO, 
W 

0.79310 
1.2609 

1.89933 
2.10067 

Zn 

ZnO 
ZnS 
ZnXH4PO4 
Zn2P2O- 

0.80337 
0.67087 
0.36640 
0.42902 

1.90491 
1.82664 
1.56396 
1.63248 

Th 

Th02 

0.87898 

1.94398 

Ti 

TiO, 

0.60051 

1.77852 

U 

U308 
U02 
U2P207 

0.84824 

0.88170 
0.73272 

1.92852 
1.94532 
1.86494' 

ZnO 

ZnS 
ZnXH4PO4 
Zn2P2O7 

0.83508 
0.45608 
0.53403 

1.92173 
1.65905 
1.72756 

V 

VA 

0.56141 

1.74928 

Zr 

ZrO2 

0.73899 

1.86864 

854 


LOGARITHMS. 


Natural 
Numbers. 

0 

1 

2 

3 

4 

5 

6 

• 
t 

8 

PROP 

9 

ORTIONAL  PARTS. 

1    2   i 

J4567 

8   9 

10 

0000 

0043 

0086 

01280170 

0212 

0253 

0294 

03340374  4  8  1 

2  17  21  25  29 

3337 

LI 

0414  0453 

0492 

0531 

0569 

0607 

0645 

0682 

07190755  4  8  l 

1  15  19  23  26 

3034 

12 

0792  0828 

0864 

08990934 

0969 

1004 

1038 

10721106  3  7  1 

0  14  17  21  24 

2831 

13 

1139 

1173 

1206 

1239  1271 

1303 

1335 

1367 

1399  1430  3   6  1 

0  13  16  19  23 

2629 

14 

1461 

1492 

1523 

1553 

1584 

1614 

1644 

1673 

1703 

1732  3    6 

9  12  15  18  21 

2427 

15 

1761 

1790 

1818 

1847 

1875 

1903 

1931 

1959 

1987 

2014  3   6 

3  11  14  17  20 

2225 

16 

2041 

2068 

2095 

2122  2148 

2175 

2201 

2227 

2253  2279       5 

5  11  13  1618 

21  24 

17 

2304 

2330 

2355 

2380  2405 

2430  2455 

2480 

25042529       5    ' 

r  10  12  15  17  20  22 

18 

2553 

2577 

2601 

26252648 

2672 

2695 

2718 

2742 

2765      5    ' 

7    91214161921 

19 

2788 

2810 

2833 

285612878 

2900 

2923 

2945 

2967 

2989       4    ' 

r    9  11  13  16  18,20 

20 

3010 

3032 

3054 

3075  3096 

3118 

3139 

3160 

3181 

3201       4    i 

5    8  11  13  15 

7  10 

21 

32223243 

3263 

32843304 

3324 

3345 

3365 

33853404       4    < 

i    8  10  12  14  16  18 

22 

3424  3444 

346434833502 

3522  3541 

3560 

3579,3598       4    ( 

5    8  10  12  14  15  17 

23 

3617  3636 

365536743692 

3711 

3729 

3747 

37663784       4    ( 

5    7    9  11  13  15  17 

24 

3802 

3820 

3838 

3856 

3874 

3892 

3909 

3927 

39453962       4    . 

5    7    9  11  12  14  16 

25 
26 

3979 
4150 

3997 
4166 

4014 
4183 

4031 
4200 

4048 
4216 

4065 
4232 

4082 
4249 

4099 
4265 

41164133       3    i 
42814298       3    l 

>    7    9  10  12  ] 
>    7    8  10  11  1 

415 
3  15 

27 

4314  4330 

4346 

4362  4378 

4393 

4409 

4425 

44404456       3    i 

>    6    8   91113  14 

28 

4472  4487 

4502 

4518 

4533 

4548 

4564 

4579 

45944609       3    i 

>    6    8    911  12  14 

29 

4624 

4639 

4654 

46694683 

4698 

4713 

4728 

4742 

4757  i   3    ^ 

t    6    7    9  10  12  13 

30 

4771 

4786 

4800 

4814J4829 

4843 

4857 

4871 

4886 

4900  i   3    A 

1    6    7    9  10  1 

1  13 

31 

4914 

4928 

4942 

49554969 

4983 

4997 

5011 

50245038  i   3    4 

i    6    7    8  10  11  12 

32 

5051 

5065 

5079 

5092 

5105 

5119 

5132 

5145 

5159 

5172  i   3    4 

57    8    9  11  12 

33 

5185 

5198 

5211 

52245237 

5250 

5263 

5276 

5289 

5302  i   3    4 

5    6    8    9  10  12 

34 

5315 

5328 

5340 

5353 

5366 

5378 

5391 

5403 

5416 

5428  i   3    4 

56891 

Oil 

35 

5441 

5453 

5465 

5478 

5490 

5502 

5514 

5527 

5539 

5551  l   2    4 

56791 

Oil 

36 

5563 

5575 

5587  5599 

5611 

5623 

5635  5647 

5658 

5670  i   2    4 

5    6   7    8  1011 

37 
38 

56S2 

5798 

5694 
5809 

5705  5717  5729 
5821  5832|5843 

5740 
5855 

5752  5763 

5866  5877 

5775 

5888 

6786  i   2    3 
5899  l   2    3 

5678 
5678 

9  10 
9  10 

39 

5911 

5922 

5933 

5944 

5955 

5966 

5977 

5988 

5999 

6010  i   2    3 

4578 

9  10 

40 

6021 

6031 

6042 

6053 

6064 

6075 

6085 

6096 

6107 

6117  l   2    a 

4568 

910 

41 

6128 

6138 

6149 

6160  6170 

6180 

6191 

6201 

6212 

6222  i   2    3 

4567 

8    9 

42 

6232 

6243 

6253  626316274 

6284 

6294  6304  6314 

6325  l   2    3 

4567 

8    9 

43 

6335 

6345 

63556365:6375 

6385  6395  6405 

6415 

6425  l   2    3 

4567 

8    9 

44 

6435 

6444 

6454 

6464 

6474 

6484 

6493 

6503 

6513 

6522  i   2    3 

4567 

8    9 

45 

6532 

6542 

6551 

6561 

6571 

65SO 

6590 

6599 

6609 

6618  i   2    3 

4567 

8   9 

46 

6628 

6637 

6646 

6656  6665 

6675  6684 

6693 

6702 

6712  i   2    3 

4567 

7    8 

47 

6721 

6730 

6739 

6749  6758 

6767 

6776 

6785 

6794 

6803  i   2    3 

4556 

7    1 

48 

6812 

6821 

6830 

6839! 

6848 

6857 

6866 

6875 

6884 

6893  124 

7    ^ 

49 

6902 

6911 

6920 

6928 

6937 

6946 

6955 

6964 

6972 

6981  l   2    3 

4456 

'      & 

7    8 

50 

6990 

6998 

7007 

7016 

7024 

7033 

7042 

7050 

7059 

7067  l   2    3 

3456' 

7    J 

51 

"076 

7084 

7093 

7101 

7110 

7118 

7126 

7135 

7143 

7152  i   2    3 

3456' 

7    8 

52 
53 

^160 
7243 

7168 
7251 

7177 
7259 

7185 

7267 

7193 
7275 

7202 

7284 

7210  7218 
72927300 

7226 
7308 

7235  l   2    2 
7316  l   2    2 

3456' 
3    4    5    6    < 

r   7 
i   7 

54 

7324 

7332 

7340 

73487356 

7364 

73727380 

7388 

7396  l   2    2 

3    4    5    6    ( 

>  7 

LOGARITHMS. 


855 


N:iliir..l  ! 

N  1  1!  1  I 

0 

1 

2 

3 

4 

0 

6 

; 

8 

9 

PROPORTIONAL  PARTS. 

1 

2 

3 

4 

ff 

6 

7 

8 

6 

55 

7404 

7412  7419 

7427|743£ 

7443 

7451 

7459 

7466  7474 

1 

2  2 

3 

4 

5 

5 

6 

7 

56 

74S2  7490  7497 

7505  7513 

7520 

752875367543 

7551 

1 

2  2 

3 

4  5 

5 

6 

7 

57 

7559  7566 

7574 

7582 

758S 

7597 

7604i  7612  7619 

7627 

1 

2 

2 

3 

4  5 

5 

6 

7 

58 

7634 

7642  7649 

7657 

7664 

7672  7679 

7686  7694 

7701 

1 

1 

2 

g 

4  4 

5 

6 

7 

59 

7709 

7716  7723 

7731 

7738 

7745 

7752 

7760 

7767 

7774 

1 

1 

2 

3 

4  4 

5 

6 

7 

60 

7782 

7789  7796 

7803 

7810 

7818 

7825 

7832 

7839 

7846 

I 

2 

3 

4  4 

5 

6 

6 

61 

62 

7860  7868  7875  7882 
79247931793879457952 

7889  7896 
7959  7966 

7903  7910  7917 
7973  7980  7987 

1 
1 

2 
2 

3 
3 

4  4 
3  4 

0 

5 

8 

6 

6 
6 

63 

7993800080078014 

8021 

8028  8035  8041 

8048  8055 

1 

2 

3 

3  4 

5 

5 

6 

64 

8062  8069  8075  8082 

8089 

8096'8102  8109 

81168122 

1 

2 

3 

3 

4 

5 

5 

6 

65 

8129813681428149 

8156 

816281698176 

8182  8189 

1 

2 

3 

3  4 

5 

5 

6 

66 

S195  8202  8209  8215  S222 

8228  8235  S241 

8248  8254 

1 

2 

3 

3  4 

5 

o 

6 

67 

8261  8267  8274  8280  SIN; 

8293  8299  8306  8312  8319 

1 

2 

3 

3  4 

5 

5 

6 

68 

83258331 

8338  8344 

8351 

8357  8363  8370  8376  8382 

1 

1 

2 

3 

3 

4 

4 

5 

6 

69 

8388  8395  8401 

8407 

8414 

8420  8426.8432 

84398445 

1 

1 

2 

2 

3 

4 

4 

0 

6 

70 

S451 

84578463 

8470  8476 

8482 

84888494 

85008506 

1 

2 

2 

3 

4 

4 

5 

6 

71 

S513  8519  8525  8531  ! 

8537 

8543 

8549  8555  8561  8567 

1 

2 

2 

3 

4 

4 

5 

5 

72 

S573  8579  8585  8591 

8597 

8603  8609  8615  8621  8627 

[ 

2 

2 

3 

4 

4 

5 

5 

73 

8633  8639  8645  8651 

8657 

8663  8669  8675  8681  8686 

1 

2 

2 

3 

4 

4 

5 

5 

74 

8692  8698 

8704  8710 

8716 

8722( 

8727 

8733 

87398745 

1 

2 

2 

3 

4  4 

5 

5 

75 

8751  8756 

87628768 

8774 

8779 

87858791 

87978802 

2 

2 

3 

3 

4 

5 

5 

76 

8808  8814,8820  8825  8831 

8837  8842  8848 

8854  8859 

2 

2 

3 

3 

4 

5 

5 

77 

88658871887688828887 

8893  8899  8904  8910  8915 

2 

2 

3 

3 

4 

4 

5 

78 
79 

S921  8927  8932  8938  8943 
8976  8982  8987  8993  8993 

8949  8954  8960  8965  8971 
9004  9009  9015  9020  9025 

2 
2 

2 
2 

3 
3 

3 
3 

4 
4 

4 
4 

5 
5 

80 

9031  9036  9042  9047  9053 

9058  9063  9069  9074  9079 

2 

2 

3 

3  4 

4 

5 

81 

9085'  9090  9096  9  101  9106 

91129117912291289133 

2 

2 

3 

3 

4 

4 

5 

82 

9138  9143  9149  9154  9159  9165  9170  9175  9180  9186 

2 

2 

3 

3 

4 

4 

5 

83 

91919196920192069212 

92179222922792329238 

2 

2 

3  3 

4 

4 

5 

84 

92439248925392589263 

9269  9274  9279  9284  9289 

2 

2 

3  3 

4 

4 

5 

85 

9294 

9299 

9304 

9309  9315 

9320  9325  9330  9335  9340 

2 

2 

3  3 

4 

4 

5 

86 

9345  9350  9355  9360  9365 

9370  9375  9380  9385  9390 

2 

2 

3  3 

4 

4 

5 

87 

9395  9400  9405  9410  9415 

9420  9425:9430  9435  9440 

0 

2 

2  3 

3 

4 

4 

88 

0445  9450  9455  9460  9465 

9469  9474  9479  9484  9489 

0 

2 

2  3 

8 

4 

4 

89' 

9494  9499  9504  9509  9513 

9518 

9523952895339538 

0 

2 

3 

3 

4 

4 

90 

)542  9547  9552  9557  9562 

9566 

9571  9576  95  si  9586 

0 

2 

2 

3 

3 

4 

4 

91 

95909595960096059609 

9614  9619  9624  9628  9633 

0 

2 

2 

3  3 

4 

4 

92 

963*  9643  9647  9652  9657 

966196669671  9675 

9680 

o 

2 

2 

3 

3 

4 

4 

93 

685  9689  9694  9699  9703 

9708971397179722 

9727 

0 

2 

2 

3 

3 

4 

4 

94 

9731 

9736  9741  9745  9750 

9754  9759  9763  9768 

9773 

0 

2 

2 

3 

3 

4 

4 

95 

777 

9782978697919795 

9800  9805  9809  9814 

9818 

0 

2 

2 

3  3 

4 

4 

96 

823 

9827,9832  9836  9841 

9845  9850  9854  9859  9863 

0 

2 

o 

3  3 

4 

4 

97 

868 

9872  9877  9881  9886 

9890  9894  9899  9903  9908 

0 

2 

2 

3 

3 

4 

4 

98 

9912J9917  9921  9926  9930 

9934  9939  9943  9948  9952 

0 

2 

2 

3 

O 

4 

4 

99 

956  9961  9965  9969  9974 

I    I 

9978  9983  9987  9991  9996 

0 

2 

2 

3 

a 

3 

4 

856 


ANTILOGARITHMS. 


1 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

PROPORTIONAL  PABTS 

1 

2 

3 

4 

5  6 

7 

8 

9 

.00 

1000 

1002 

1005  1007 

1009 

1012 

!  1014 

1016 

1019 

1021 

0 

0 

1 

1 

1 

1 

2 

2 

2 

.01 

1023 

1026  1028 

1030 

1033 

10351038 

1040 

1042 

1045 

0 

0 

1 

1 

1  1 

2 

2 

2 

.02 

1047 

1050  1052 

1054 

1057 

1059 

1062 

1064 

1067 

1069 

0 

0 

1 

1 

1 

1 

2 

2 

2 

.03 

1072 

1074  1076 

10791081 

1084 

|1086 

1089 

1091 

1094 

0 

0 

1 

1 

1 

1 

2 

2 

2 

04 

1096 

10991102 

11041107 

1109 

1112 

1114 

1117 

1119 

0 

1 

1 

1 

1 

2 

2 

2 

2 

.05 

1122 

11251127 

11301132 

1135 

1138 

1140 

1143 

1146 

0 

1 

i 

1 

1 

2 

2 

2 

2 

.06 

1148 

1151  1153 

11561159 

1161 

1164 

1167 

1169 

1172 

0 

1 

i 

1 

1 

2 

2 

2 

2 

.07 

1175 

1178  1180 

1183  1186 

1189 

1191 

1194 

1197 

1199 

0 

1 

i 

1 

1 

2 

2 

2 

2 

.08 

1202 

1205  1208 

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1216 

1219 

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1 

i 

1 

1 

2 

2 

2 

3 

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1230 

1233  1236 

12391242 

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1250 

1253 

1256 

0 

1 

i 

1 

1 

2 

2 

2 

3 

.10 

1259 

1262  1265 

1268 

1271 

1274 

1276 

1279 

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1285 

n 

1 

i 

1 

1 

2 

2 

2 

3 

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1288 

1291  1294 

1297  1300 

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1309 

1312 

1315 

0 

1  1  1 

2 

2  2 

2 

3 

.12- 

1318 

1321  1324 

13271330 

1334 

1337 

1340 

1343 

1346 

0111 

2 

2  2 

2 

3 

.13 

1349 

13521355 

1358  1361 

1365 

1368 

1371 

1374  1377 

0 

1 

i  i 

2 

2 

2 

3  3 

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13841387 

1390 

1393 

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0 

1 

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1 

2 

2 

2 

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1416  1419 

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1435  1439  1442 

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1 

2 

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2 

3  3 

.16 

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1449  1452 

1455  1459 

1462 

1466  1469  1472;  1476 

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2 

3  3 

.17 

1479 

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1479  1493 

1496 

15001503:1507  1510 

0111 

2  2 

2 

3  3 

.18 

1514 

1517  1521 

1524  152? 

1531 

15351538  15421545 

0111 

2  2 

2 

3 

3 

.19 

1549 

15521556 

15601563 

1567 

157015741578  15  1 

0 

1  1  1 

2  2 

3 

3 

3 

.20 

1585 

15891592 

1596  1600 

1603 

1607  1611 

1614  1618 

0 

1 

i  i 

2 

2 

3 

3 

3 

.21 

1622  1626  1629 

1633  1637 

1641 

1644  1648  1652  1656 

0  1 

i 

•-) 

2  2 

3 

3 

3 

.22 

1660  1663  1667 

1671  1675 

1679 

1683  168716901694 

o 

1 

1  2 

2 

2 

3  3 

3 

.23 

1698  1702  1706 

17101714 

1718 

1722|172617301734 

0 

1 

1  2 

2 

2 

3 

3 

4 

.24 

1738 

1742  1746 

17501754 

1758 

1762 

176617701774 

0 

1 

1 

2 

2 

2 

3 

3 

4 

.25 

1778 

1782  1786 

1791  1795 

1799 

1803 

1807 

1811  1816 

0 

1 

1  2 

2  !  2 

3 

3 

4 

.26 

1820  1824  1828 

18321837 

1841 

1845  1849  1854  1858 

0 

1 

1 

2 

2^3 

3 

3 

4 

.27 

1862  1866  1871 

1875  1879 

1884 

1888  1892i  1897  1901 

0 

1 

1  2 

2  3 

3 

3 

4 

.28 

190519101914 

1919  1923 

19281932193619411945 

0 

1 

1 

2 

2  3 

3 

4 

4 

.i>9 

1950^954 

1959 

1963  1968 

1972 

1977 

198219861991 

0 

1 

1 

2 

23 

3 

4 

4 

.30 

1995  2000 

2004 

2009  2014 

2018 

2023  2028  2032  2037 

0 

1 

1 

2 

2  3 

3 

4 

4 

.31 

2042  2046 

2051 

20562061 

2065207020752080 

2034 

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2  3 

3 

4 

4 

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2089  2094  2099  2104  2109 
2138  2143  2148  2153:2158 

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2239  2244 

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2254  2259 

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2 

3  3 

4 

4 

5 

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2323 

2377 

2328  2333 

2382:2388 

2339 
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1 

1 

1 

2  2 
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3  3 
3  3 

4 

4 

4 
4 

5 
5 

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23992404 

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1 

2 

2 

:;  :; 

4 

4 

5 

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2455  2460 

2466 

2472:2477 

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248924952500 

2506 

1 

1 

2  2 

3  3 

4 

5 

5 

.40 

25122518 

2523 

2529  ;  2535 

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25472553:2559 

2564 

1 

1 

2 

2 

3  4 

4 

5 

5 

.41 

2570  2576 

2582 

25882594 

2600  2606!  2612:2618 

2624 

1 

122 

:;  4 

4 

5 

5 

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26302636 

264226492655 

2661 

26671  2673:  2679 

26S5 

1 

1  2 

2 

34 

4 

5 

6 

.43 
.44 

26922698 
27542761 

2704 
2767 

27102716 
2773  2780 

2723 

2786 

2729  2735 
2793  2799 

2742 
2805 

2748 
2812 

1 
1 

1 
1 

2 
2 

3 
3 

3 
3 

4 
4 

4 
4 

5 
5 

6 
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,45 

2818  2825 

2831 

2838  2844 

2851 

2858 

2864 

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1 

1 

2 

3 

3 

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5 

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8 

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2884  2891 

2897 

29042911 

2917 

2924 

2931 

2938 

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1 

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4 

5 

5 

6 

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2965 

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29852992 

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2 

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31263133 

3141 

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1 

2 

3 

4 

4 

5 

6 

6 

ANTILOGARITHMS. 


857 


1 

0 

1 

• 
2 

3 

4 

56789 

PROPORTIONAL  PARTS. 

1 

3 

3 

4 

5 

6 

7 

8   9 

.50 

3162 

31703177 

31843192 

3199320632143221322 

1    2 

3 

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6  7 

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3 

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6  7 

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3350  3357  3365  3373  338 

2    2 

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3388  3396  3404  3412  3420 

342^  3436  3443  3451  3459 

2    2 

0 

4 

6 

6   7 

.54 

3467 

34753483 

3491 

3499 

3598  3516  3524  3532  3540 

2     2 

3 

4 

G 

6  7 

.55 

3548 

35563565 

35733531 

35  9  3597  3606  3614  3622 

2 

2 

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4 

6 

7    7 

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3631 

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3656  3664 

3673  3681  3699  3698  3707 

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6 

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3750 

375    3767  3776;3784  3793 

2 

3 

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4 

6 

7    8 

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38023811 

3819  3828  3837 

3846  3855  3864  3873  3882 

2|3 

4 

4 

6 

7    8 

.59 

389038993908 

39173926 

39363945395439633972 

2 

3 

4 

5 

6 

7    8 

.60 

39sl 

39903999 

4009  401f 

4027  4036  4046  4055  4064 

2 

3 

4 

5    6 

6 

7    8 

.61 

4074 

4083409341024111 

1121  4130414041534159 

2    3 

4 

5    6 

7 

8    9 

.62 

41694178418841984207 

42  17  4227  4236  424t; 

2 

3 

4 

5    6 

7 

8    9 

.63 

4266  4276  42.85  4295  4305 

4315  4325  4335  4345  4355 

2 

3 

4 

5    6 

7 

8    9 

.&4 

4365  4375  4385 

4395  4406 

4416  4426  4436  4446  4457 

1 

2 

3 

4 

5    6 

7 

8    9 

| 

.65 

4467 

4477  44X7 

449  >  450? 

4519  4529  4539  4550  4560 

1    2 

3 

4 

5    6 

7 

8    9 

.66 

4571 

45X1 

4.59246034613 

4624  4634  4645  4656  4667 

1    2    3J4 

5    6 

7 

910 

.67 

4677  46.88 

4699 

47104721 

4732  4742  4753  4764  4775 

1234 

5    7 

8 

9  10 

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478647974803 

4819 

4831 

4842  4853  4864  4S75  4887 

1234 

6  7 

S 

9  10 

.69 

4898  4909  4920 

49324943 

4955  4966  4977  4989  5000 

1235 

6 

7 

8 

910 

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591  2  5023 

5935 

50475958 

5070  5032  5093  5105  5117 

1 

2 

4 

5 

6 

1 

8 

911 

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51295140515251645176 

5188  5200  5212  5224  5236 

1245 

6^7 

81011 

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524S  .5260  5272  5284 

5297 

53095321533353465358 

1245 

6    7 

9 

1011 

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5370  53X3 

5395  5408 

5420 

5433  5445  5458  5470  5483 

1 

3  U|5 

6 

8 

9 

1011 

.74 

5495  5508 

552! 

5534 

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5559  5572  5585  5598  5610 

1 

3 

4 

5 

6 

8 

9  10  12 

.75 

5623 

5636  5649  5662 

5675 

5689  5702  5715  5728  .5741 

1 

3 

4 

0 

7 

8 

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10  12 

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5821  5834  5848  5  361  5X75 

1 

3    4 

6 

7 

8 

9 

11  12 

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5888 

5902 

5916  5929 

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59575970598459986012 

1 

3    4 

5 

7 

8  iclll  12 

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6026  6039  6053  60671  6081 

6095  6109  6124  6138  6152 

1 

3    4|6 

7 

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1011 

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6166 

6180 

6194 

6209 

6223 

6237  6252  6266  6281  6295 

1 

3 

4    6 

7 

9 

10  11 

13 

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6310 

6324 

6339 

6353 

6368 

6383  6397  6412  6427  6442 

1 

3 

4    G 

7 

9 

10 

12 

13 

.81 

6457 

6471 

6486  6501 

6516 

6.531  6546  6561  6577  6592 

2 

356 

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11  12 

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6683  6699  6714  6730  6745 

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11 

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2 

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0  12  14  I.-,  17 

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8.511 

8531 

8551 

s.5708590 

8610  8630  8650  8670  8690 

2 

4 

G 

8 

0  12  14  16  18 

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8710 

8730 

8750 

8770 

8790 

8810  .8831  8851  8872  8892 

2 

4 

6 

8 

0 

12 

14 

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18 

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8933 

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8974 

8995  9916  9036  9057  9078  9099 

) 

4 

8 

8 

0 

12 

15 

17 

19 

.96 

91209141 

9162  9183  9204  9226  9247  9268  9290  9311 

2 

4 

6 

& 

1 

13 

15 

17 

19 

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9333  9354  9376  9397 

9419:9441  9462  9484  9506  9528 

2 

4 

7 

8 

1 

13 

15 

17 

20 

.98 
.99 

9550  9572  9594^616  9638] 
9772  9795  9817  9840  9863 

9661  9683  9705  9727  9750 
9886  9908  9931  9954  9977 

t 
-> 

4 
5 

7 
7 

0 
0 

1 
1 

13 
14 

16 
16 

IS 
IS 

20 
20 

INDEX  OF  AUTHORS. 


A. 

PAGE 

Alefeld,  E.,  Determination  of  chlorine 708 

Allner,  Determination  of  carbon  monoxide 762 

Andrew,  L.  W.,  Use  of  potassium  iodate 672 

Andrews,  Volumetric  estimation  of  nickel 720 

sulphuric  acid 716 

Anneler,  Determination  of  ozone 677 

Arndt,  Combustion  of  nitric  oxide 803 

Arthur,  Volumetric  estimation  of  nickel 720 

Augenot,  Separation  of  tungsten  from  tin 300 

Austin,  M.,  Determination  of  magnesium 66 

manganese 120,  126 

zinc .  140 


B. 

Bamber,  Method  for  determining  sulphur  in  iron  and  steel 354 

Barnebey,  O.  L.,  Determination  of  titanium 118 

Baubigny,  Determination  of  antimony 222 

Beckett/E.  G 223,  224 

Beilsu-in,  Electrolytic  determination  of  cadmium 189 

Belhoubek,  Volumetric  determination  of  uranium 621 

Benz,  Determination  of  thorium 510 

Bergmann,  Electrolysis  of  nickel  solutions 131 

Screen  for  reading  burettes 529 

Berthelot,  Absorption  of  benzene 753 

Distribution  of  iodine  between  two  solvents 658 

Berzelius,  Alkalies  in  silicates 499 

Mercurous  tungstate  precipitation 289 

Silicic  acid  precipitation 473,  489 

Vanadium  determination 304 

Biltz,  W.,  Separation  of  halides  from  sulphides 329 

Black,  Colorimetric  determination  of  arsenic 208 

859 


86o  INDEX  OF  AUTHORS. 


Blair,  A,  A.,  Condition  of  sulphur  in  iron  and  steel 352 

Direct  combustion  of  steel 413 

Manganese  in  steel,  iron,  ores,  etc 616 

Phosphorus  in  iron  and  steel 637 

Vanadium,  molybdenum,  chromium,  and  nickel  in  steel.  .  .    313 

Blasdale,  W.  C.,  Separation  of  calcium  and  magnesium 77 

Bloxam,  Electrolytic  determination  of  arsenic 212 

Bockmann,  Sulphur  in  insoluble  sulphides 358 

Bong,  G.,  Decomposition  of  silicates 490 

Borchers,  W.,  Sulphocyanic  acid 341 

and  hydrocyanic  acids 342 

Borda,  Method  of  weighing 9 

Boreli,  Electrolytic  determination  of  mercury 173 

Bormann,  Expelling  ammonia  with  magnesium  oxide 59 

Borntrager,  H.,  Determination  of  tungsten 300 

Recovery  of  molybdenum  residues 447 

Boudet,  Error  in  measuring  alcoholic  soap  solutions 536- 

Boutron,  Error  in  measuring  alcoholic  soap  solutions 536 

Braun,  Effect  of  carbon  disulphide  on  antimony  sulphide 242 

Brauner,  B.,  Separation  of  selenium  and  tellurium 279,  280 

Brearley,  Volumetric  estimation  of  nickel 720 

Bretschger,  M.,  Density  of  acetylene 754 

ethylene 751 

Pipette  for  gas  analysis 796 

Preparation  of  acetylene 754 

Brodie,  B.  C.,  Ozone 676 

Bruhns,  Volumetric  estimation  of  sulphuric  acid 716 

Brunck,  O.,  Combustion  of  hydrogen 772 

Determination  of  antimony 222 

nickel 129 

Separation  of  iron  and  manganese 153 

nickel 166 

nickel  and  cobalt 162 

zinc 165 

Volumetric  estimation  of  hydrogen  sulphide 688 

Brunner,  Analysis  of  tungsten  bronze 299 

Determination  of  arsenic 205 

Separation  of  zinc  from  nickel,  cobalt,  and  manganese 158 

Brush,  Water  in  silicates 512 

Bunsen,  Alkalimeter 376 

Analysis  of  chromite 509 

Combustion  of  nitrogen 807 

Determination  of  antimony 222 

arsenic 205 

Igniting  precipitates 22 


INDEX  OF  AUTHORS.  86 1 


Bunsen,  lodimetric  analysis  of  peroxides 661 

Separation  of  arsenic  and  antimony 241 

Solubility  of  nitrous  oxide 801 

Type  of  burette 514 

Valve 87,  98,  601 

Volumetric  estimation  of  bromides 659 

sulphurous  acid 692 

Bunte,  Apparatus  for  gas  analysis 798 

Burgstaller,  A.,  Determination  of  chlorine 708 

Busch,  M.,  Determination  of  nitric  acid 451 

Busvold,  X.,  Analysis  of  gases  rich  in  chlorine 810 

Density  of  hydrogen  chloride 814 

Experiments  with  chlorine , 812 

C. 

Campagne,  Chromium  in  steel 315 

Campbell,  Absorption  of  hydrogen 771 

Determination  of  nickel 720 

Direct  combustion  of  steel 413 

Carius,  Method  for  decomposing  organic  substances 325,  370 

Carnot,  A.,  Determination  of  lithium 56 

Cavendish,  Rare  gases  in  commercial  nitrogen 807 

Chancel,  Determination  of  aluminium 83 

Separation  of  iron  and  titanium 115 

Chapin,  W.  H.,  Boric  acid  in  silicates 590 

Christ ensen,  A..  Metallic  iron  in  the  presence  of  oxide 612 

Christie,  W.  A.  K.,  Computations  in  gas  analysis 782 

Density  of  carbon  dioxide 750 

chlorine 808 

Standardization  of  permanganate 98 

Clarke,  F.  W.,  Electrolytic  determination  of  mercury 172 

Separation  of  antimony  and  tin 248 

Classen,  A.,  Determination  of  antimony 224,  226,  227 

carbon  dioxide 380 

zinc "  .  .    147 

Electrolytic  apparatus 134 

determination  of  mercury 172 

tin 234 

Standardization  of  permanganate 92,  600 

Comment,  Colorimetric  determination  of  arsenic 208 

Contat,  Valve 98,  602 

Cooke,  J.  P.,  Ferrous  iron  in  silicates 503 

Corleis,  Carbon  in  iron  and  steel 399 

Cormimboef,  H.,  Nickel  determination 138 


862  INDEX  OF  AUTHORS. 

D. 

PAGE 

Dakin,  H.  D.,  Determination  of  zinc 140 

Daniel,  D.,  Determination  of  iron 89 

Davila,  Density  of  nitric  oxide 802 

Deckert,  Apparatus  for  chlorine  determination 813 

de  Haen,  Titration  of  ferrocyanic  acid 632 

de  Koninck,  Determination  of  ferrous  iron 504 

nitric  acid 460 

Dennis,  Technical  gas  analysis 794 

Dennstedt,  M.,  Combustion  in  open  tube 421 

Deussen,  E.,  Fusion  with  potassium  acid  fluoride 109 

Devada,  Determination  of  nitric  acid 454 

Deventer,  Preparation  of  nitric  oxide 802 

Deville,  St.  Claire,  Analysis  of  commercial  platinum 272 

Water-jacketed  tube 732 

Dexter,  Determination  of  antimony 224 

Diefenthaler,  O.,  Volumetric  estimation  of  ferricyanic  acid 695 

Diehl,  Volumetric  analysis  of  lead  peroxide 675 

Diethelm,  Burette  float 529 

Dietz,  H.,  Analysis  of  iodides 671 

Dittmar,  Determination  of  potassium 45,  48 

Ditz,  H.,  Analysis  of  chlorates 669 

Divers,  E.,  Absorption  of  nitric  oxide 803 

Donath,  Determination  of  ammonia 824 

Separation  of  tungsten  and  tin *. 301 

Dormaar,  O.  M.  M.,  Determination  of  antimony 227 

Drechsel,  G.,  Volumetric  estimation  of  chlorine 707 

Drehschmidt,  Analysis  of  gas  mixtures 784 

Apparatus  for  gas  analysis 785 

Pipette 743,  766 

Platinum  capillary 743,  766 

Drouin,  Qualitative  detection  of  carbon  monoxide 769 

Drown,  T.  M.,  Determination  of  silicon  in  iron  and  steel 442 

Dudley,  Determination  of  phosphorus  in  bronze 239 

Dulin,  Determination  of  copper 724 

Dumas*  Method  for  determining  nitrogen 422 

Dupasquier,  Volumetric  estimation  of  sulphurous  acid 692 

Dupre,  Determination  of  potassium 44,  45,  46,  48 

Durkes,  Benzidine  hydrochloride 714 

Duschak,  Determination  of  sulphuric  acid 465,  469 

E. 

Eddy,  Determination  of  magnesium 69 

Eggertz,  Precipitation  of  ammonium  phosphomolybdate 436 

Emich,  Determination  of  nitric  oxide 802 


INDEX  OF  AUTHORS.  863 

PAOE 

Engel,  Electrolytic  determination  of  tin 235 

Engels,  Determination  of  potassium 48 

Engler,  Theory  of  oxidation 605 

Erdmann,  Determination  of  mercury 172 

Euler,  Separation  of  vanadic  and  molybdic  acids 666,  667 

F. 

Fairbanks,  Volumetric  estimation  of  molybdenum 667,  668 

Feldhaus,  Gravimetric  estimation  of  cyanogen ....  * 337 

Ferchland,  P.,  Determination  of  chlorine 812 

Finch,  G.,  Analysis  of  fuming  sulphuric  acid 577 

Finkener,  Determination  of  antimony 224 

ferrocyanic  acid 343 

phosphoric  acid 439 

Precipitation  of  potassium 47 

Volumetric  estimation  of  sulphurous  acid 692 

Fischer,  A.,  Determination  of  antimony 225,  227 

E.,  Separation  of  arsenic  and  antimony 245 

N.  W.,  Separation  of  nickel  and  cobalt 162 

Fitzenkam,  R.,  Determination  of  potassium 51 

Folin,  O.,  Determination  of  sulphuric  acid 469 

Follenius,  Determination  of  cadmium 191 

Fones,  Determination  of  boric  acid 428 

Fordos,  Volumetric  estimation  of  sulphurous  acid 692 

Forster,  F.,  Analysis  of  ferrum  reductum 612 

Electrolytic  deposition  of  copper 187 

Determination  of  antimony 227 

Testing  of  commercial  platinum 276 

Franzen,  Use  of  hydrosulphite 760 

Fresenius,  Alkalimeter 376 

Determination  of  carbon  dioxide 380 

potassium 45 

sulphur  in  sulphides 357 

Drying  precipitates 21 

Electrolysis  of  nickel  solutions 131 

Separation  of  barium,  calcium,  and  strontium 79 

and  strontium 80 

bismuth  and  copper 198 

calcium  and  magnesium 76 

copper  and  cadmium 202 

Solubility  of  barium  chromate 75 

strontium  carbonate 73 

sulphate 73 

Volumetric  estimation  of  ferric  iron .  697 


864  INDEX  OF  ALTHORS. 

PAGE 

Fresenius,  Volumetric  estimation  of  iodides 657 

nitrates 634 

pyrolusite 625 

Friedberger,  O.,  lodates  and  periodates 670 

Friedheim,  Determination  of  sulphuric  acid 714 

vanadium 666,  667 

Separation  of  arsenic  and  antimony 245 

vanadic  and  molybdic  acids 666,  667 

Friend,  J.  A.,  Titration  with  permanganate 604,  607 

Funk,  W.,  Separation  of  iron  and  molybdenum 153 

G. 

Gaihlot,  Determination  of  nitrous  and  nitric  acids 826 

Gay-Lussac,  Volumetric  estimation  of  silver 702 

Geissler,  Alkalimeter 376 

Bulb  for  absorbing  gases 416 

Gelis,  Volumetric  estimation  of  sulphurous  acid 692 

Gerlinger,  Determination  of  nitrous  and  nitric  acids •. 826 

Gerster,  M.,  Preparation  of  fuming  sulphuric  acid  mixtures 580 

Gibbs,  W.,  Electrolytic  determination  of  copper 187 

nickel 131 

Determination  of  magnesium 68 

manganese 126 

Gilbert,  Determination  of  silica 487 

vanadium 304 

Gladding,  Modification  of  Kjeldahl's  method 63 

Gladstone,  Density  of  methane 774 

Preparation  of  ethylene 751 

Glaser,  Determination  of  sulphur  in  sulphides 358 

Indicators 548 

Gockel,  Screen  for  reading  burettes 529 

Valve 98,  602 

Gooch,  F.  A.,  Crucible  for  asbestos  filters 24,  25 

Determination  of  boric  acid 428 

magnesium 66,  69 

manganese 120,  126 

phosphoric  acid 434 

vanadium 304 

Separation  of  aluminium  and  titanium 116 

iodine  and  chlorine 331 

lithium  from  sodium  and  potassium 53 

Volumetric  estimation  of  copper 682 

molybdenum 667,  668 

Gott,  Direct  combustion  of  steel 413 

Goutal,  Determination  of  nickel 720 


INDEX  OF  AUTHORS.  865 

PAGE 

Grandeau,  Determination  of  nitric  acid 456 

Gray,  Density  of  nitric  oxide 802 

Grossly.  A.,  Separation  of -vanadic  and  phosphoric  acids 307 

.  Peter,  Colorimetric  determination  of  nitrous  acid 344 

Grimm,  Error  in  measuring  instruments 536 

Meniscus  corrections 744 

Groger,  M.,  Determination  of  sulphur  in  sulphides 367 

<  ii  i>«!nann,  Determination  of  nickel 720 

Grund,  R.,  Magnesite  cupels 259 

Gutbier,  Determination  of  nitric  acid 451 

tellurium 279 

Separation  of  tellurium  and  antimony 281 

Guye,  P.  A.,  Density  of  acetylene 754 

ethylene 751 

nitric  oxide 802 

Preparation  of  acetylene 755 

H. 

Haber,  Absorption  of  oxygen 759 

Combustion  of  carbon  monoxide 766 

Computations  in  gas  analysis 782 

Determination  of  benzene 753 

ethylene 752,  818 

Separation  of  benzene  and  ethylene 757 

Hackford,  E.,  Electrolytic  determination  of  arsenic 212 

Haen,  Volumetric  estimation  of  copper 682 

Haidlen,  Separation  of  bismuth  and  copper 198 

copper  and  cadmium 202 

Hampe,  Determination  of  copper 185 

manganese  in  steel 619 

Separation  of  antimony,  arsenic,  and  tin 257 

antimony  and  tin 252 

arsenic  and  tin 255 

Handy,  J.  O.,  Phosphorus  in  steel 588 

Harbeck,  Separation  of  benzene  and  acetylene 756 

Harding,  Hydrogen  sulphide  from  insoluble  sulphides 368 

Hart,  Absorption  of  hydrogen 771 

Hauer,  Determination  of  vanadium 304 

Heath,  Volumetric  estimation  of  copper 682 

Heberlein,  Determination  of  sulphocyanic  acid 340,  341 

Hefti,  F.,  Colorimetric  estimation  of  arsenic 208 

Determination  of  arsenic  as  arsine 214 

Electrolytic  determination  of  arsenic ". . .  212 

Results  obtained  in  determining  arsenic 218 

Helmer,  Determination  of  sulphur  in  iron  and  steel 364 


866  INDEX  OF  AUTHORS. 

PAGE 

Hempel,  Absorption  of  oxygen 759 

Analysis  of  illuminating  gas 775 

Apparatus  for  gas  analysis 743,  785 

Combustion  of  carbon  monoxide 766 

Detection  of  carbon  monoxide 768 

Determination  of  carbon  in  iron  and  steel 404 

fluorine  as  silicon  fluoride . 829 

Rare  gases  in  commercial  nitrogen 807 

Solubility  of  acetylene  in  acetone 754 

Technical  gas  analysis 786 

Henry,  Non-explosibility  of  carbon  monoxide  and  nitric  oxide  mixtures . . .  804 

Henz,  Determination  of  antimony 218,  222,  225,  226,  227 

boric  acid 432 

tin 233,  235 

Separation  of  antimony  and  tin 248 

Herold,  Analysis  of  ferrum  reductum 612 

Hibbert,  Volumetric  estimation  of  hydrogen  peroxide 700 

iron 699 

persulphuric  acid 701 

Hill,  A.  E.,  Volumetric  estimation  of  chlorine 707 

HiUebrand,  W.  F.,  Analysis  of  silicates 487,  491 

Dehydration  of  silica 493 

Determination  of  chlorine  in  minerals 324 

ferrous  iron  in  silicates 504 

iron  in  silicates 109 

sulphur  and  zirconium 505 

titanium . 100 

uranium 106,  621 

vanadium  and  chromium 310 

Precipitation  of  vanadium 304 

Removal  of  melts  from  crucibles 488 

Testing  silicates  for  barium 495 

Hinder,  Decomposition  of  silicates 501 

Hinman,  Volumetric  estimation  of  sulphuric  acid 716 

Hinrichsen,  Titration  of  hydrofluosilicic  acid 582 

Hintz,  Determination  of  sulphuric  acid 469 

Hofmann,  A.  W.,  Separation  of  copper  and  cadmium 200 

Hoitsema,  Determination  of  silver 706 

Hollard,  Determination  of  antimony 225 

Holliger,  Volumetric  estimation  of  sulphuric  acid 716 

Holthof,  Determination  of  copper 186 

Holverscheidt,  Determination  of  vanadium 304,  306,  666 

Hommel,  W.,  Separation  of  molybdenum  and  Tungsten 294 

Honig,  M.,  Titration  of  boric  acid 5C9 

Hopkins,  Permanence  of  permanganate  solution 90 


INDEX  OF  AUTHORS.  867 

PAGE 

Huldschinsky,  Separation  of  cobalt  and  nickel 165 

Hulett,  Apparatus  for  distilling  mercury 748 

Determination  of  sulphuric  acid 465,  469 

Hundeshagen,  Precipitation  of  ammonium  phosphcmolybdate ! . .  436 

I 

Ilinsky,  Separation  of  nickel  and  cobalt 165 

Ilosvay  von  Nagy  Ilosva,  Determination  of  Acetylene 755 

Nitrous  acid 345 

Inglis,  Determination  of  ozone 677 

Inhelder,  A.,  Determination  of  antimony 227 

Preparation  of  sodium  sulphide  reagent 225 

Isham,  R.  M.,  Determination  of  titanium 118 

Isler,  Specific  gravity  of  strong  acids 838,  839 

J 

Jager,  Oxidation  of  gases 797 

Jamieson,  Volumetric  estimation  of  copper % 672 

Janini,  Determination  of  zinc 146 

Jannasch,  P.,  Decomposition  of  silicates 490 

Determination  of  alkalies  in  silicates 501 

chlorine  in  apatite 323 

water  in  fluosilicates 484 

silicates 512 

Separation  of  bismuth  from  lead 195 

iodine  from  chlorine 332 

selenium  and  tellurium 279 

Jarvinen,  K.  K.,  Determination  of  phosphoric  acid 434 

Jarvis,  Volumetric  estimation  of  nickel 720 

Jawein,  Electrolytic  determination  of  cadmium 189 

Jeffery,  J.  H.,  Titration  with  permanganate 604,  607 

Johnson,  Volumetric  estimation  of  nickel 720 

Johnston,  Determination  of  sulphur  in  iron  and  steel 366 

Jones,  Reductor 608,  637 

Jones,  C.  C.,  Titration  with  permanganate 604,  607 

Jones,  H.,  Titration  of  boric  acid 589 

Jones,  W.  A.,  Carbon  monoxide-copper  compound 763 

Jordis,  Dehydration  of  silicic  acid 487 

Jorgensen,  G.,  Determination  of  magnesium 67 

phosphoric  acid 434 

Titration  of  boric  acid 589 

Jungfleisch,  Distribution  of  iodine  between  two  solvents 658 

K 

Kanter,  Dehydration  of  silicic  acid 487 

Kassner,  Titration  of  barium  peroxide 628 


863  INDEX  OF  AUTHORS. 

PAGE 

Keen,  W.  H.,  Electric  furnace 414 

Keller,  E.,  Determination  of  selenium  and  tellurium  in  copper 284 

Separation  of  selenium  and  tellurium 282 

from  copper 280,  282 

Kempf,  R.,  Titration  of  persulphates . 630 

Kerner,  Separation  of  halogens  and  cyanogen 339 

Kessler,  Titration  with  permanganate 604 

Kingzett,  Determination  of  hydrogen  peroxide 680 

Kinnicutt,  Oxidation  of  carbon  monoxide 767 

Kjeldahl,  Method  for  determining  nitrogen 62 

Klapproth,  Determination  of  antimony 225 

Kling,  Determination  of  potassium 48 

Knecht,  Volumetric  estimation  of  hydrogen  peroxide 700 

iron  . 699 

persulphuric  acid 701 

Knerr,  Electrolytic  determination  of  mercury 172 

Knop,  Determination  of  ammonia 822 

Knorre,  Separation  of  nickel  and  cobalt 165 

Koch,  A.  A.,  Determination  of  fluorine 476,  830 

Separation  of  phosphoric  and  hydrofluoric  acids 474 

Kohlrausch,  F.,  Reduction  of  weights  to  vacuo 13,  14 

Testing  of  weights 15 

Koppeschaar,  W.,  Determination  of  phenol 69o 

Korbuly,  M.,  Absorption  of  benzene 753 

Kramers,  G.  H.,  Precipitation  of  zinc  sulphide 160 

Kraut,  K.,  Determination  of  nickel 129 

Kreider,  Preparation  of  perchloric  acid 51 

Kreitlina',  Use  of  burette  floats 529 

Krug,  Determination  of  sulphur  in  iron  and  steel « 365 

Kiister,  Determination  of  sulphuric  acid 467 

Sodium  hydroxide  solution  free  from  alkali  carbonates 555 

Titration  of  alkali  carbonates 562,  564 

Kuzma,  B.,  Separation  of  selenium  and  tellurium  from  metals 280 

L 

Ladenburg,  R.,  Determination  of  ozone 677,  680 

Lagutt,  E.,  Determination  of  silver 318 

Langbeck,  H.  W.,  Ortho-nitrophenol  as  indicator 543 

Leberle,  Determination  of  iron 89 

Lecco,  Calorimetric  determination  of  iodine 661 

Lecrenier,  Determination  of  antimony 225 

Leduc,  Density  of  carbon  dioxide 750 

chlorine 808 

hydrogen  chloride 814 

hydrogen  sulphide 348,  816 


INDEX  OF  AUTHORS.  869 

PAGE 

Ledllc,  Density  of  sulphur  dioxide . 815 

Lefort,  J.,  Aqua  regia  for  dissolving  sulphides 362 

Lenssen,  Titration  with  permanganate 604 

Volumetric  estimation  of  ferricyanic  acid 694 

Leutold,  Combustion  of  hydrogen 773 

Levol,  Determination  of  arsenic 206 

manganese  in  pyrolusite 624 

Solubility  of  magnesium  ammonium  arsenate 208 

Levy,  Volumetric  estimation  of  copper 672 

Lieben,  Volumetric  estimation  of  formic  acid 626 

Liebig,  J.,  Determination  of  carbon  and  hydrogen  in  organic  compounds.  414 

sulphur  in  organic  compounds 371 

Separation  of  nickel  from  cobalt 163,  164 

Titration  of  cyanogen 711 

Liechti,  Determination  of  nitric  acid 460 

Lindeiuann,  Absorption  of  oxygen  by  phosphorus 759 

Lorkemann,  Determination  of  arsenic 208 

Loose,  H.,  Methyl  red 543 

Losekann,  G.,  Determination  of  zinc 140 

Low,  A.  H.,  Analysis  of  copper  ores 725 

Determination  of  copper  by  iodide  method 682 

potassium  cyanide 724 

lead 726 

Lowe,  Separation  of  bismuth  and  lead 195 

Lowenthal,  J.,  Precipitation  of  tin .- 232 

Titratioii  with  permanganate 604 

Luckow,  Determination  of  antimony 224 

zinc 147 

Electrolytic  determination  of  mercury 172 

Lunge,  G.,  Analysis  of  fuming  acids 579 

pyrolusite 624 

Dehydration  of  silicic  acid 487 

Determination  of  carbon  dioxide 388,  391 

nitrous  acid 345,  626 

sulphur  in  pyrite 362 

-Rey  pipette 575 

Separation  of  ethylene  and  benzene 756 

Soluble  and  insoluble  silicic  acid 507 

Standardization  of  permanganate 92 

Specific  gravities  of  ammonia  solutions 841 

strong  acids 838 

Universal  apparatus 387,  823 

Luther,  Analysis  of  chlorates 669 

O/one 677 

Lux,  Analysis  cf  red  lead 623 


870  INDEX  OF  AUTHORS. 

M 

PAGE 

Manchot,  Titration  with  permanganate 005 

Marchand,  Determination  of  mercury 172 

Marchlewski,  Determination  of  carbon  dioxide 388 

Specific  gravities  of  strong  acids 838,  839 

Margosches,  B.  M.,  Analysis  of  iodides 671 

Margueritte,  Volumetric  estimation  of  iron 89,  99,  603 

Marquardt,  Analysis  of  ferrum  reductum 612 

Marshall,  M.,  Colorimetric  determination  of  manganese 128 

Martz,  E.,  Oxygen  in  sea  water 740 

Mascazzini,  Determination  of  antimony 224 

Massaciu,  Determination  of  chromium 103 

May,  W.  C.,  Drying  lead  peroxide  deposits 178 

Mayer,  L.,  Determination  of  lithium . 56 

Purification  of  mercury 747 

Mayr,  D.  K.,  Determination  of  antimony 227 

McArthur,  Determination  of  potassium 45,  48 

McKay,  Determination  of  arsenic 205 

Mensel,  Volumetric  analysis  of  iodides 656 

Merck,  Determination  of  metallic  iron  in  the  presence  of  oxide 611 

Merling,  Determination  of  lithium 56 

Metzl,  Determination  of  antimony 221 

Separation  of  antimony  and  tin 248,  250 

Meyer,  J.,  Determination  of  manganese 125 

Meyer,  V.,  Nitrous  oxide 800 

Michaelis,  Separation  of  arsenic  and  antimony 245 

Millberg,  Dehydration  of  silicic  acid 487 

Separation  of  soluble  and  insoluble  silicic  acid 507 

Miller,  Notes  on  assaying 264 

Miolati,  A.,  Electrolytic  determination  of  mercury 173 

Misteli,  W.,  Preparation  of  ethylene 751 

Mitscherlich,  Determination  of  ferrous  iron  in  silicates 504 

Mittash,  Separation  of  iron  and  manganese 154 

Mohr,  Definition  of  liter 521 

Titration  with  pyrolusite 625 

Mohr,  C.,  Volumetric  estimation  of  iron .  , 681 

ferricyanic  acid 694 

Mohr,  F.,  Determination  of  bromine 709 

chlorine 708 

Preparation  of  litmus  solution 545 

M  oiler,  Titration  of  fluosilicic  acid 583 

Moore,  Volumetric  determination  of  nickel 720 

Moore,  C.  J.,  Purification  of  mercury 747 

Morse,  Permanence  of  permanganate  solutions 90 

Morton,  Volumetric  determination  of  sulphuric  acid 716 


INDEX  OF  AUTHORS.  871 

PAGE 

Miiller,  E.,  Analysis  of  iodates  and  periodates 670 

Determination  of  ferricyanic  acid 095 

Miiiler,  M.,  Separation  of  tellurium  and  selenium 279 

tungsten  and  tin 301 

Miiller,  W.,  Use  of  benzidine  hydrochloride 714 

Munroe,  C.  E.,  Crucible  with  platinized  felt 27 

Muthmann,  Separation  of  tellurium  and  antimony 281 

Mylius,  Testing  commercial  platinum 276 

N. 

Naef ,  Specific  gravities  of  strong  acids 838,  839 

Neher,  F.,  Determination  of  arsenic 205 

Separation  of  arsenic  and  antimony 243 

tin 255 

Neubauer,  Determination  of  cyanogen  and  halogens 339 

phosphoric  acid 434 

Precipitation  of  potassium 47,  48 

Xernst,  Molecular  volumes 782 

Neuman,  Electrolytic  separation  of  copper  and  cadmium 203 

IS'icloux,  Oxidation  of  carbon  monoxide 767 

Noyes,  W.  A.,  Determination  of  sulphur  in  iron  and  steel 364 

Nydegger,  Determination  of  sulphuric  acid  with  benzidine 714 

O. 

Oberer,  Separation  of  copper  and  cadmium 010 

Oechelhauser,  Determination  of  ethylene 752 

Separation  of  benzene  and  ethylene 757 

Oettel,  Determination  of  fluorine  as  silicon  fluoride 829 

phosphorus  in  bronze 238 

OfTerhaus,  Determination  of  chlorine  gas  by  titration 811 

Ohm,  Law  of 133 

Oppenheim,  C.,  Absorption  of  nitric  oxide 803 

Orsat,  Apparatus  for  gas  analysis 797 

Osann,  Volumetric  determination  of  silver 706 

Ost,  Determination  of  antimony 225,  227 

Ostwald,  Definition  of  mol 455 

solubility  product 156 

Washing  precipitates 18 


P. 

Palmera,  W.,  Iridium  in  commercial  platinum 275 

Panting,  Determination  of  carbon  monoxide 762 


872  INDEX  OF  AUTHORS. 


Parr,  Volumetric  determination  of  copper 673 

Parrodi,  Electrolytic  determination  of  antimony 224 

Pattinson,  J.,  Determination  of  manganese  in  iron  and  steel 642 

sulphuric  acid 467 

Paul,  Drying  oven 33,  34 

Heating  antimony  pentasulphide 220 

Pease,  Determination  of  phosphorus  in  bronze 239 

Pechard,  Analysis  of  wolframite 296 

Pelouze,  Titration  of  nitrates 634 

Penfield,  S.  L.,  Determination  of  fluorine 476 

water  in  silicates 512 

Titration  of  hydrofluosilicic  acid 582 

Pennock,  Volumetric  determination  of  sulphuric  acid 716 

Penny,  Determination  of  iron  by  dichromate 641 

stannous  chloride 697 

Perillon,  Separation  of  tungsten  and  silicon 302 

Pettenkofer,  Determination  of  carbon  dioxide  in  air 593 

Pettersson,  O.,  Determination  of  carbonic  acid 384 

carbon  in  steel 405 

Pfeiffer,  Separation  of  benzene  and  ethylene 756,  757 

Philipp,  R.,  Analysis  of  tungsten  bronzes 298 

Determination  of  sulphocyanic  acid 340 

Colorimetric  determination  of  cadmium 189 

Separation  of  copper  and  cadmium 203 

Phillips,  Condition  of  sulphur  in  iron  and  steel 352 

Determination  of  silicon  in  presence  of  silicic  acid 513 

Philosophoff,  P.,  Analysis  of  chlorine  gas 812 

Piloty,  O.,  Separation  of  arsenic  and  antimony 245 

Pincus,  Determination  of  phosphoric  acid 718 

Poggiale,  Titration  of  pyrolusite 624 

Pollak,  L.,  Explosibility  of  carbon  monoxide — nitric  oxide  mixtures 804 

Permanence  of  ammoniacal  copper  solution 756 

Solubility  of  nitrous  oxide 801 

Potain,  Determination  of  carbon  monoxide  in  air 769 

Preusser,  Analysis  of  iron-tungsten  alloys 298 

Prince,  Analysis  of  iodides 671 

Q. 

Quasig,  Ozone  determination 677 

R. 

Rammage,  Determination  of  manganese 616 

Rammelsberg,  Analysis  of  wolframite 297 

Separation  of  lithium,  sodium  and  potassium 55 


INDEX  OF  AUTHORS.  873 

PAGE 

Raschig,  Titration  of  hydroxylamine 631 

sulphuric  acid  by  benzidine 714 

sulphurous  acid 693 

Rayleigh,  Density  of  carbon  dioxide 386,  750 

hydrogen 770 

nitrous  oxide 800 

Recoura,  Determination  of  sulphuric  acid 467 

Reddrop,  Determination  of  manganese 616 

Regnault,  Tension  of  aqueous  vapor 842-845 

Reich,  Determination  of  nitric  acid 453 

sulphur  dioxide 815 

Reinhardt,  Titration  with  permanganate 607,  609 

Retgers,  K.,  Determination  of  manganese 125 

Reuter,  M.,  Determination  of  sulphuric  acid 716 

Richards,  T.  W.,  Precipitation  of  calcium  in  presence  of  magnesium.  ...      76 

Solubility  of  calcium  oxalate 70 

Testing  weights 15 

Ricketts,  Notes  on  assaying 264 

Riegler,  E.,  Standardization  of  permanganate 599 

Riess,  Determination  of  antimony 224 

Ritter,  Determination  of  nitric  acid 460 

Rittener,  Determination  of  carbonic  acid 391 

Rivot,  Analysis  of  iron  ores 88 

Determination  of  copper 186 

Separation  of  copper  from  cadmium 202 

Rohmer,  M.,  Separation  of  arsenic  and  antimony 245,  246 

Romijn,  Titration  of  formaldehyde 694 

hydroxylamine  salt 581 

Rosanoff,  M.  A.;  Determination  of  chlorine 707 

Roscoe,  Precipitation  of  vanadium 304,  305 

Rose,  H.,  Determination  of  ammonium  as  chloroplatinate 58 

bismuth 181 

cyanogen 338 

ferricyanide 343 

sulphur  in  sulphides 359 

water  in  fluosilicates 484 

Precipitation  of  vanadium 304,  306 

Reduction  of  mercuric  salts 171 

Separation  of  antimony  and  tin 250 

,  arsenic,  and  tin 256 

barium,  calcium,  and  strontium 79 

copper  and  cadmium 202 

molybdenum  and  tungsten 296 

phosphoric  and  hydrofluoric  acids 474 

Volumetric  estimation  of  sulphurous  acid 692 


874  INDEX  OF  AUTHORS. 

PAOB 

Rosenbladt,  Determination  of  boric  acid 428 

Rosenheim,  Separation  of  copper  and  nickel 165 

Rossing,  Determination  of  antimony 222 

Separation  of  antimony  and  tin 248,  255 

Rothe,  Solution  of  ferric  chloride  in  ether 167 

Rothmund,  Determination  of  .chlorine 708 

Ruegenberg,  M.,  Separation  of  molybdenum  and  tungsten 293 

Riiderff,  Electrolytic  determination  of  mercury 172 

Rupp,  Methyl  red -. 543 

Volumetric  estimation  of  sulphurous  acid 692 

Rutter,  Analysis  of  chlorates 669 

S.' 

Sahlbom,  Titration  of  hydrofluosilicic  acid 582 

Sand,  H.  G.  S.,  Electrolytic  determination  of  arsenic 212 

Sanger,  C.  R.,  Colorimetric  determination  of  arsenic 208,  210 

Sarnstrom,  Determination  of  carbon  in  steel 399 

Schaffgottsche,  Precipitation  of  magnesium  ammonium  carbonate 69 

Scheen,  Determination  of  antimony 227 

Schellbach,  Burette 529 

Schiff,  H.,  Azotometer 423 

Schirm,  E.,  Determination  of  aluminium 85 

chromium 103 

Schloetter,  Examination  of  electrolytic  chlorine 812 

Schlosser,  W.,  Correction  tables 519,  533 

Draining  of  burettes 528 

Error  in  measuring  vessels 536 

Meniscus  corrections 745 

Schlossing,  Separation  of  potassium  and  sodium 50 

Schmidt,  E.,  Analysis  of  ferrum  reductum 612 

H.,  Separation  of  barium  and  strontium 81 

Schmitz,  B.,  Method  of  precipitating  magnesium 67 

phosphoric  acid 434 

Schneider,  Direct  combustion  of  steel 413 

Manganese  determination 616 

Scholler,  Determination  of  chromium 103 

Schonbein,  Estimation  of  ozone 676 

Schrauth,  Determination  of  chromium 103 

Schroder,  Separation  of  tellurium  and  antimony 281 

Schroter,  A.,  Dehydration  of  silicic  acid 487 

Schrotter,  Alkalimeter 376 

Schucht,  Titration  of  hydrofluosilicic  acid 583 

Schudel,  Determination  of  manganese 120 

Schudl,  Standardization  of  permanganate 97 

Schulze,  Determination  of  nitric  acid 456 


INDEX   OF  AUTHORS.  875 


PAGE 


Schulze,  Titration  of  pyridine  bases 561 

Schweitzer,  P.,  Solubility  of  barium  chromate 75 

Seeman,  L.,  Precipitation  of  gold  by  hydrogen  peroxide 258 

Separation  of  gold  and  platinum 272 

Shimer,  P.  \V.,  Direct  combustion  of  steel 413 

Smith,  Method  for  separating  zinc  from  nickel,  etc 158 

Smith,  E.  F.,  Electrolytic  determination  of  cadmium 190 

mercury 172 

Separation  of  molybdenum  and  tungsten 293 

Smith,  J.  L.,  Determination  of  alkalies  in  silicates 496 

Smitt,  A.,  Determination  of  carbon  in  steel 405 

Snelling,  W.  O.,  Use  of  a  Munroe  crucible 27 

Sonnenschein,  Determination  of  phosphoric  acid 436 

Sonnstadt,  Separation  of  potassium  and  sodium 50 

Sorensen,  Standardization  of  acids 548 

permanganate 597 

Soret,  Determination  of  ozone 680 

Soxhlet,  Fat  extraction  apparatus 236 

Spear,  E.  B.,  Electrolytic  determination  of  zinc 146 

Spitz,  G.,  Titration  of  boric  acid 589 

Spitzer,  Determination  of  zinc . , 145 

Stahrfoss,  M.,  Density  of  acetylene 754 

ethylene 751 

Preparation  of  acetylene 755 

Stas.  .Analysis  of  commercial  platinum 272 

Steen,  O.,  Separation  of  lead  and  bismuth 197 

Steffan,  Determination  of  phosphoric  acid 439,  440 

Volumetric  separation  of  vanadium  and  molybdenum 667,  668 

Stieglitz,  J.,  Theory  of  indicators 540 

Steinbeck,  Determination  of  copper 724 

Steiner,  O.,  Determination  of  chlorine  by  titration 811 

Stillman,  T.  B.,  Analysis  of  incandescent  mantles 512 

Stock,  A.,  Determination  of  aluminium 84 

chromium 103 

Separation  of  arsenic  and  antimony 245 

Stoffel,  M.,  Determination  of  lead 179 

Stokes,  Absorption  of  benzene 753 

ethylene 752 

Determination  of  carbon  monoxide 764 

ferrous  iron 504 

Separation  of  ethylene  and  benzene 756 

Stromayer,  Separation  of  barium,  calcium,  and  strontium 79 

iron  and  titanium 115 

Subech,  A.,  Silicic  acid  in  clay 508 

Swett,  O.  D.,  Use  of  Munroe  crucible 27 


876  INDEX  OF  AUTHORS. 

T. 

PAGE 

Talbot,  H.  P.,  Removal  of  melt  from  crucible 488 

Tamm,  H.,  Determination  of  manganese 121 

zinc 140 

Than,  C.,  Standardization  of  sodium  thiosulphate  solution 647 

Thenard,  Ozone 680 

Thiel,  Determination  of  sulphuric  acid 467 

Thiele,  J.,  Apparatus  for  chlorine  determination 813 

Determination  of  arsenic 205 

antimony 223 

Thomsen,  J.,  Heats  of  combustion 845 

Thomson,  W.,  Electrolytic  determination  of  arsenic 212 

Thorpe,  T.  E.,  Electrolytic  determination  of  arsenic 212 

Tiemann,  Determination  of  nitric  acid . 456 

Tomicek,  Determination  of  arsenic 205 

Tootmann,  S.  R.,  Determination  of  arsenic 212 

Topf ,  Volumetric  determination  of  lead  peroxide 675 

Toth,  J.,  Titration  of  phenol 607 

Treadwell,  F.  P.,  Absorption  of  benzene 753 

ethylene 752 

Apparatus  for  chlorine  determination 808 

Collection  of  gas  samples 733 

Colorimetric  determination  of  arsenic 208 

Computations  in  gas  analysis 782 

Density  of  carbon  monoxide 764 

chlorine 808 

Determination  of  carbon  monoxide 764 

ferrous  iron  in  silicates 503 

hydrofluoric  acid 476 

iron 89 

nickel 137,  138 

ozone 677 

Gases  from  defibrinated  blood 740 

Separation  of  ethylene  and  benzene 756 

Titration  of  hydrofluosilicic  acid 582 

Treubert,  Determination  of  bismuth 181 

Tribe,  Density  of  methane 774 

Preparation  of  ethylene 751 

Tromsdorff,  Determination  of  nitrous  acid  in  water 346 

Tschugaeff,  L.,  Determination  of  nickel 129 

Separation  of  nickel  and  cobalt 161 

manganese 165 

zinc.  .                                           .  165 


INDEX   OF  AUTHORS.  877 

U. 

PAGE 

Ukena,  Determination  of  manganese  in  steel 619 

Urech,  W.,  Determination  of  bismuth 182 

V. 

Vanino,  L.,  Determination  of  bismuth 181 

Precipitation  of  gold  by  hydrogen  peroxide : . . . .   258 

Separation  of  gold  and  platinum 272 

Van  Name,  R.  G.,  Determination  of  copper 186 

van't  Kruys,  M.  J.,  Determination  of  sulphuric  acid 464 

Vernon,  R.  H.,  Analysis  of  fuming  acids 577 

Viogili,  J.  F.,  Solubility  of  mangesium  ammonium  arsenate 208 

Vogel,  Detection  of  carbon  monoxide 768 

cobalt 165 

Voigt,  Determination  of  zinc 140 

Volhard,  Analysis  of  sulphocyanate  and  halogens 342 

Determination  of  manganese  as  sulphate 120 

Precipitation  of  mercuric  sulphide 168 

Standardization  of  sodium  thiosulphate 648 

Titration  of  bromine 709 

chlorine 707 

cyanogen : 710 

iodine 709 

manganese 612 

silver 705 

sulphurous  acid 692 

sulphocyanide 712 

with  permanganate 604 

Transformation  of  chloride  into  oxide 142 

von  Girsewald,  Electrolytic  determination  of  cadmium 189 

Separation  of  copper  and  cadmium 202 

v.  Jiiptner,  H.,  Determination  of  phosphorus 436 

v.  Knorre,  Combustion  of  gases  using  copper  oxide 797 

nitric  oxide 803 

Determination  of  manganese  in  steel 620 

sulphuric  acid 714 

tungsten 290,  291 

v.  der  Pfordten,  O.,  Absorption  of  oxygen 760 

Precipitation  of  mercurous  tungstate 289 

v.  Rath,  G.,  Precipitation  of  mercuric  sulphide 194 

Vortmann,  G.,  Electrolytic  determination  of  mercury 172 

Determination  of  antimony 221 

Removal  of  sulphur  from  precipitates 169 

Separation  of  antimony  and  tin 248,  250 


878  INDEX  OF  AUTHORS. 

W. 

PAGE 

Wade,  Determination  of  carbon  monoxide . 762 

Wagner,  Determination  of  ammonia  in  ammonium  salts 822 

Draining  of  burettes 528 

Indicators 548 

Standardization  of  sodium  thiosulphate  solution 647 

Walker,  Permanence  of  permanganate  solutions 90 

Wallace,  Volumetric  estimation  of  iron 697 

Walters,  J.  H.,  Jr.,  Determination  of  titanium 101 

H.  E.,  Colorimetric  determination  of  manganese 128 

Warder,  R.  B.,  Behavior    of    sodium    bicarbonate     solutions     toward 

phenolphthalein 562 

Titration  of  alkali  carbonate  and  bicarbonate 566 

carbonate  and  hydroxide 564 

Washburn,  E.  W.,  lodimetric  titration  of  arsenic 650 

Weber,  H.,  Determination  of  sulphuric  acid 469 

Weber,  J.,  Ignition  of  calcium  sulphate 71 

Wegelin,  A.,  Composition  of  sodium  sulphide  solution 226 

Determination  of  iron 89 

nitric  acid 458 

Weitnauer,  H.,  Determination  of  manganese 126 

Waller,  A.,  Determination  of  titanium 100,  504 

Titration  of  antimony 687 

Wells,  Determination  of  copper  by  potassium  iodate 672 

Wense,  Separation  of  potassium  and  sodium 50,  51 

Wherry,  E.  T.,  Determination  of  boric  acid  in  minerals 590 

Whitby,  G.  S.,  Solubility  of  silver  chloride 317 

Wiernik,  Specific  gravity  of  ammonia  solutions 841 

Wiborgh,  J.,  Determination  of  carbon  in  steel 405 

sulphur  in  iron  and  steel 354 

Wilfarth,  Modification  of  Kjeldahl  method 63 

Will,  Alkalimeter 376 

Titration  of  pyrolusite .   625 

WTilner,  Determination  of  metallic  iron  in  the  presence  of  oxide 611 

Windelschmidt,  A.,  Determination  of  nickel 137,  138 

WTinkler,  C.,  Absorption  of  benzene 753 

Combustion  of  hydrogen 772 

Detection  of  carbon  monoxide 769 

Determination  of  chlorine  by  titration 811 

Titration  of  alkali  carbonate  and  bicarbonate 565 

hydroxide 563 

bicarbonate  in  the  presence  of  carbonate 592 

Use  of  gauze  electrodes 134 

Winkler- Dennis,  Method  of  technical  gas  analysis 794 

Winkler,  L.  W.,  Absorption  coefficient  of  hydrogen 771 


INDEX  OF  AUTHORS.  879 

PAGE 

Winkler,  L.  W.,  Absorption  coefficient  of  methane 774 

nitric  oxide 803 

nitrogen 807 

Determination  of  absorbed  oxygen 760 

carbon  monoxide 762 

Solubility  of  oxygen 757 

Winteler,  Determination  of  mercury 172 

Titration  of  hydrofluoric  acid 581 

Wohl,  A.,  Computations  in  gas  analysis 782 

Wohler,  Determination  of  carbon  in  steel 406 

Tungsten  bronzes 298 

Wolff,  O.,  Determination  of  antimony 227 

Wolfrum,  L.,  Analysis  of  ferrum  reductum 612 

Wolter,  L.,  Determination  of  tungsten  in  steel 292 

Woy,  Determination  of  phosphorus 436 

as  phosphomolybdic  anhydride 440 

Method  of  precipitating  ammonium  phosphomolybdate 437 

Wynkoop,  G.,  Determination  of  aluminium 85 

Y. 

Young,  S.  W.,  Standardization  of  iodine  solution 651 

Z. 

Zawadzki,  Precipitation  of  sulphides 158 

Zimmermann,  Determination  of  uranium 106 

Precipitation  of  zinc  as  sulphide 158 

Reduction  of  ferric  salts 609 

Titration  with  permanganate 604,  605,  609 

Volumetric  determination  of  uranium .  .                               .  621 


INDEX  OF  SUBJECTS. 


A. 

Acceptor,  definition  of 605 

Acetic  acid, 371,583 

anhydride 583 

Acetylene 754 

determination  in  presence  of  ethylene 821 

preparation  of 755 

Acidimetry 537,  571 

Acids,  specific  gravities  of. 838-389 

volumetric  determination  of 571,  575,  583 

Adsorption 18 

Air  bath 27,  32 

Air,  determination  of  carbon  dioxide  in 397,  593 

Alkali  bicarbonates,  volumetric  determination  of 564 

in  presence  of  carbonates 565 

Alkali  carbonates,  volumetric  determination  of 562 

in  presence  of  bicarbonates 565 

hydroxides 563 

Alkalies 38 

determination  in  lepidolite 502 

silicates 496.  499 

separation  from  metals  of  Group  III 107,  147 

molydebnum 286 

(caustic),  specific  gravities  of 840-841 

volumetric  determination  of 558 

Alkali  hydroxides,  volumetric  determination  of 558 

in  presence  of  carbonates .  .    563 

Alkalimeter 376 

Alkalimetry 537,  558 

Alkaline  earths 70 

separation  from  the  alkalies,  and  magnesium 78 

metals  of  Group  III 107,  147 

molybdenum 287 

one  another 79 

Alkaline  earth  bicarbonates,  volumetric  determination  of 568 

carbonates,  volume*  -'  *  determination  of 566, 567 

881 


882  INDEX  OF  SUBJECTS. 

PAGE 

Alkaline  earth  hydroxides 566 

salts,  titration  of •. 570 

Alkali  sulphides,  analysis  of 689 

and  hydrogen  sulphide 691 

sulphydrates,  titration  of 689 

Aluminium 82 

determination  in  bronzes 236 

silicates 492 

separation  from  alkalies  and  alkaline  earths 107 

chromium 114 

iron 107 

iron  and  phosphoric  acid Ill 

manganese,  nickel,  cobalt  and  zinc 149,  152 

metals  of  Group  II 192,  235 

titanium 116 

uranium 119 

Ammonia  (reagent),  necessity  for  redistilling 82 

preparation  free  from  carbonate 149 

specific  gravity  of 841 

titration  of 560 

Ammonium 57 

colorimetric  determination 60 

gas-volumetric  determination 822 

in  drinking  water 60 

volumetric  determination 560 

Ammonium  molybdate,  preparation  of  the  reagent 437,  638 

Analysis 1 

direct 2 

gravimetric 1,  38 

indirect 2 

volumetric 2 

Anions,  gravimetric  determination  of 320 

calculation  of 737 

Antimony m 218 

determination  in  bearing  metal 252 

electrolytic  determination 224 

removal  from  electrodes 227 

separation  from  arsenic 241 

arsenic  and  tin 256 

mercury,  lead,  etc 235 

metals  of  Group  III 235 

selenium  and  tellurium 281 

tin 248 

volumetric  determination 685,  687 

Apparent  iron  value  of  wire  for  standardization, 98,  601 


INDEX   OF  SUBJECTS.  883 

PAGE 

Arsenic 205 

colorimetric  determination 208 

determination  as  arsine 214 

in  commercial  sulphuric  acid 248 

mispickel 218 

vanadinite 309 

electrolytic  determination 212 

separation  from  antimony 241 

antimony  and  tin 256 

mercury,  lead,  etc 235 

metals  of  Group  III 235 

molybdenum 287 

selenium  and  tellurium 281 

tin 255 

Asbestos  niters 26 

Assay  for  gold  and  silver 259 

Assay  ton 268 

Atomic  weight  table 849 

Azotometer 423 

B. 

Balance,  accuracy  of 6 

sensitiveness  of 7 

Baking  powders,  determination  of  carbonic  acid  in 377 

Barium 74 

detection  in  calcium  precipitates 495 

determination  in  silicates  and  rocks 496,  507 

separation  from  calcium  and  strontium 79 

magnesium 79 

strontium 80 

metals  of  Group  III 107,  147 

metals  of  Group  II 192 

volumetric  determination 566 

Barium  carbonate  method 149 

hydroxide,  normal  solution  of 557 

Basic  acetate  method 152 

Bearing  metal,  analysis  of 252 

Benzene,  determination  in  gas  mixtures 752 

separation  from  ethylene 756 

Benzidine  hydrochloride,  reagent 291,  714,  715 

Bicarbonates,  in  presence  of  carbonates 592 

volumetric  estimation  of 591 

Bichromate,  determination  of  chromium  in 105 

Bismarck  brown 344 

Bismuth .179 


884  INDEX  OF  SUBJECTS. 

PAGE 

Bismuth,  determination  in  bearing  metal 252 

separation  from  arsenic,  antimony,  and  tin 235 

copper  and  cadmium 198 

lead 195 

mercury 194 

metals  of  Groups  III,  IV,  and  V 192 

molybdenum 287 

selenium  and  tellurium 280 

Bitter-almond  water 337 

Blick,  the 261 

Blood,  gases  from  defibrinated 740 

Boric  acid 428 

in  mineral  waters 431 

silicates  and  enamels 431 

volumetric  determination  of 588 

Brass,  analysis  of 193 

Bromine,  determination  in  mineral  waters 660 

non-electrolytes 325,  329 

soluble  bromides 659 

gravimetric  determination  of  329 

separation  from  chlorine 334 

chlorine  and  iodine 336 

iodine 335 

volumetric  determination 655,  660,  709 

Bronzes,  analysis  of . 236 

Burettes 514 

allowable  error  in 530 

calibration  of 527 

draining  of 527 

floats  for , 528 

reading  of 528 

C. 

Cadmium 189 

colorimetric  determination 189 

electrolytic  determination 189 

precipitation  as  sulphide 191 

separation  from  arsenic,  antimony,  and  tin 235 

copper 200-204 

lead 200 

mercury 194 

metals  of  Groups  III,  IV,  and  V 192 

molybdenum 287 

selenium  and  tellurium 280 


INDEX  OF  SUBJECTS.  885 

PAGE 

Calcium 70 

determination  in  sib'cates 494 

separation  from  barium  and  strontium 79 

magnesium 76 

metals  of  Group  III 107,  147 

metals  of  Group  II .    192 

volumetric  estimation 566,  623 

chloride,  presence  of  free  b'me  in 377 

precipitates,  testing  for  barium  in 495 

Calibrated  flasks,  testing  of , 524 

Calibration  of  burettes 527 

gas-measuring  instruments 743 

flasks 522 

pipettes 524 

Carbon,  determination  in  iron  and  steel 398 

nitrogenous  organic  substances 419 

organic  substances 414 

dioxide  (see  Carbonic  acid),  determination  of 750,  778,  788 

in  electrolytic  chlorine 808 

Carbonates,  in  presence  of  bicarbonate 592 

volumetric  estimation  of 591 

Carbonic  acid 375 

combined,  calculation  of 736 

determination  in  air 397,  593 

baking  powders 377 

carbonates 591 

chlorine , 397,  808 

illuminating  gas 778,  788 

mineral  waters 382,  736 

presence  of  bicarbonate 591 

cyanic  and  hydrocyanic  acids  371 

N2O,  NO,  and  N 806 

free,  calculation  of 738 

determination  of 590 

Carbon  monoxide,  determination  of 762,  779,  781,  789 

qualitative  detection  in  air 768 

Cathodes,  cleaning  of 136,  227 

Cations,  calculation  of  those  present  in  water 737 

gravimetric  determination  of 38 

Ceric  oxide,  iodimetric  titration  of 665 

Cerium,  determination  in  soluble  salts 828 

Charcoal,  testing  the  reducing  power  of 265 

Chemical  factors,  table  of 850 

Chlorates,  analysis  of 460,  633,  669 

Chloric  acid,  gravimetric  determination 460 


886 


INDEX  OF  SUBJECTS. 


Chloric  acid,  in  the  presence  of  perchloric  acid 463 

and  hydrochloric  acids 463 

reduction  of 461 

volumetric  determination  of 633,  669 

Chlorine,  free,  determination  by  titration 809 

gravimetric  determination 324 

volumetric  determination 654 

gravimetric  determination 320 

in  aqueous  solutions 320 

commercial  tin  chloride 321 

insoluble  chlorides 323 

organic  substances 325 

vanadinite 308 

separation  from  bromine .  .  . 334 

carbon  dioxide 397 

chlorate  and  perchlorate 463 

cyanogen 339,  711 

fluorine 482 

iodine 331,  335 

iodine  and  bromine 336 

volumetric  determination 654,  707,  708 

Chlorine  gas,  examination  of 808 

Chloroplatinates,  conversion  of  chlorides  into 43 

Chromates,  gravimetric  analysis  of 105 

volumetric  analysis  of 641,  649,  664,  675 

Chromic  acid.     See  chromates. 

compounds 102 

Chromite,  analysis  of 509 

determination  of  chromium  in 675 

Chromium 102 

determination  in  iron  ores  and  rocks 310 

chromite 510,  675 

pig  iron 312 

steel 313 

separation  from  alkaline  earths  and  magnesium 107 

aluminium 114 

iron 113 

nickel,  cobalt,  manganese  and  zinc 149 

volumetric  determination  of 664,  675 

Clay,  soluble  silicic  acid  in 508 

Closet,  drying 24,  25 

Cobalt 138 

electrolytic  determination 138 

separation  from  alkaline  earths  and  magnesium 147 

manganese 161 


INDEX  OF  SUBJECTS.  887 

PAGE 

Cobalt,  separation  frommetals  of  Group  III 149,  152 

metals  of  Group  II 192 

nickel.  . 161-164 

zinc 156 

Coil  for  heating  crucibles  by  steam 32 

Collection  and  confinement  of  gases 730 

Combustion  (elementary  analysis) 414 

of  organic  substances  containing  halogen 421 

metal 422 

sulphur 422 

furnace 415 

tube 416 

of  gases 764 

(a)  by  explosion 765 

(6)  method  of  Dreschmidt 766 

(c)  by  method  of  Winkler-Dennis 794 

(d)  fractional - 766 

Cone  for  holding  crucible  on  water  bath 31 

Copper 182 

analysis  of  for  selenium  and  tellurium 284 

determination  in  bearing  metal 252 

brass. 193 

bronze 236 

ores 673,  683,  725 

electrolytic  determination 187 

separation  from  arsenic,  antimony  and  tin  235 

bismuth 198 

cadium 200 

lead 198 

mercury 194 

metals  of  Groups  III,  IV  and  V 192 

volumetric  determination 672,  682,  724 

Crucible,  Gooch 24,    25 

Crystals,  size  of 36 

Cupellation 259 

Cyanic  acid 371 

in  the  presence  of  carbonic  and  hydrocyanic  acids , .  371 

Cyanogen 337 

in  the  presence  of  halogens , . .  339,  711 

sulphocyanogen 342,  712 

chlorine  and  sulphocyanogen 713 

volumetric  determination 710 

D. 

Defibrinated  blood,  gases  from 740 

Desiccator 23 


888  INDEX  OF  SUBJECTS. 

PAGE 

Dichromate  methods  ............................................  641 

Dimethyl  glyoxime,  reagent  .......................................    129 

Direct  methods  of  analysis  .........................................       2 

Distribution  coefficient  ...........................................  658 

Double  weighing  .................................................       9 

Drying  apparatus  ................................................   416 

closet  ..............................................   24,  25,  33 

of  precipitates  ............................................     21 

of  substances  in  currents  of  gases  ........................   33,  220 

ovens  .....................................  21,  24,  25,  28,  33,  34 

E. 

Electric  furnace  ..................................................  28 

Electrodes  .........................  ..........................   93,  134 

cleaning  of  .........................................    136,  227 

Electrolytic  determination  of  antimony  .............................  224 

arsenic  ................................  212 

cadmium  ..............................  ISO 

cobalt  ................................  138 

copper  ................................  187 

lead  ..................................  177 

mercury  ...............................  172 

nickel  .................................  131 

tin  ...................................  234 

zinc  ..............................  ....  145 

iron,  for  standardizing  permanganate  solution  ............  600 

preparation  of  ....................................  93 

outfit,  ............................................    132,  178 

Elementary  analysis  ..............................................  414 

Ethylene,  determination  of  ...................................   751,  818 

separation  from  acetylene  ................................  821 

benzene  .............................   757,  820 

Evaporat  ion  of  liquids  .......................................... 

Explosion  pipette  ...........................  •  ...............            •  '"** 


F 

Factors,  table  of  chemical  .................. 

Fahlerz,  analysis  of  ............................................ 

Feme  chloride,  solution  in  ether  .......................... 

iron,  volumetric  determination  of  .................   99,  681,  697,  699 

Ferric  salts,  reduction  of  solutions  of  ............... 

by  hydrogen  sulphide  ..............  .   99,  112 

metals  .............................. 

stannous  chloride  ..........................   609 

sulphurous  acid  ...........................  607 


INDEX  OF  SUBJECTS.  889 

PAGE 

Ferricyanic  acid,  gravimetric  estimation  of 344 

volumetric  estimation  of 633,  694 

Ferrocyaiiic  acid,  gravimetric  estimation 342 

volumetric  estimation 632 

Ferrous  iron,  volumetric  determination  of 89,  603,  607,  641 

Ferrum  reductum,  analysis  of 611,  612 

Filters 18 

of  asbestos 26 

size  of 20 

Filtration  and  washing  precipitates 18 

Flask--,  calibrated,  permissible  error  in 524 

calibration  of 522 

Floats  for  burettes 528 

Fluorine,  determination  as  calcium  fluoride 471 

hydrofluosilic  acid 476 

silicon  fluoride 475,  829 

in  calcium  fluoride 472 

lepidolite 502 

mineral  waters 480 

presence  of  phosphoric  acid 474 

separation  from  acids 482 

metals 481 

Formaldehyde  (formalin)  volumetric  determination  of 694 

Formic  acid,  volumetric  determination  of 626 

Fractional  combustion  of  gases 766 

Fuming  acid,  analysis  of 575 

Fuming  sulphuric  acid 577 

Furnace,  electric 28 

G. 

Gas  analysis 729 

exact 775 

technical 786 

Gas-combustion  pipette 792,  794 

Gases,  collection  and  confinement  of 730 

transference  of 742 

Gas  measuring  instruments,  calibration  of 743 

pipettes 786 

Gas-volumetric  methods 822 

Gauze  platinum  electrodes 134 

Generator  gas,  analysis  of 775,  783 

Gold,  determination  in  solutions 257 

ores 263 

precipitation  by  hydrogen  peroxide 258 

separation  from  platinum 271 


890  INDEX  OF  SUBJECTS. 

PAGE 

Gold,  separation  from  selenium  and  tellu»fum 282 

silver 262 

Gram-equivalent,  definition  of 530 

Graphite  in  cast  iron 414 

Gravimetric  analysis,  methods  of 2,  38 

H. 

Halogens,  determination  by  indirect  analysis 334 

gravimetric  determination  in  presence  of  cyanide 339 

separation  from  cyanide  and  sulphocyanide 342 

hydrofluoric  acid 482 

one  another 331 

Hardness  of  water 568,  569 

Hematite,  determination  of  iron  in 610,  698 

Heats  of  combustion  of  gases 845 

Hood 30,  31 

Hydriodic  acid 330 

separation  from  hydrobromic  and  hydrochloric  acids,  331,  336 

hydrocyanic  acid 339 

volumetric  determination 709 

Hydrobromic  acid 329 

determination  in  mineral  waters 660 

separation  from  hydrochloric  acid 334 

and  hydriodic  acids 336 

hydrocyanic  acid 339 

hydriodic  acid 335 

volumetric  estimation 655,  659 

Hydrocarbons,  heavy 751,  779,  788 

separation  of 756 

Hydrochloric  acid .  320 

gas,  determination  of 814 

normal  solution  of 549 

separation  from  chloric  and  perchloric  acid 463 

hydriodic  acid 335 

hydrobromic  acid 334 

hydrogen  sulphide 329 

hydrocyanic  acid 339,  711 

and  sulphocyanic  acids  .  713 

sulphocyanic  acid 342,  713 

volumetric  determination 571,  707,  708 

Hydrocyanic  acid 337 

determination  in  bitter  almond  water 337 

separation  from  cyanic  and  carbonic  acids 371 

halogen  hydride 339,  711 

hydrochloric  and  sulphocyanic  acids.  .   713 


INDEX  OF  SUBJECTS.  891 

PAGE 

Hydrocyanic  acid,  separation  from  sulphocyanic  acid 342,  712 

volumetric  determination  of 710,  711 

Hydroferricyanic  acid 344,  633,  694 

Hydroferrocyanic  acid 342,  632 

Hydrofluoric  acid 471 

determination  as  calcium  fluoride 471 

hydrofluosilic  acid 476 

silicon  fluoride 475,  829 

in  calcium  fluoride 472 

lepidolite 502 

mineral  waters 480 

separation  from  boric  acid 483 

hydrochloric  acid 482 

metals 481 

phosphoric  acid 474 

volumetric  determination 581 

Hydrofluosilic  acid 483 

analysis  of  its  salts 484 

determination  as  calcium  fluoride 483 

potassium  silicofluoride 484 

volumetric  determination 581 

Hydrogen 770 

determination  in  nitrogenous  organic  substances 419 

organic  substances 414 

Hydrogen  peroxide,  iodimetric  titration 680 

methods 826 

titration  by  permanganate 626 

titanous  chloride 700 

sulphide,  colorimetric  determination 354 

expulsion  from  insoluble  sulphides 367 

evolution  and  absorption 350 

determination  in  mineral  waters 349,  688 

gas  mixtures 816 

gravimetric  determination 347 

titration  of 687 

separation  fiom  alkali  sulphydrate 691 

thiosulphate , 691 

Hydrosulphite  of  sodium 760 

Hydrosulphuric  acid  (see  Hydrogen  sulphide) 347 

Hydroxylamine 581,  631 

Hypochlorous  acid 344,  669,  701 

determination  in  the  presence  of  chlorine 655 

Hypophosphorous  acid 372 

separation  from  phosphorous  acid 374 


892  INDEX   OF  SUBJECTS. 


Igniting  precipitates,  method  of 21,  28 

Illuminating  gas,  analysis  of 775,  786 

Incandescent  mantles,  analysis  of 512 

Indicators 538 

Indirect  analysis 2,  56 

determination  of  halogens  by 334 

SO3  content  of  fuming  sulphuric  acid .   579 

error  in -. .  .       6 

Inquartation 259 

International  atomic  weights 849 

lodic  acid 433 

determination  in  the  presence  of  periodates 670 

Iodides,  analysis  of  (see  Iodine) 671 

lodimetry 644 

Iodine,  determination  by  gravimetric  methods 330 

in  mineral  waters 660 

non-electrolytes 328 

soluble  iodides 656 

free,  titration  of , 654 

preparation  of  pure 646 

separation  from  bromine .   335 

bromine  and  chlorine 336 

chlorine 331 

solution,  standardization  of 649 

volumetric  determination 654,  709 

lodo-starch  reaction,  sensitiveness  of 652 

Iron 87 

electrolytic 93,  603 

determination  in  bearing  metal 252 

brass 193 

ferrum  reductum 611 

hematite 610,  698 

presence  of  oxide 611 

silicates 493,  502 

separation  from  alkaline  earths  and  magnesium 107 

aluminium 107 

and  phosphoric  acid Ill 

chromium 113 

manganese 153 

nickel,  cobalt  and  zinc 149,  152,  155 

nickel 166 

metals  of  Group  II 192 

titanium 114 

uranium 119 


INDEX  OF  SUBJECTS.  893 


Iron,  volumetric  determination  by  permanganate  dichromate  method 641 

iodimetric  method 68 1 

permanganate 603,  607 

stannous  chloride 697 

titanous  chloride 699 

ore,  determination  of  vanadium  in 310 

and  chromium  in 312 

wire,  determination  of  apparent  iron  value 98,  601 

L. 

Lacmoid 544 

Lead 174 

determination  in  bearing  metal 252 

brass 193 

bronze 236 

vanadinite J508 

electrolytic  determination  as  peroxide 177 

separation  from  arsenic,  antimony,  and  tin 235 

bismuth 195 

cadmium 200 

copper .  . 198 

mercury 194 

metals  of  Groups  II,  IV,  and  V 192 

molybdenum 287 

peroxide,  analysis  of 675 

sulphate,  separation  from  barium  sulphate  and  silica 176 

volumetric  determination  by  molybdate 726 

Lepidolite,  analysis  of 502 

Lime  method  for  halogens  in  organic  substances 329 

Liquids,  evaporation  of 30 

Liter,  definition  of 516,  521 

Litharge,  testing  of 264 

Lithium 53 

determination  in  lepidolite 502 

indirect  determination  of 56 

separation  from  sodium  and  potassium 53 

Litmus 544 

Logarithms,  tables  of 854,  855 

M. 

Magnesia  mixture,  preparation  of 206 

Magnesium 65 

determination  in  silicates 495 

separation  from  alkalies 68 

barium 79 


894  INDEX  OF  SUBJECTS. 

PAGE 

Magnesium,  separation  from  calcium 76 

metals  of  Group  II 192 

Group  III 107,  147 

strontium 78 

Manganese 120 

colorimetric  determination 127 

determination  in  bronze 237 

iron  and  steel: 

by  bismuthate  method 616 

Volhard's  method 615 

v.  Knorre's  method 620 

Pattinson's  method 642 

Williams'  method 619 

pyrolusite 624,  663 

separation  from  alkaline  earths  and  magnesium 147 

iron 153 

metals  of  Group  II 192 

nickel  and  cobalt 161 

trivalent  metals 149,  155 

zinc 156 

volumetric  determination 612,  616,  619,  620,  663 

Measuring  flasks 515 

calibrating 522 

instruments 514 

for  gas  analysis 743 

Melt,  removal  from  the  crucible 488 

Meniscus  corrections « 745 

Mercury 168 

determination  in  organic  substances 170 

electrolytic  determination 172 

purification  of 747 

separation  from  arsenic,  antimony  and  tin 235 

lead,  bismuth,  copper,  and  cadmium 194 

metals  of  Groups  II,  IV,  and  V 192 

selenium  and  tellurium .  .  .  281 

Metallic  iron,  determination  in  presence  of  oxide 611 

Metalloids,  gravimetric  determination  of 320 

Metaphosphoric  acid 433 

Methane 774 

determination  in  gas  mixtures 780,  781,  790 

separation  from  hydrogen 790 

carbon  monoxide  and  hydrogen 781 

Methods,  gravimetric 2 

volumetric 514 

Methyl  orange :'. 539 


INDEX  OF  SUBJECTS.  895 

PAGE 

Methyl  red 543 

Minium  (red  lead),  volumetric  determination  of 623,  675 

Mispickel,  determination  of  arsenic  in 218 

Mol,  definition  of 455 

Molybdenum 284,  666 

determination  in  steel 313 

residues,  recovery  of  molybdenum  from 447 

separation  from  the  alkalies 286 

the  alkaline  earths 287 

the  metals  of  Groups  II  and  III 287 

phosphoric  acid 288 

tungsten 293 

vanadium 308,  667 

Molybdic  acid,  gravimetric  determination 284 

volumetric  determination 666 

Moment,  statical 8* 

Monazite,  determination  of  thorium  in 510 

Munroe  crucible .  .  27 


N 

Nickel 129 

determination  in  brass 193 

.steel 166,313,  723 

electrolytic  determination 131,  136 

separation  from  alkaline  earths  and  magnesium 147 

cobalt 161,  162,  163,  164 

iron 166 

,  aluminium,  titanium,  and  uranium 149 

,  aluminium,     chromium,     titanium,     and 

uranium 149 

manganese 161,  165 

zinc 156,   165 

volumetric  determination 720 

Nickel-chromium  alloy  for  crucible  triangles 29 

Niobium,  determination  in  wolframite 297 

Nitre,  testing  the  oxidising  power  of 266 

Nitric  acid 451 

determination  as  ammonia 453 

nitric  oxide 456,  825 

nitrogen  pentoxide 453 

nitron  nitrate 451 

in  drinking  water 460 

normal  solution  of 552 

volumetric  determination 571,  634 


896  INDEX   OF  SUBJECTS. 

PAGE 

Nitric  oxide 802 

separation  from  nitrous  oxide 804 

and  nitrogen 805 

nitrogen,  and  carbon  dioxide .  806 

Nitron 451 

Nitrous  acid,  colorimetric  determination 344 

determination  as  nitric  oxide 825 

volumetric  determination 626 

oxide 800 

determination  in  the  presence  of  nitric  oxide 804 

and  nitrogen.  .  805 
nitrogen,  and 

carbon  dioxide  806 

Nitrogen,  determination  by  Dumas  method .  .  . 422 

Kjeldahl  method 62 

in  organic  substances 422 

properties  and  method  of  preparation 806 

separation  from  nitrous  and  nitric  oxides , 805 

oxide  and  carbon  dioxide  .  .  806 

Nitrophenol  as  indicator . 543 

Normal  solutions 530,  548 

of  barium  hydroxide 557 

hydrochloric  acid 549 

nitric  and  sulphuric  acids 552 

oxalic  acid 552 

sodium  hydroxide 553 

preparation  of 532 

standardization  in  acidimetry  and  alkalimetry 548 

volume  and  temperature 516 


O. 

Oil,  removal  from  borings 236 

Oleum,  analysis  of 575 

Operations 6 

Organic  acids,  titration  of 583 

substances,  determination  of  carbon  in 414,  419 

chlorine  in 325,  329 

hydrogen  in^ 414,  419 

nitrogen  in 62,  422 

sulphur  in 370 

Orthoclase,  analysis  of 491 

Orthophosphoric  acid 434 

volumetric  estimation  of 718 

Oven  for  drying 24,  25,  28,  33,  34,  220 


INDEX  OF  SUBJECTS.  897 


PAO.TS 


Oxalic  acid 427 

normal  solution  of 552 

volumetric  determination  of 622 

Oxidation  methods 596 

Oxygen 757 

determination  in  illuminating  gas 779,  789 

presence  of  hydrogen  in 417 

Ozone,  determination  of 676 

P. 

Partition,  law  of  (see  law  of  distribution). 

Paul's  drying  oven 33,  34 

Percarbonates,  analysis  of .  . 628 

Perchloric  acid 462 

determination  in  the  presence  of  chloric  acid 463 

hydrochloric  acid 463 

preparation  of 51 

Perhydrol 230 

Periodic  acid  (and  periodates) 670 

Permanence  of  ammoniacal  copper  solution 756 

permanganate  solutions 90,  603 

sodium  thiosulphate  solution 649 

oxalic  acid  solutions 599 

Permanganate  methods 596 

solution,  permanence  of 90,  603 

preparation  of 596 

standardization  of 91,  597,  827 

uses  of ,603 

Peroxides,  analysis  of 627,  680,  661 

Persulphuric  acid  (and  persulphates),  analysis  by  permanganate 629 

potassium  hydroxide.  .   595 

titanous  chloride 701 

Phenol,  volumetric  estimation  of 695 

Phenolphthalein 545,  554 

Phosphoric  acid 434 

determination  in  calcium  phosphate 720 

mineral  water 447 

silicates 447 

^  vanadinite 309 

separation  from  alkaline  earths  and  alkalies 449 

chromic  acid 499 

iron  and  aluminium Ill 

metals  of  Groups  I,  II,  and  III 448 

vanadium .   307 


•898  INDEX   OF  SUBJECTS. 

PAGE 

Phosphoric  acid,  volumetric  determination 587,  588,  718 

Phosphorous  acid 374 

determination  in  the  presence  of  hypophosphorous  acid.    374 

Phosphorus,  determination  in  bronze 238,  239 

iron  and  steel 440,  443,  445,  588,  637 

organic  substances 448 

Pipettes 514 

calibration  of 524 

permissible  error  in 526 

Platinum 268 

action  of  ferric  chloride  on 1 10 

analysis  of  commercial  platinum 272 

brass  cone  lined  with 31 

capillary  for  use  in  gas  analysis 743,  766 

determination  in  alloys 270 

separation  from  gold  and  silver 270,  271 

Potassium 38 

determination  in  mineral  water 50 

indirect  determination 56 

separation  from  lithium 53 

sodium 43,  49,  50 

Potassium  bichromate,  determination  of  chromium  in 105 

potassium  in 40,  41 

biiodate  solution 647 

dichromate  solution 532,  641,  649 

percarbonate,  analysis  of '. 62b 

permanganate  solution 90,  531,  597,  827 

persulphate,  analysis  of 62i) 

Precipitates,  drying  and  igniting  of 21 

filtration  and  washing  of 18 

method  of  igniting  when  wet 2* 

Precipitation  analyses,  (volumetric) 702 

Preparation  cf  the  substance  for  analysis 35 

Primary  oxides 605 

Producer  gas,  analysis  of 775,  783,  784 

Protoxides,  separation  from  the  sesquioxides 149 

Prussian  blue,  analysis  of 343 

Prussic  acid  (see  Hydrocyanic  acid). 

Pyridine  bases,  titration  of 561 

Pyrite,  determination  of  sulphur  in 362 

Pyrogallol  solution,  preparation  of 758 

Pyrolusite,  analysis  of *.  .   624,  663 

Q. 

Quartation 259 


INDEX  OF  SUBJECTS.  899 

R. 

PAGE 

Hr<  rystallization 35 

Red  lead  (minium),  analysis  of 623,  676 

Reduction  methods  of  volumetric  analysis 697 

of  ferric  salts 607 

weighings  to  vacuo 13 

Resorcin  blue  as  indicator 544 

Rutile,  determination  of  titanium  in 118 


S. 

Salting  out  method  for  precipitating  zinc 160 

Selenium 277,  374 

determination  in  crude  copper 284 

separation  from  gold  and  silver 282 

metals  of  Groups  II,  III,  IV,  and  V 280,  281 

tellurium 279,  282 

Selenous  acid 277,  374 

Sensitiveness,  or  sensibility,  of  the  balance 7 

Sesquioxides  of  Group  III 82 

separation  from  the  protoxides 149 

Silica,  separation  from  tungsten 302 

testing  the  purity  of 487 

triangles  for  platinum  crucibles 29 

Silicates,  analysis  of 491 

decomposable  by  acids 485 

determination  of  alkalies  in 496 

ferrous  iron  in 502 

water  in 484,  512 

not  decomposable  by  acids 491 

Silicon,  determination  in  iron  and  steel 441,  442 

the  presence  of  silica 513 

Silver 317 

determination  in  alloys 259,  703 

ores  (see  Gold) 268 

separation  from  other  metals 318 

selenium  and  tellurium 282 

volumetric  determination 702,  705 

Silver  chloride,  solubility  of 317 

Sodium 43 

determination  in  silicates 496 

indirect  determination  of 56 

separation  from  lithium 53 

potassium 43-50 


900  INDEX  OF  SUBJECTS. 


Sodium  hydroxide,  determination  in  commercial  caustic  soda 558 

caustic  soda  solution 558 

normal  solution  of 553 

preparation  of  a  solution  free  from  carbonates 555 

sulphide,  reagent,  preparation  of 225 

succinate  method  of  separation 155 

thiosulphate  solution,  normal  solution  of 645 

permanence  of 649 

Solubility  product,  definition  of 156 

Solution  of  sulphides,  explanation  of  the  process 157 

Specific  gravity  tables  of  acid  and  alkali 838-841 

Stannic  chloride,  analysis  of 321,  573 

Starch  solution 652 

Statical  moment  of  a  balance 8 

Steel,  determination  of  carbon  in  398 

manganese  in 615,  616,  619,  620,  642 

nickel 166,  723 

nickel,  manganese,  chromium,  and  vanadium. .  .  .   313 

phosphorus 440,  443,  445,  588,  637 

Stibnite,  determination  of  antimony  in 686 

Streak  of  gold  alloys 261 

Strontium 72 

separation  from  barium 80 

calcium 79 

magnesium 78 

metals  of  Group  II 192 

metals  of  Group  III 107,  147 

Substitution,  weighing  by 9 

Sulpho-acids 205 

separation  from  one  another 241 

Sulpho-bases 168 

separation  from  one  another 194 

Sulphides,  determination  in  the  presence  of  sulphates 470 

theory  of  their  solubility  in  acid 157 

titration  of 689 

Sulphocyanic  acid 339,  712 

separation  from  halogen  hydrides 342 

hydrocyanic  and  hydrochloric  acids.  .   713 

Sulphydrates,  analysis   of 689,  691 

Sulphur,  colorimetric  determination 354 

determination  in  insoluble  sulphides 357,  367,  368 

iron  and  steel 352,  354,  364,  365 

mineral  waters 688 

organic  substances 370 

pyrite 357,  362,  716 


INDEX  OF  SUBJECTS.  9°I 

PAGE 

Sulphur,  determination  in  rocks ., 505 

sulphides  soluble  in  acids 350 

water £49 

dioxide,  gravimetric  determination 373 

volumetric  determination 587,  692,  815 

removal  from  precipitated  sulphides 169,  180,  223 

volumetric  determination  in  gas  mixtures 687,  816 

Sulphuretted  hydrogen  (see  Hydrogen  sulphide  and  Sulphur). 

Sulphuric  acid 464 

determination  of  arsenic  in 248 

in  the  presence  of  soluble  sulphides 470 

preparation  of  concentrated  acid  of  definite  strength 580 

normal  solution  of 551 

volumetric  determination  of 571,  577,  714,  716 

Sulphurous  acid  (see  Sulphur  dioxide) 373 

volumetric  determination 587,  692 

Swings,  weighing  by  the  method  of 10 

T. 

Tables  * 517,  519,  520,  522,  533,  534,  535,  838-857 

Tantalum,  determination  of  wolframite 297 

Tare 9 

Tartar  emetic,  analysis  of 685 

Tartaric  acid 433 

Teclu  burner 94 

Telluric  acid  (see  Tellurium) 664 

Tellurium,  determination  in  crude  copper 284 

precipitation  with  sulphurous  acid 279 

separation  from  antimony,  tin,  and  arsenic 281 

copper,  bismuth,  and  cadmium 280 

gold  and  silver 282 

mercury 281 

metals  of  Group  II 280 

Groups  III,  IV,  and  V 279 

selenium 282 

volumetric  estimation 664 

Tellurous  acid  (see  Tellurium) 374 

Temperature,  taken  as  normal  in  volumetric  work 516 

Testing  of  weights 15 

Tetrahedrite,  analysis  of 359 

Thallium,  determination  of 318 

Thiocyanic  acid  (see  Sulphocyanic  acid) . 

*  The  tables  in  this  book  can  be  purchased  separately  in  flexible  cloth  binding.     Price, 
thirty-five  cents. 


902  INDEX  OF  SUBJECTS. 

PAGE 

Thiosulphuric  acid  (thiosulphate) 450,  645 

determination  in  presence  of  sulphide 691 

Thorium,  determination  in  monazite 510 

Tin 228 

determination  in  bearing  metal 252 

bronze 236 

tin  chloride 321,  573 

separation  from  alkaline  earths  and  magnesium 235 

antimony , 248,  256 

arsenic 255 

mercury,  lead,  bismuth,  copper,  and  cadmium 235 

metals  of  Group  III 235 

phosphoric  acid 238,  239 

silicic  acid 298 

tungsten 297,  300 

Titanium 100 

Titanium,  colorimetric  determination 100 

determination  in  rocks 504 

rutile  and  iron  ores 118 

separation  from  alkaline  earths 107 

aluminium 116 

iron 114 

manganese,  nickel,  cobalt  and  zinc 149,  152 

Titanous  chloride,  as  reagent  in  volumetric  analysis 700 

Total  carbon  in  iron  and  steel 399 

Tungsten 288 

determination  in  steel 291 

wolframite 296 

separation  from  molybdenum 293 

silica 302 

tin 297,  300 

Tungsten  bronzes,  analysis  of 298 

U. 

Uranium 106 

separation  from  alkaline  earths  and  magnesium 107 

aluminium  and  iron 119 

metals  of  Group  II 192,  235 

nickel,  cobalt,  manganese  and  zinc 149 

volumetric  determination 621 

V. 

Vacuo,  reduction  of  weighings  to .      13 

Valve,  Bunsen 87,  98,  601 

Contat-Gockel 304,  602 

Vanadic  acid  (see  Vanadium). 


INDEX  OF  SUBJECTS.  903 

PAGE 

Vanadium 303 

determination  in  ores  and  rocks 310 

pig  iron 312 

steel 313 

vanadinite 309 

separation  from  arsenic 306 

molybdenum 308,  313,  667 

phosphoric  acid 307 

volumetric  determination 636,  665 

Vanadinite,  analysis  of 308 

Vapors,  determination  in  gases 831 

Volume,  normal 516 

Volumetric  analysis 1,  514 

w. 

Water  bath 31 

Water,  density  at  different  temperatures 517 

determination  of  absorbed  oxygen  in 760 

in  fluosilicates 484 

lepidolite 502 

silicates 512 

hardness  of 569,  570 

vapor,  tension  of 842 

Washing  precipitates 18 

Weighing 6 

double 9 

by  substitution 9 

swings 10 

reduction  to  vacuo 13 

Weights,  testing  of 15 

Wet  precipitates,  method  of  igniting 28 

Welsbach  mantles,  analysis  of 512 

White  lead,  analysis  of 379 

wolframite  (Wolfram)  analysis  of 296 

Z. 

Zinc 140 

electrolytic  determination 145 

separation  from  alkaline  earths  and  magnesium 147 

metals  of  Group  II 192,  235 

nickel  cobalt,  and  manganese 156 

trivalent  metals  of  Group  III 149-155 

determination  in  bearing  metal 252 

brass 193 

bronze 237 

Zirconium,  determination  in  rocks .   505 


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OCT  19  1956 


Rt. 

JAN  10  1357 


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FEB    41955 
IAN  2  3  195S 


LD  21-95m-ll,'50(2877sl6)476 


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